为了熟悉或者回顾 React 及其周边的相关知识，想必这份来自 Github 的 React 自问自答清单会很适合你，后期我会持续对这份清单做翻译，以减轻阅读难度
React清单
Core React
简单介绍下React?
React 是前端的一个开源库，用来构建交互式应用并在SPA中应用广泛。在 web 和移动端的分层结构中，它充当展示层的角色。React 由在 Facebook 工作的 Jordan Walke 所创建。2011年，Facebook的 News Feed 使用了它，过了一年后，它在 Instagram 也得到了应用。
React的主要特性有哪些?
React 的主要特性包括:
- 使用虚拟DOM来代替真实的DOM操作，以提升性能
- 支持服务端渲染.
- 遵循单向数据流.
- 使用高复用 UI 组件来构建页面
讲讲什么是JSX?
JSX 是一种类似于 XML 的 JS 语法扩展。React 提供有
React.createElement()这样的语法糖来转化 JSX 语法。它的具体格式是由 JS 和 HTML 构成，看起来像模版语言。下面
<h1>标签 通过 render 返回后，最终会由React.createElement()转化为虚拟DOM1
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9class App extends React.Component {
render() {
return(
<div>
<h1>{'Welcome to React world!'}</h1>
</div>
)
}
}React中的元素和组件有啥区别?
React 中的元素是一个普通对象，它描述着屏幕上要展示的内容。而元素对象可以嵌套元素，嵌套的元素在props字段中。创建元素的方式很简单。下面是一个典型的 React 元素
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5const element = React.createElement(
'div',
{id: 'login-btn'},
'Login'
)上面的
React.createElement()函数返回的对象长这样:1
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7{
type: 'div',
props: {
children: 'Login',
id: 'login-btn'
}
}最终会通过
ReactDOM.render()渲染到真实的DOM中:1
<div id='login-btn'>Login</div>
而组件的创建方式就很多了。可以是Class或者函数形式。通常它会有props作为变量输入，然后返回 JSX 语法树
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2const Button = ({ onLogin }) =>
<div id={'login-btn'} onClick={onLogin} />然后 JSX 树会被
React.createElement()函数转移成对象树:1
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5const Button = ({ onLogin }) => React.createElement(
'div',
{ id: 'login-btn', onClick: onLogin },
'Login'
)在React中咋创建组件?
有两种方式创建组件.
- 函数组件: 创建组件最简单的方式. 通常是纯函数，接收 props 作为函数的第一个参数，然后返回 React 元素
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3function Greeting({ message }) {
return <h1>{`Hello, ${message}`}</h1>
}- 类组件: 你也可以通过ES6形式定义组件。上面的组件可转为类组件的写法如下:
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5class Greeting extends React.Component {
render() {
return <h1>{`Hello, ${this.props.message}`}</h1>
}
}类组件和函数组件咋做选择?
如果函数组件内需要state或者生命周期方法则选择类组件，否则选择函数组件。而 React16 后有了 HOOKS 的支持，更推荐函数组件的写法
PureComponent是用来干啥的?
React.PureComponent和React.Component很像，除此之外，它对shouldComponentUpdate()方法做了优化。当props或者state改变时，PureComponent 会针对 props 和 state 做一层浅比较。而普通的组件 则不会做这层比较，默认 props 或 state 的变化会触发重新渲染。讲讲React中的state?
组件中 State 是对象类型，包含着组件生命周期所需要的一些信息。我们应该尽量只在 state 中保存简单的数据，不应该将所有状态都保存在组件内，使得组件保持纯净。下面是一个存有 message 的state
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17class User extends React.Component {
constructor(props) {
super(props)
this.state = {
message: 'Welcome to React world'
}
}
render() {
return (
<div>
<h1>{this.state.message}</h1>
</div>
)
}
}
讲讲React中的props?
Props 是组件的输入数据。它可能是一个字符串或者数字，或者是包含着很多数据的对象，它的写法就像 HTML 的标签属性一样。通常父组件给子组件传递数据时需要用到。
它的功能如下
- 传递自定义的数据到子组件
- 传递函数，间接触发 父组件 state 的变化，使得子组件保持独立性
- 在子组件中可通过类似于
this.props.reactProp这样的方式调用 props 上的变量
比如创建一个有
reactProp变量的 props:1
<Element reactProp={'1'} />
reactProp就成为了 Element 组件上 props 的一个属性。1
props.reactProp
state与props有啥区别?
props 和 state 都是普通的 JS 对象，都包含着渲染相关的数据。它们的不同在于，组件的 props 就像函数的形参，组件的 state 就像函数内部的变量
通过setState更新state要注意啥?
如果你尝试直接修改 state 上的变量，并不会触发重新渲染
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2//Wrong
this.state.message = 'Hello world'应该使用
setState()方法更新 state1
2//Correct
this.setState({ message: 'Hello World' })提示: 只能在 constructor 函数中直接设置 state，这是 state 初始化的地方。
setState中的回调是用来干啥的?
当setState操作结束后且组件渲染更新后触发setState的回调函数，即在componentDidUpdate后触发。因为
setState()是一步操作，因此可用于做一些后期操作。提示: 不推荐使用该回调函数，可以在生命周期方法中做更新之后的操作
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setState({ name: 'John' }, () => console.log('The name has updated and component re-rendered'))
HTML和React的事件处理有啥区别?
- 在 HTML 中，事件名应该小写：
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<button onclick='activateLasers()'>
而在 React 中，遵循的是驼峰原则：
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<button onClick={activateLasers}>
- 在 HTML 中，可以通过 return
false阻止默认行为：
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<a href='#' onclick='console.log("The link was clicked."); return false;' />
而在 React 中，必须调用
preventDefault()阻止：1
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4function handleClick(event) {
event.preventDefault()
console.log('The link was clicked.')
}在JSX中咋绑定事件回调或者组件方法?
有三种方法
- 在类组件的构造方法中绑定。 通常的做法是在构造函数中绑定，因为这样只需要绑定一次
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10class Component extends React.Componenet {
constructor(props) {
super(props)
this.handleClick = this.handleClick.bind(this)
}
handleClick() {
// ...
}
}- 类方法中使用箭头函数。箭头函数中的 this 指向的就是组件实例了
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3handleClick = () => {
console.log('this is:', this)
}1
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3<button onClick={this.handleClick}>
{'Click me'}
</button>- 在事件回调中使用箭头函数绑定
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3<button onClick={(event) => this.handleClick(event)}>
{'Click me'}
</button>咋给事件回调传参?
可以在事件处理外包裹箭头函数，并将参数传入
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<button onClick={() => this.handleClick(id)} />
这和
.bind是等价的1
<button onClick={this.handleClick.bind(this, id)} />
React中的合成事件是啥?
SyntheticEvent是一个跨浏览器原生事件包装器。 具有与浏览器原生事件相同的接口，包括stopPropagation()和preventDefault()等, 这些事件兼容所有的浏览器。React中的行列式条件表达式是啥?
JS 的写法可以通过 if 语句或者是三元表达式返回需要渲染的内容，JSX中则可用大括号包裹内容并加上逻辑操作
&&，当然也可以使用三元表达式，而不能使用 if 语句。1
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7<h1>Hello!</h1>
{
messages.length > 0 &&
<h2>
You have {messages.length} unread messages.
</h2>
}
在数组中key的作用是啥?
key一般用在列表渲染中给每个 item 加上唯一的标识，这样可以帮助 React 识别哪些 item 是需要更新的，或者是新加的，或者应该删除的1
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5const todoItems = todos.map((todo) =>
<li key={todo.id}>
{todo.text}
</li>
)当列表中没有唯一的ID作为 key，可以使用 index 作为最终的方案，但是有一定的风险。
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5const todoItems = todos.map((todo, index) =>
<li key={index}>
{todo.text}
</li>
)提示:
- 不推荐在动态列表中使用 index 作为 key，可能会影响组件的渲染性能
- 如果 item 被单独抽取出来作为一个组件，应该将key加在这个组件上而不是里面的 li 标签上
- 如果不加 key，会有 console 提示
refs是啥?
ref 用来获取元素实例。有时要获取DOM元素或者组件实例还是很有用的
咋创建refs?
有两种方法
通过
React.createRef()创建实例，并将该实例挂载到元素的ref属性上。要在整个组件中使用，则在组件的构造函数创建即可1
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9class MyComponent extends React.Component {
constructor(props) {
super(props)
this.myRef = React.createRef()
}
render() {
return <div ref={this.myRef} />
}
}也可以通过 ref 回调函数来创建
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21class SearchBar extends Component {
constructor(props) {
super(props);
this.txtSearch = null;
this.state = { term: '' };
this.setInputSearchRef = e => {
this.txtSearch = e;
}
}
onInputChange(event) {
this.setState({ term: this.txtSearch.value });
}
render() {
return (
<input
value={this.state.term}
onChange={this.onInputChange.bind(this)}
ref={this.setInputSearchRef} />
);
}
}
当然将箭头函数的回调放在元素上能达到同样的效果
提示: 官方不推荐创建字符串类型的 ref，可能会导致一些问题forwardRefs又是啥?
Ref forwarding 可以将创建的 ref 传递到子组件，然后可以在父组件获取该 ref 绑定的元素
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9const ButtonElement = React.forwardRef((props, ref) => (
<button ref={ref} className="CustomButton">
{props.children}
</button>
));
// Create ref to the DOM button:
const ref = React.createRef();
<ButtonElement ref={ref}>{'Forward Ref'}</ButtonElement>CallbackRefs和findDOMNode咋选择?
findDOMNode()获取的是真实的DOM，而 ref 既可以获取真实 DOM，也可以findDOMNode方式:1
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9class MyComponent extends Component {
componentDidMount() {
findDOMNode(this).scrollIntoView()
}
render() {
return <div />
}
}推荐的 ref 方式:
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9class MyComponent extends Component {
componentDidMount() {
this.node.scrollIntoView()
}
render() {
return <div ref={node => this.node = node} />
}
}为什么不建议创建String类型的refs?
React 中还有一个创建 ref 的方式如
ref={'textInput'}，通过this.refs.textInput可访问该元素。但是不建议通过这种方式创建 ref，它有下面几个问题。并且 React16 也将废弃这种方式。They force React to keep track of currently executing component. This is problematic because it makes react module stateful, and thus causes weird errors when react module is duplicated in the bundle.
They are not composable — if a library puts a ref on the passed child, the user can’t put another ref on it. Callback refs are perfectly composable.
They don’t work with static analysis like Flow. Flow can’t guess the magic that framework does to make the string ref appear on
this.refs, as well as its type (which could be different). Callback refs are friendlier to static analysis.It doesn’t work as most people would expect with the “render callback” pattern (e.g.
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13class MyComponent extends Component {
renderRow = (index) => {
// This won't work. Ref will get attached to DataTable rather than MyComponent:
return <input ref={'input-' + index} />;
// This would work though! Callback refs are awesome.
return <input ref={input => this['input-' + index] = input} />;
}
render() {
return <DataTable data={this.props.data} renderRow={this.renderRow} />
}
}
简单介绍下虚拟DOM?
虚拟DOM(VDOM)是真实DOM的一种表达方式，并且会和真实的DOM同步更新。虚拟DOM的运行机制是发生在 render 函数被调用和页面视觉发生更新之间，这个过程叫做 reconciliation 即调和阶段。
虚拟DOM的原理是啥?
虚拟DOM 的运行流程分为三步
当下面的任何数据发生变化时，对应的虚拟DOM会同步变化
接着便是 diff，对之前的虚拟DOM和最新的虚拟DOM做对比
diff 完成后，只更新真实DOM中真正发生变化的部分
ShadowDOM和VirtualDOM有啥区别?
The Shadow DOM is a browser technology designed primarily for scoping variables and CSS in web components. The Virtual DOM is a concept implemented by libraries in JavaScript on top of browser APIs.
讲讲ReactFiber?
Fiber 是 React16 中一种的新的调和机制。它的设计目的是为了解决的场景包括页面动画、布局、手势、中止及暂停能力，这些在 React16 之前可能会造成页面卡顿，并且会给这些任务分配优先级，以优化这些场景的页面体验。
ReactFiber的设计理念是啥?
React Fiber的设计目的是为了解决场景包括页面动画、布局、手势的卡顿现象。它的核心思想是分片渲染，即将渲染任务分成一系列小的任务，并分配到多个帧中去执行，以充分利用空闲帧，减少卡顿。
啥是可控组件?
可控组件中的数据和组件的 state 挂钩，便于处理和获取
比如下面栗子在input的输入回调中处理输入的数据，存入state中
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3handleChange(event) {
this.setState({value: event.target.value.toUpperCase()})
}啥是不可控组件?
不可控组件则需要操作原生DOM才可获取元素的数据，通过会绑定ref获取原生DOM。
下面的栗子中, 获取input中的值是通过ref来获取，和可控组件通过state方式不同
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24class UserProfile extends React.Component {
constructor(props) {
super(props)
this.handleSubmit = this.handleSubmit.bind(this)
this.input = React.createRef()
}
handleSubmit(event) {
alert('A name was submitted: ' + this.input.current.value)
event.preventDefault()
}
render() {
return (
<form onSubmit={this.handleSubmit}>
<label>
{'Name:'}
<input type="text" ref={this.input} />
</label>
<input type="submit" value="Submit" />
</form>
);
}
}大多数情况下，建议通过可控组件来获取表单类的数据
createElement与cloneElement有啥区别?
JSX elements will be transpiled to
React.createElement()functions to create React elements which are going to be used for the object representation of UI. WhereascloneElementis used to clone an element and pass it new props.React中state中的数据共享是啥?
当几个组件需要共享同一个数据的时候，推荐的做法是将该数据提升到它们共同的父级，而不是将该数据保存在各自组件中。
讲讲组件的生命周期?
有四种不同的阶段。
Initialization: 这个阶段中，组件开始创建初始的 state 及 默认的 props。
Mounting: 组件即将挂载到真实的DOM上。这个阶段包括
componentWillMount()和componentDidMount()生命周期方法。Updating: 这个阶段，组件有两种更新方式包括传递新的props或者改变state。该阶段包括
shouldComponentUpdate(),componentWillUpdate()和componentDidUpdate()生命周期方法。Unmounting: 这个是组件从 DOM 中销毁时触发的阶段， 包括
componentWillUnmount()生命周期函数。
React有哪些生命周期钩子?
后期补充 React16 钩子
- componentWillMount: 在 render 之前触发，仅触发一次。常用于组件级别的配置工作。React16中废弃了该方法。
- componentDidMount: 在 第一次 render 之后执行，常用于 AJAX 请求、DOM 或者 state 更新以及事件的监听。
- componentWillReceiveProps: 在 props 发生变化之前触发，常用于将 props 转化为 state。
- shouldComponentUpdate: 决定了组件是否更新。默认返回true触发更新。如果确定某些 state 或者 props 的改变不需要触发重新渲染，则可以返回false来阻止更新。常用于组件的渲染性能提升。
- componentWillUpdate: 当组件重新渲染时，并且在
shouldComponentUpdate()返回 true 之后触发。 - componentDidUpdate: 当组件更新之后触发，即在更新 render 之后将真实的 DOM 渲染到页面上。
- componentWillUnmount: 当组件即将销毁时触发。常用于网络请求或者移除当前组件所有的事件监听。
HOC是啥?
高阶组件是一个函数，接受一个组件作为入参，通过处理后返回一个新的组件。
通常也叫做纯粹型组件。因为它接受任何组件但并不会修改或者复制该组件的任何行为。
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const EnhancedComponent = higherOrderComponent(WrappedComponent)
HOC 常用于以下几个方面
- Code reuse, logic and bootstrap abstraction.
- Render hijacking.
- State abstraction and manipulation.
- Props manipulation.
在HOC组件中咋创建props代理?
使用 props 代理模式可以添加或者修改输入组件的 props
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14function HOC(WrappedComponent) {
return class Test extends Component {
render() {
const newProps = {
title: 'New Header',
footer: false,
showFeatureX: false,
showFeatureY: true
}
return <WrappedComponent {...this.props} {...newProps} />
}
}
}讲讲React中的context?
Context 提供了一种自上而下跨多层级传递数据的方式，而不是 props 形式一层一层传递。适用于数据需要多个组件访问的场景如用户权限、UI主题等。
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const {Provider, Consumer} = React.createContext(defaultValue)
React中children的prop是啥?
Children 是 prop 上的一个属性 (
this.prop.children)，用来将组件作为数据传递给其他组件，和 props 的性质一样。组件的起始和闭合标签之间的部分就是 Children，在组件中可以通过this.prop.children访问到。这种类型的 prop 有很多相关 API 包括
React.Children.map,React.Children.forEach,React.Children.count,React.Children.only,React.Children.toArray。
下面是一个简单的栗子1
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13const MyDiv = React.createClass({
render: function() {
return <div>{this.props.children}</div>
}
})
ReactDOM.render(
<MyDiv>
<span>{'Hello'}</span>
<span>{'World'}</span>
</MyDiv>,
node
)React中怎么注释代码?
JSX 中的注释类似于 JavaScript 的多行注释，然后再用大括号包裹起来即可
Single-line comments:
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4<div>
{/* Single-line comments(In vanilla JavaScript, the single-line comments are represented by double slash(//)) */}
{`Welcome ${user}, let's play React`}
</div>Multi-line comments:
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5<div>
{/* Multi-line comments for more than
one line */}
{`Welcome ${user}, let's play React`}
</div>在构造函数中使用super(props)有啥作用?
在子组件构造函数中调用了
super()之后才可访问到 this。传递 props 给super()是为了在组件构造函数中能访问到this.props传 props:
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7class MyComponent extends React.Component {
constructor(props) {
super(props)
console.log(this.props) // prints { name: 'John', age: 42 }
}
}不传 props:
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15class MyComponent extends React.Component {
constructor(props) {
super()
console.log(this.props) // prints undefined
// but props parameter is still available
console.log(props) // prints { name: 'John', age: 42 }
}
render() {
// no difference outside constructor
console.log(this.props) // prints { name: 'John', age: 42 }
}
}上面的栗子说明，在构造函数中
super()不传 props，也能在组件构造函数外访问到this.props讲讲reconciliation?
当组件的 props 或者 state 变化的时候，React 通过对比 render 之后新的组件元素和变化前的组件元素来决定真实DOM元素是否需要更新，当元素变化前后不一致时则会触发DOM的更新，这个过程叫调和。
setState中咋使用动态的key?
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3handleInputChange(event) {
this.setState({ [event.target.id]: event.target.value })
}函数被作为props传递给组件有啥常见问题?
传递给组件时不能是函数的调用状态。
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4render() {
// Wrong: handleClick is called instead of passed as a reference!
return <button onClick={this.handleClick()}>{'Click Me'}</button>
}而应该是这样
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4render() {
// Correct: handleClick is passed as a reference!
return <button onClick={this.handleClick}>{'Click Me'}</button>
}React的lazy支持引入多个组件中一个吗?
不支持。目前 React.lazy 只支持引入默认的一个组件。如果想要引入多个组件中的一个，则可以通过一个文件作为中间过度。据一个栗子。
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3// MoreComponents.js
export const SomeComponent = /* ... */;
export const UnusedComponent = /* ... */;再在 MoreComponents.js 中引入需要的那个组件
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2// IntermediateComponent.js
export { SomeComponent as default } from "./MoreComponents.js";最后再通过 lazy 引入该组件
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2import React, { lazy } from 'react';
const SomeComponent = lazy(() => import("./IntermediateComponent.js"));为啥React使用className而不是class属性?
class是 JavaSript 中的保留关键字， 而 JSX 又是 JavaSript 的一种扩展，需要遵循这个规则，因此不能使用 class 属性。1
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3render() {
return <span className={'menu navigation-menu'}>{'Menu'}</span>
}讲讲fragments?
React 中的组件常常需要返回一组元素，这种情况下使用 Fragments 包裹这组元素，不需要添加额外的标签。
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9render() {
return (
<React.Fragment>
<ChildA />
<ChildB />
<ChildC />
</React.Fragment>
)
}也有简写语法， 但是兼容心不是很好
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9render() {
return (
<>
<ChildA />
<ChildB />
<ChildC />
</>
)
}其实 Fragments 不一定要包裹多个元素，包裹一个也是同样的道理。
为啥fragments比div包裹要好?
- Fragments 不需要创建额外的DOM元素，也就不需要内存，所以创建更快。只在规模大的组件中会有真正的影响。
- 某些CSS布局模式如 Flexbox 和 CSS Grid 需要元素间有特定的父子关系，而用 div 包裹会破坏这种关系。
React中的Portals是啥?
Portal is a recommended way to render children into a DOM node that exists outside the DOM hierarchy of the parent component.
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ReactDOM.createPortal(child, container)
The first argument is any renderable React child, such as an element, string, or fragment. The second argument is a DOM element.
啥是无状态组件?
如果组件内不需要维护状态，则就是无状态组件即没有state的组件。可以是函数组件也可以是类组件。除非需要在组件中使用生命周期函数，则建议使用函数组件，不仅容易编写，也容易理解，并且容易测试，相比类组件也会更快一点，同时不需要绑定 this。
啥是状态型组件?
如果组件的行为依赖于状态，则就是状态型组件即有state的组件。通常这些状态型组件是类组件，并且有一个初始化 state 的构造函数。
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10class App extends Component {
constructor(props) {
super(props)
this.state = { count: 0 }
}
render() {
// ...
}
}而 React16 中有了 Hooks 后，也能在函数组件中创建状态型组件。
React中咋检查props变量类型?
When the application is running in development mode, React will automatically check all props that we set on components to make sure they have correct type. If the type is incorrect, React will generate warning messages in the console. It’s disabled in production mode due performance impact. The mandatory props are defined with
isRequired.The set of predefined prop types:
PropTypes.numberPropTypes.stringPropTypes.arrayPropTypes.objectPropTypes.funcPropTypes.nodePropTypes.elementPropTypes.boolPropTypes.symbolPropTypes.any
We can define
propTypesforUsercomponent as below:1
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18import React from 'react'
import PropTypes from 'prop-types'
class User extends React.Component {
static propTypes = {
name: PropTypes.string.isRequired,
age: PropTypes.number.isRequired
}
render() {
return (
<>
<h1>{`Welcome, ${this.props.name}`}</h1>
<h2>{`Age, ${this.props.age}`}</h2>
</>
)
}
}Note: In React v15.5 PropTypes were moved from
React.PropTypestoprop-typeslibrary.React的优势有哪些?
- Increases the application’s performance with Virtual DOM.
- JSX makes code easy to read and write.
- It renders both on client and server side (SSR).
- Easy to integrate with frameworks (Angular, Backbone) since it is only a view library.
- Easy to write unit and integration tests with tools such as Jest.
React有哪些劣势?
- React is just a view library, not a full framework.
- There is a learning curve for beginners who are new to web development.
- Integrating React into a traditional MVC framework requires some additional configuration.
- The code complexity increases with inline templating and JSX.
- Too many smaller components leading to over engineering or boilerplate.
React16中的边缘错误是做啥的?
Error boundaries are components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of the component tree that crashed.
A class component becomes an error boundary if it defines a new lifecycle method called
componentDidCatch(error, info):1
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21class ErrorBoundary extends React.Component {
constructor(props) {
super(props)
this.state = { hasError: false }
}
componentDidCatch(error, info) {
// Display fallback UI
this.setState({ hasError: true })
// You can also log the error to an error reporting service
logErrorToMyService(error, info)
}
render() {
if (this.state.hasError) {
// You can render any custom fallback UI
return <h1>{'Something went wrong.'}</h1>
}
return this.props.children
}
}After that use it as a regular component:
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3<ErrorBoundary>
<MyWidget />
</ErrorBoundary>React15是咋处理边缘错误的?
React v15 provided very basic support for error boundaries using
unstable_handleErrormethod. It has been renamed tocomponentDidCatchin React v16.React中推荐使用哪些静态类型检查工具?
Normally we use PropTypes library (
React.PropTypesmoved to aprop-typespackage since React v15.5) for type checking in the React applications. For large code bases, it is recommended to use static type checkers such as Flow or TypeScript, that perform type checking at compile time and provide auto-completion features.ReactDOM包提供了哪些能力?
The
react-dompackage provides DOM-specific methods that can be used at the top level of your app. Most of the components are not required to use this module. Some of the methods of this package are:render()hydrate()unmountComponentAtNode()findDOMNode()createPortal()
ReactDOM中的render方法是做啥的?
This method is used to render a React element into the DOM in the supplied container and return a reference to the component. If the React element was previously rendered into container, it will perform an update on it and only mutate the DOM as necessary to reflect the latest changes.
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ReactDOM.render(element, container[, callback])
If the optional callback is provided, it will be executed after the component is rendered or updated.
ReactDOMServer是啥?
The
ReactDOMServerobject enables you to render components to static markup (typically used on node server). This object is mainly used for server-side rendering (SSR). The following methods can be used in both the server and browser environments:renderToString()renderToStaticMarkup()
For example, you generally run a Node-based web server like Express, Hapi, or Koa, and you call
renderToStringto render your root component to a string, which you then send as response.1
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11// using Express
import { renderToString } from 'react-dom/server'
import MyPage from './MyPage'
app.get('/', (req, res) => {
res.write('<!DOCTYPE html><html><head><title>My Page</title></head><body>')
res.write('<div id="content">')
res.write(renderToString(<MyPage/>))
res.write('</div></body></html>')
res.end()
})在React中咋使用innerHTML?
The
dangerouslySetInnerHTMLattribute is React’s replacement for usinginnerHTMLin the browser DOM. Just likeinnerHTML, it is risky to use this attribute considering cross-site scripting (XSS) attacks. You just need to pass a__htmlobject as key and HTML text as value.In this example MyComponent uses
dangerouslySetInnerHTMLattribute for setting HTML markup:1
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7function createMarkup() {
return { __html: 'First · Second' }
}
function MyComponent() {
return <div dangerouslySetInnerHTML={createMarkup()} />
}在React中咋使用styles?
The
styleattribute accepts a JavaScript object with camelCased properties rather than a CSS string. This is consistent with the DOM style JavaScript property, is more efficient, and prevents XSS security holes.1
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8const divStyle = {
color: 'blue',
backgroundImage: 'url(' + imgUrl + ')'
};
function HelloWorldComponent() {
return <div style={divStyle}>Hello World!</div>
}Style keys are camelCased in order to be consistent with accessing the properties on DOM nodes in JavaScript (e.g.
node.style.backgroundImage).React中的事件有哪些不同?
Handling events in React elements has some syntactic differences:
- React event handlers are named using camelCase, rather than lowercase.
- With JSX you pass a function as the event handler, rather than a string.
在constructor中使用setState会发生啥?
When you use
setState(), then apart from assigning to the object state React also re-renders the component and all its children. You would get error like this: Can only update a mounted or mounting component. So we need to usethis.stateto initialize variables inside constructor.数组的index作为key会有啥影响?
Keys should be stable, predictable, and unique so that React can keep track of elements.
In the below code snippet each element’s key will be based on ordering, rather than tied to the data that is being represented. This limits the optimizations that React can do.
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6{todos.map((todo, index) =>
<Todo
{...todo}
key={index}
/>
)}If you use element data for unique key, assuming todo.id is unique to this list and stable, React would be able to reorder elements without needing to reevaluate them as much.
1
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4{todos.map((todo) =>
<Todo {...todo}
key={todo.id} />
)}建议在componentWillMount中使用setState吗，为啥?
It is recommended to avoid async initialization in
componentWillMount()lifecycle method.componentWillMount()is invoked immediately before mounting occurs. It is called beforerender(), therefore setting state in this method will not trigger a re-render. Avoid introducing any side-effects or subscriptions in this method. We need to make sure async calls for component initialization happened incomponentDidMount()instead ofcomponentWillMount().1
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8componentDidMount() {
axios.get(`api/todos`)
.then((result) => {
this.setState({
messages: [...result.data]
})
})
}初始化的state变量中使用props会有什么影响?
If the props on the component are changed without the component being refreshed, the new prop value will never be displayed because the constructor function will never update the current state of the component. The initialization of state from props only runs when the component is first created.
The below component won’t display the updated input value:
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14class MyComponent extends React.Component {
constructor(props) {
super(props)
this.state = {
records: [],
inputValue: this.props.inputValue
};
}
render() {
return <div>{this.state.inputValue}</div>
}
}Using props inside render method will update the value:
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13class MyComponent extends React.Component {
constructor(props) {
super(props)
this.state = {
record: []
}
}
render() {
return <div>{this.props.inputValue}</div>
}
}在JSX中咋使用条件语句?
In some cases you want to render different components depending on some state. JSX does not render
falseorundefined, so you can use conditional short-circuiting to render a given part of your component only if a certain condition is true.1
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8const MyComponent = ({ name, address }) => (
<div>
<h2>{name}</h2>
{address &&
<p>{address}</p>
}
</div>
)If you need an
if-elsecondition then use ternary operator.1
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9const MyComponent = ({ name, address }) => (
<div>
<h2>{name}</h2>
{address
? <p>{address}</p>
: <p>{'Address is not available'}</p>
}
</div>
)在DOM元素上平铺props会有啥影响?
When we spread props we run into the risk of adding unknown HTML attributes, which is a bad practice. Instead we can use prop destructuring with
...restoperator, so it will add only required props. For example,1
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5const ComponentA = () =>
<ComponentB isDisplay={true} className={'componentStyle'} />
const ComponentB = ({ isDisplay, ...domProps }) =>
<div {...domProps}>{'ComponentB'}</div>React中咋使用装饰器?
You can decorate your class components, which is the same as passing the component into a function. Decorators are flexible and readable way of modifying component functionality.
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21@setTitle('Profile')
class Profile extends React.Component {
//....
}
/*
title is a string that will be set as a document title
WrappedComponent is what our decorator will receive when
put directly above a component class as seen in the example above
*/
const setTitle = (title) => (WrappedComponent) => {
return class extends React.Component {
componentDidMount() {
document.title = title
}
render() {
return <WrappedComponent {...this.props} />
}
}
}Note: Decorators are a feature that didn’t make it into ES7, but are currently a stage 2 proposal.
React中咋缓存组件?
There are memoize libraries available which can be used on function components. For example
moizelibrary can memoize the component in another component.1
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11import moize from 'moize'
import Component from './components/Component' // this module exports a non-memoized component
const MemoizedFoo = moize.react(Component)
const Consumer = () => {
<div>
{'I will memoize the following entry:'}
<MemoizedFoo/>
</div>
}React中咋做服务端渲染或者SSR?
React is already equipped to handle rendering on Node servers. A special version of the DOM renderer is available, which follows the same pattern as on the client side.
1
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4import ReactDOMServer from 'react-dom/server'
import App from './App'
ReactDOMServer.renderToString(<App />)This method will output the regular HTML as a string, which can be then placed inside a page body as part of the server response. On the client side, React detects the pre-rendered content and seamlessly picks up where it left off.
在React中咋启用生产环境模式?
You should use Webpack’s
DefinePluginmethod to setNODE_ENVtoproduction, by which it strip out things like propType validation and extra warnings. Apart from this, if you minify the code, for example, Uglify’s dead-code elimination to strip out development only code and comments, it will drastically reduce the size of your bundle.使用脚手架CRA构建应用有哪些好处?
The
create-react-appCLI tool allows you to quickly create & run React applications with no configuration step.Let’s create Todo App using CRA:
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11Installation
npm install -g create-react-app
Create new project
create-react-app todo-app
cd todo-app
Build, test and run
npm run build
npm run test
npm start在挂载阶段依次执行的生命周期是哪些?
The lifecycle methods are called in the following order when an instance of a component is being created and inserted into the DOM.
constructor()static getDerivedStateFromProps()render()componentDidMount()
React16即将废弃哪些生命函数钩子?
The following lifecycle methods going to be unsafe coding practices and will be more problematic with async rendering.
componentWillMount()componentWillReceiveProps()componentWillUpdate()
Starting with React v16.3 these methods are aliased with
UNSAFE_prefix, and the unprefixed version will be removed in React v17.函数钩子getDerivedStateFromProps的作用是啥?
The new static
getDerivedStateFromProps()lifecycle method is invoked after a component is instantiated as well as before it is re-rendered. It can return an object to update state, ornullto indicate that the new props do not require any state updates.1
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5class MyComponent extends React.Component {
static getDerivedStateFromProps(props, state) {
// ...
}
}This lifecycle method along with
componentDidUpdate()covers all the use cases ofcomponentWillReceiveProps().函数钩子getSnapshotBeforeUpdate的作用是啥?
The new
getSnapshotBeforeUpdate()lifecycle method is called right before DOM updates. The return value from this method will be passed as the third parameter tocomponentDidUpdate().1
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5class MyComponent extends React.Component {
getSnapshotBeforeUpdate(prevProps, prevState) {
// ...
}
}This lifecycle method along with
componentDidUpdate()covers all the use cases ofcomponentWillUpdate().Hooks会替换掉render模式或者HOC吗?
In JSX the React element is transpiled to
React.createElement()which represents an UI element. WhereasReact.cloneElement()is used in order to clone an element and pass it new props.组件的命名有啥推荐方式?
It is recommended to name the component by reference instead of using
displayName.Using
displayNamefor naming component:1
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4export default React.createClass({
displayName: 'TodoApp',
// ...
})The recommended approach:
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3export default class TodoApp extends React.Component {
// ...
}在class组件中推荐书写函数钩子的顺序是咋样的?
Recommended ordering of methods from mounting to render stage:
staticmethodsconstructor()getChildContext()componentWillMount()componentDidMount()componentWillReceiveProps()shouldComponentUpdate()componentWillUpdate()componentDidUpdate()componentWillUnmount()- click handlers or event handlers like
onClickSubmit()oronChangeDescription() - getter methods for render like
getSelectReason()orgetFooterContent() - optional render methods like
renderNavigation()orrenderProfilePicture() render()
讲讲开关型组件是咋样的?
A switching component is a component that renders one of many components. We need to use object to map prop values to components.
For example, a switching component to display different pages based on
pageprop:1
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22import HomePage from './HomePage'
import AboutPage from './AboutPage'
import ServicesPage from './ServicesPage'
import ContactPage from './ContactPage'
const PAGES = {
home: HomePage,
about: AboutPage,
services: ServicesPage,
contact: ContactPage
}
const Page = (props) => {
const Handler = PAGES[props.page] || ContactPage
return <Handler {...props} />
}
// The keys of the PAGES object can be used in the prop types to catch dev-time errors.
Page.propTypes = {
page: PropTypes.oneOf(Object.keys(PAGES)).isRequired
}为啥推荐传递函数给setState来处理state?
The reason behind for this is that
setState()is an asynchronous operation. React batches state changes for performance reasons, so the state may not change immediately aftersetState()is called. That means you should not rely on the current state when callingsetState()since you can’t be sure what that state will be. The solution is to pass a function tosetState(), with the previous state as an argument. By doing this you can avoid issues with the user getting the old state value on access due to the asynchronous nature ofsetState().Let’s say the initial count value is zero. After three consecutive increment operations, the value is going to be incremented only by one.
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5// assuming this.state.count === 0
this.setState({ count: this.state.count + 1 })
this.setState({ count: this.state.count + 1 })
this.setState({ count: this.state.count + 1 })
// this.state.count === 1, not 3If we pass a function to
setState(), the count gets incremented correctly.1
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4this.setState((prevState, props) => ({
count: prevState.count + props.increment
}))
// this.state.count === 3 as expected在React中咋使用严格模式?
React.StrictModeis an useful component for highlighting potential problems in an application. Just like<Fragment>,<StrictMode>does not render any extra DOM elements. It activates additional checks and warnings for its descendants. These checks apply for development mode only.1
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16import React from 'react'
function ExampleApplication() {
return (
<div>
<Header />
<React.StrictMode>
<div>
<ComponentOne />
<ComponentTwo />
</div>
</React.StrictMode>
<Footer />
</div>
)
}In the example above, the strict mode checks apply to
<ComponentOne>and<ComponentTwo>components only.<StrictMode>currently helps with:- Identifying components with unsafe lifecycle methods.
- Warning about legacy string ref API usage.
- Detecting unexpected side effects.
- Detecting legacy context API.
React中的Mixins是啥?
Mixins are a way to totally separate components to have a common functionality. Mixins are should not be used and can be replaced with higher-order components or decorators.
One of the most commonly used mixins is
PureRenderMixin. You might be using it in some components to prevent unnecessary re-renders when the props and state are shallowly equal to the previous props and state:1
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7const PureRenderMixin = require('react-addons-pure-render-mixin')
const Button = React.createClass({
mixins: [PureRenderMixin],
// ...
})
`
为啥isMounted是一种反模式写法?
The primary use case for
isMounted()is to avoid callingsetState()after a component has been unmounted, because it will emit a warning.1
2
3if (this.isMounted()) {
this.setState({...})
}Checking
isMounted()before callingsetState()does eliminate the warning, but it also defeats the purpose of the warning. UsingisMounted()is a code smell because the only reason you would check is because you think you might be holding a reference after the component has unmounted.An optimal solution would be to find places where
setState()might be called after a component has unmounted, and fix them. Such situations most commonly occur due to callbacks, when a component is waiting for some data and gets unmounted before the data arrives. Ideally, any callbacks should be canceled incomponentWillUnmount(), prior to unmounting.React中支持哪些pointer事件?
Pointer Events provide a unified way of handling all input events. In the olden days we have a mouse and respective event listeners to handle them but nowadays we have many devices which don’t correlate to having a mouse, like phones with touch surface or pens. We need to remember that these events will only work in browsers that support the Pointer Events specification.
The following event types are now available in React DOM:
onPointerDownonPointerMoveonPointerUponPointerCancelonGotPointerCaptureonLostPointerCapturonPointerEnteronPointerLeaveonPointerOveronPointerOut
为啥React组件的首字母需要大写?
If you are rendering your component using JSX, the name of that component has to begin with a capital letter otherwise React will throw an error as unrecognized tag. This convention is because only HTML elements and SVG tags can begin with a lowercase letter.
You can define component class which name starts with lowercase letter, but when it’s imported it should have capital letter. Here lowercase is fine:
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7class myComponent extends Component {
render() {
return <div />
}
}
export default myComponentWhile when imported in another file it should start with capital letter:
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import MyComponent from './MyComponent'
React16支持自定义属性吗?
Yes. In the past, React used to ignore unknown DOM attributes. If you wrote JSX with an attribute that React doesn’t recognize, React would just skip it. For example, this:
1
<div mycustomattribute={'something'} />
Would render an empty div to the DOM with React v15:
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<div />
In React v16 any unknown attributes will end up in the DOM:
1
<div mycustomattribute='something' />
This is useful for supplying browser-specific non-standard attributes, trying new DOM APIs, and integrating with opinionated third-party libraries.
constructor和getinitialState有啥区别?
You should initialize state in the constructor when using ES6 classes, and
getInitialState()method when usingReact.createClass().Using ES6 classes:
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5
6class MyComponent extends React.Component {
constructor(props) {
super(props)
this.state = { /* initial state */ }
}
}Using
React.createClass():1
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5const MyComponent = React.createClass({
getInitialState() {
return { /* initial state */ }
}
})Note:
React.createClass()is deprecated and removed in React v16. Use plain JavaScript classes instead.咋强制渲染一个组件不需要通过setState?
By default, when your component’s state or props change, your component will re-render. If your
render()method depends on some other data, you can tell React that the component needs re-rendering by callingforceUpdate().1
component.forceUpdate(callback)
It is recommended to avoid all uses of
forceUpdate()and only read fromthis.propsandthis.stateinrender().在React中使用super和super（props）有啥不同?
When you want to access
this.propsinconstructor()then you should pass props tosuper()method.Using
super(props):1
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6class MyComponent extends React.Component {
constructor(props) {
super(props)
console.log(this.props) // { name: 'John', ... }
}
}Using
super():1
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6class MyComponent extends React.Component {
constructor(props) {
super()
console.log(this.props) // undefined
}
}Outside
constructor()both will display same value forthis.props.在JSX咋使用循环?
You can simply use
Array.prototype.mapwith ES6 arrow function syntax. For example, theitemsarray of objects is mapped into an array of components:1
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3<tbody>
{items.map(item => <SomeComponent key={item.id} name={item.name} />)}
</tbody>You can’t iterate using
forloop:1
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5<tbody>
for (let i = 0; i < items.length; i++) {
<SomeComponent key={items[i].id} name={items[i].name} />
}
</tbody>This is because JSX tags are transpiled into function calls, and you can’t use statements inside expressions. This may change thanks to
doexpressions which are stage 1 proposal.JSX的元素属性中咋拼接字符串?
React (or JSX) doesn’t support variable interpolation inside an attribute value. The below representation won’t work:
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<img className='image' src='images/{this.props.image}' />
But you can put any JS expression inside curly braces as the entire attribute value. So the below expression works:
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<img className='image' src={'images/' + this.props.image} />
Using template strings will also work:
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<img className='image' src={`images/${this.props.image}`} />
React中的PropTypes的shape字段是啥?
If you want to pass an array of objects to a component with a particular shape then use
React.PropTypes.shape()as an argument toReact.PropTypes.arrayOf().1
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6ReactComponent.propTypes = {
arrayWithShape: React.PropTypes.arrayOf(React.PropTypes.shape({
color: React.PropTypes.string.isRequired,
fontSize: React.PropTypes.number.isRequired
})).isRequired
}JSX中的class咋使用判断语句?
You shouldn’t use curly braces inside quotes because it is going to be evaluated as a string.
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<div className="btn-panel {this.props.visible ? 'show' : 'hidden'}">
Instead you need to move curly braces outside (don’t forget to include spaces between class names):
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<div className={'btn-panel ' + (this.props.visible ? 'show' : 'hidden')}>
Template strings will also work:
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<div className={`btn-panel ${this.props.visible ? 'show' : 'hidden'}`}>
React和ReactDOM有啥区别?
The
reactpackage containsReact.createElement(),React.Component,React.Children, and other helpers related to elements and component classes. You can think of these as the isomorphic or universal helpers that you need to build components. Thereact-dompackage containsReactDOM.render(), and inreact-dom/serverwe have server-side rendering support withReactDOMServer.renderToString()andReactDOMServer.renderToStaticMarkup().为啥ReactDOM要和React分开在两个包中?
The React team worked on extracting all DOM-related features into a separate library called ReactDOM. React v0.14 is the first release in which the libraries are split. By looking at some of the packages,
react-native,react-art,react-canvas, andreact-three, it has become clear that the beauty and essence of React has nothing to do with browsers or the DOM. To build more environments that React can render to, React team planned to split the main React package into two:reactandreact-dom. This paves the way to writing components that can be shared between the web version of React and React Native.在React中咋使用label标签?
If you try to render a
<label>element bound to a text input using the standardforattribute, then it produces HTML missing that attribute and prints a warning to the console.1
2<label for={'user'}>{'User'}</label>
<input type={'text'} id={'user'} />Since
foris a reserved keyword in JavaScript, usehtmlForinstead.1
2<label htmlFor={'user'}>{'User'}</label>
<input type={'text'} id={'user'} />在JSX元素style属性中咋合并多个行内样式?
You can use spread operator in regular React:
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<button style={{...styles.panel.button, ...styles.panel.submitButton}}>{'Submit'}</button>
If you’re using React Native then you can use the array notation:
1
<button style={[styles.panel.button, styles.panel.submitButton]}>{'Submit'}</button>
当浏览器窗口大小调整时咋触发页面渲染?
You can listen to the
resizeevent incomponentDidMount()and then update the dimensions (widthandheight). You should remove the listener incomponentWillUnmount()method.1
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21class WindowDimensions extends React.Component {
componentWillMount() {
this.updateDimensions()
}
componentDidMount() {
window.addEventListener('resize', this.updateDimensions)
}
componentWillUnmount() {
window.removeEventListener('resize', this.updateDimensions)
}
updateDimensions() {
this.setState({width: $(window).width(), height: $(window).height()})
}
render() {
return <span>{this.state.width} x {this.state.height}</span>
}
}setState和replaceState有啥区别?
When you use
setState()the current and previous states are merged.replaceState()throws out the current state, and replaces it with only what you provide. UsuallysetState()is used unless you really need to remove all previous keys for some reason. You can also set state tofalse/nullinsetState()instead of usingreplaceState().咋监听state的变化?
The following lifecycle methods will be called when state changes. You can compare provided state and props values with current state and props to determine if something meaningful changed.
1
2componentWillUpdate(object nextProps, object nextState)
componentDidUpdate(object prevProps, object prevState)React中咋移除数组中的元素?
The better approach is to use
Array.prototype.filter()method.For example, let’s create a
removeItem()method for updating the state.1
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5removeItem(index) {
this.setState({
data: this.state.data.filter((item, i) => i !== index)
})
}在render中可以不使用HTML元素吗?
It is possible with latest version (>=16.2). Below are the possible options:
1
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3render() {
return false
}1
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3render() {
return null
}1
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3render() {
return []
}1
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3render() {
return <React.Fragment></React.Fragment>
}1
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3render() {
return <></>
}Returning
undefinedwon’t work.在React中咋优雅地打印JSON数据?
We can use
<pre>tag so that the formatting of theJSON.stringify()is retained:1
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13const data = { name: 'John', age: 42 }
class User extends React.Component {
render() {
return (
<pre>
{JSON.stringify(data, null, 2)}
</pre>
)
}
}
React.render(<User />, document.getElementById('container'))React中为啥不能修改props?
The React philosophy is that props should be immutable and top-down. This means that a parent can send any prop values to a child, but the child can’t modify received props.
页面加载时咋聚焦input元素?
You can do it by creating ref for
inputelement and using it incomponentDidMount():1
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21class App extends React.Component{
componentDidMount() {
this.nameInput.focus()
}
render() {
return (
<div>
<input
defaultValue={'Won\'t focus'}
/>
<input
ref={(input) => this.nameInput = input}
defaultValue={'Will focus'}
/>
</div>
)
}
}
ReactDOM.render(<App />, document.getElementById('app'))React有那几种方式更新state中的对象变量?
Calling
setState()with an object to merge with state:Using
Object.assign()to create a copy of the object:1
2const user = Object.assign({}, this.state.user, { age: 42 })
this.setState({ user })Using spread operator:
1
2const user = { ...this.state.user, age: 42 }
this.setState({ user })
Calling
setState()with a function:1
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6this.setState(prevState => ({
user: {
...prevState.user,
age: 42
}
}))
为啥建议传递给setState参数是函数而不是对象?
React may batch multiple
setState()calls into a single update for performance. Becausethis.propsandthis.statemay be updated asynchronously, you should not rely on their values for calculating the next state.This counter example will fail to update as expected:
1
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4// Wrong
this.setState({
counter: this.state.counter + this.props.increment,
})The preferred approach is to call
setState()with function rather than object. That function will receive the previous state as the first argument, and the props at the time the update is applied as the second argument.1
2
3
4// Correct
this.setState((prevState, props) => ({
counter: prevState.counter + props.increment
}))React在浏览器端运行时咋打印版本号?
You can use
React.versionto get the version.1
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6const REACT_VERSION = React.version
ReactDOM.render(
<div>{`React version: ${REACT_VERSION}`}</div>,
document.getElementById('app')
)在create-react-app中有那几种方式引入polyfills?
Manual import from
core-js:Create a file called (something like)
polyfills.jsand import it into rootindex.jsfile. Runnpm install core-jsoryarn add core-jsand import your specific required features.1
2
3import 'core-js/fn/array/find'
import 'core-js/fn/array/includes'
import 'core-js/fn/number/is-nan'Using Polyfill service:
Use the polyfill.io CDN to retrieve custom, browser-specific polyfills by adding this line to
index.html:1
<script src='https://cdn.polyfill.io/v2/polyfill.min.js?features=default,Array.prototype.includes'></script>
In the above script we had to explicitly request the
Array.prototype.includesfeature as it is not included in the default feature set.
在create-react-app中咋切换http和https?
You just need to use
HTTPS=trueconfiguration. You can edit yourpackage.jsonscripts section:1
2
3"scripts": {
"start": "set HTTPS=true && react-scripts start"
}or just run
set HTTPS=true && npm start在create-react-app中如何避免import相对路径?
Create a file called
.envin the project root and write the import path:1
NODE_PATH=src/app
After that restart the development server. Now you should be able to import anything inside
src/appwithout relative paths.在ReactRouter中咋引入谷歌分析?
Add a listener on the
historyobject to record each page view:1
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4history.listen(function (location) {
window.ga('set', 'page', location.pathname + location.search)
window.ga('send', 'pageview', location.pathname + location.search)
})咋实现每秒更新组件的效果?
You need to use
setInterval()to trigger the change, but you also need to clear the timer when the component unmounts to prevent errors and memory leaks.1
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7componentDidMount() {
this.interval = setInterval(() => this.setState({ time: Date.now() }), 1000)
}
componentWillUnmount() {
clearInterval(this.interval)
}在inline样式中如何写兼容样式?
React does not apply vendor prefixes automatically. You need to add vendor prefixes manually.
1
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5<div style={{
transform: 'rotate(90deg)',
WebkitTransform: 'rotate(90deg)', // note the capital 'W' here
msTransform: 'rotate(90deg)' // 'ms' is the only lowercase vendor prefix
}} />React和ES6咋引入和导出组件?
You should use default for exporting the components
1
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12import React from 'react'
import User from 'user'
export default class MyProfile extends React.Component {
render(){
return (
<User type="customer">
//...
</User>
)
}
}With the export specifier, the MyProfile is going to be the member and exported to this module and the same can be imported without mentioning the name in other components.
React的组件命名有哪些非硬性规定?
In JSX, lowercase tag names are considered to be HTML tags. However, capitalized and lowercase tag names with a dot (property accessors) aren’t.
<component />compiles toReact.createElement('component')(i.e, HTML tag)<obj.component />compiles toReact.createElement(obj.component)<Component />compiles toReact.createElement(Component)
为啥组件的构造函数只被调用了一次?
React’s reconciliation algorithm assumes that without any information to the contrary, if a custom component appears in the same place on subsequent renders, it’s the same component as before, so reuses the previous instance rather than creating a new one.
React中如何定义常量?
You can use ES7
staticfield to define constant.1
2
3class MyComponent extends React.Component {
static DEFAULT_PAGINATION = 10
}Static fields are part of the Class Fields stage 3 proposal.
React中咋手动触发事件?
You could use the ref prop to acquire a reference to the underlying
HTMLInputElementobject through a callback, store the reference as a class property, then use that reference to later trigger a click from your event handlers using theHTMLElement.clickmethod. This can be done in two steps:Create ref in render method:
1
<input ref={input => this.inputElement = input} />
Apply click event in your event handler:
1
this.inputElement.click()
React中咋使用async和await?
If you want to use
async/awaitin React, you will need Babel and transform-async-to-generator plugin. React Native ships with Babel and a set of transforms.React项目的文件结构一般是啥样的?
There are two common practices for React project file structure.
Grouping by features or routes:
One common way to structure projects is locate CSS, JS, and tests together, grouped by feature or route.
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18common/
├─ Avatar.js
├─ Avatar.css
├─ APIUtils.js
└─ APIUtils.test.js
feed/
├─ index.js
├─ Feed.js
├─ Feed.css
├─ FeedStory.js
├─ FeedStory.test.js
└─ FeedAPI.js
profile/
├─ index.js
├─ Profile.js
├─ ProfileHeader.js
├─ ProfileHeader.css
└─ ProfileAPI.jsGrouping by file type:
Another popular way to structure projects is to group similar files together.
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15api/
├─ APIUtils.js
├─ APIUtils.test.js
├─ ProfileAPI.js
└─ UserAPI.js
components/
├─ Avatar.js
├─ Avatar.css
├─ Feed.js
├─ Feed.css
├─ FeedStory.js
├─ FeedStory.test.js
├─ Profile.js
├─ ProfileHeader.js
└─ ProfileHeader.css
**[⬆ 回顶部](#React清单)**React动画类有哪些包?
React Transition Group and React Motion are popular animation packages in React ecosystem.
Styles模块的写法有哪些好处?
It is recommended to avoid hard coding style values in components. Any values that are likely to be used across different UI components should be extracted into their own modules.
For example, these styles could be extracted into a separate component:
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13export const colors = {
white,
black,
blue
}
export const space = [
0,
8,
16,
32,
64
]And then imported individually in other components:
1
import { space, colors } from './styles'
**[⬆ 回顶部](#React清单)**React有哪些流行的代码检查工具?
ESLint is a popular JavaScript linter. There are plugins available that analyse specific code styles. One of the most common for React is an npm package called
eslint-plugin-react. By default, it will check a number of best practices, with rules checking things from keys in iterators to a complete set of prop types. Another popular plugin iseslint-plugin-jsx-a11y, which will help fix common issues with accessibility. As JSX offers slightly different syntax to regular HTML, issues withalttext andtabindex, for example, will not be picked up by regular plugins.AJAX请求应该放在哪个生命周期?
You can use AJAX libraries such as Axios, jQuery AJAX, and the browser built-in
fetch. You should fetch data in thecomponentDidMount()lifecycle method. This is so you can usesetState()to update your component when the data is retrieved.For example, the employees list fetched from API and set local state:
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41class MyComponent extends React.Component {
constructor(props) {
super(props)
this.state = {
employees: [],
error: null
}
}
componentDidMount() {
fetch('https://api.example.com/items')
.then(res => res.json())
.then(
(result) => {
this.setState({
employees: result.employees
})
},
(error) => {
this.setState({ error })
}
)
}
render() {
const { error, employees } = this.state
if (error) {
return <div>Error: {error.message}</div>;
} else {
return (
<ul>
{employees.map(item => (
<li key={employee.name}>
{employee.name}-{employees.experience}
</li>
))}
</ul>
)
}
}
}render的props是啥?
Render Props is a simple technique for sharing code between components using a prop whose value is a function. The below component uses render prop which returns a React element.
1
2
3<DataProvider render={data => (
<h1>{`Hello ${data.target}`}</h1>
)}/>Libraries such as React Router and DownShift are using this pattern.
React Router
简单介绍下ReactRouter?
React Router is a powerful routing library built on top of React that helps you add new screens and flows to your application incredibly quickly, all while keeping the URL in sync with what’s being displayed on the page.
ReactRouter和history库有啥区别?
React Router is a wrapper around the
historylibrary which handles interaction with the browser’swindow.historywith its browser and hash histories. It also provides memory history which is useful for environments that don’t have global history, such as mobile app development (React Native) and unit testing with Node.ReactRouter4中的
有哪些组件? React Router v4 provides below 3
<Router>components:<BrowserRouter><HashRouter><MemoryRouter>
The above components will create browser, hash, and memory history instances. React Router v4 makes the properties and methods of the
historyinstance associated with your router available through the context in therouterobject.history库中的push和replace是干啥的?
A history instance has two methods for navigation purpose.
push()replace()
If you think of the history as an array of visited locations,
push()will add a new location to the array andreplace()will replace the current location in the array with the new one.ReactRouter4中咋手动调用跳转?
There are three different ways to achieve programmatic routing/navigation within components.
Using the
withRouter()higher-order function:The
withRouter()higher-order function will inject the history object as a prop of the component. This object providespush()andreplace()methods to avoid the usage of context.1
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10import { withRouter } from 'react-router-dom' // this also works with 'react-router-native'
const Button = withRouter(({ history }) => (
<button
type='button'
onClick={() => { history.push('/new-location') }}
>
{'Click Me!'}
</button>
))Using
<Route>component and render props pattern:The
<Route>component passes the same props aswithRouter(), so you will be able to access the history methods through the history prop.1
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12import { Route } from 'react-router-dom'
const Button = () => (
<Route render={({ history }) => (
<button
type='button'
onClick={() => { history.push('/new-location') }}
>
{'Click Me!'}
</button>
)} />
)Using context:
This option is not recommended and treated as unstable API.
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16const Button = (props, context) => (
<button
type='button'
onClick={() => {
context.history.push('/new-location')
}}
>
{'Click Me!'}
</button>
)
Button.contextTypes = {
history: React.PropTypes.shape({
push: React.PropTypes.func.isRequired
})
}
ReactRouter4中咋获取query?
The ability to parse query strings was taken out of React Router v4 because there have been user requests over the years to support different implementation. So the decision has been given to users to choose the implementation they like. The recommended approach is to use query strings library.
1
2const queryString = require('query-string');
const parsed = queryString.parse(props.location.search);You can also use
URLSearchParamsif you want something native:1
2const params = new URLSearchParams(props.location.search)
const foo = params.get('name')You should use a polyfill for IE11.
什么时候会抛出Router可能只有一个子元素提示?
You have to wrap your Route’s in a
<Switch>block because<Switch>is unique in that it renders a route exclusively.At first you need to add
Switchto your imports:1
import { Switch, Router, Route } from 'react-router'
Then define the routes within
<Switch>block:1
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3
4
5
6<Router>
<Switch>
<Route {/* ... */} />
<Route {/* ... */} />
</Switch>
</Router>ReactRouter4中咋传递参数到history.push中?
While navigating you can pass props to the
historyobject:1
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3
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5this.props.history.push({
pathname: '/template',
search: '?name=sudheer',
state: { detail: response.data }
})The
searchproperty is used to pass query params inpush()method.ReactRouter4中咋添加默认或者空页面?
A
<Switch>renders the first child<Route>that matches. A<Route>with no path always matches. So you just need to simply drop path attribute as below1
2
3
4
5<Switch>
<Route exact path="/" component={Home}/>
<Route path="/user" component={User}/>
<Route component={NotFound} />
</Switch>ReactRouter4中咋获取history?
Create a module that exports a
historyobject and import this module across the project.For example, create
history.jsfile:1
2
3
4
5import { createBrowserHistory } from 'history'
export default createBrowserHistory({
/* pass a configuration object here if needed */
})You should use the
<Router>component instead of built-in routers. Imported the abovehistory.jsinsideindex.jsfile:1
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9import { Router } from 'react-router-dom'
import history from './history'
import App from './App'
ReactDOM.render((
<Router history={history}>
<App />
</Router>
), holder)You can also use push method of
historyobject similar to built-in history object:1
2
3
4// some-other-file.js
import history from './history'
history.push('/go-here')
ReactRouter4中login后咋自动重定向?
The
react-routerpackage provides<Redirect>component in React Router. Rendering a<Redirect>will navigate to a new location. Like server-side redirects, the new location will override the current location in the history stack.1
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12import React, { Component } from 'react'
import { Redirect } from 'react-router'
export default class LoginComponent extends Component {
render() {
if (this.state.isLoggedIn === true) {
return <Redirect to="/your/redirect/page" />
} else {
return <div>{'Login Please'}</div>
}
}
}
React Redux
讲讲flux是啥?
Flux is an application design paradigm used as a replacement for the more traditional MVC pattern. It is not a framework or a library but a new kind of architecture that complements React and the concept of Unidirectional Data Flow. Facebook uses this pattern internally when working with React.
The workflow between dispatcher, stores and views components with distinct inputs and outputs as follows:
介绍下Redux是啥?
Redux is a predictable state container for JavaScript apps based on the Flux design pattern. Redux can be used together with React, or with any other view library. It is tiny (about 2kB) and has no dependencies.
Redux的核心理念是啥?
Redux follows three fundamental principles:
- Single source of truth: The state of your whole application is stored in an object tree within a single store. The single state tree makes it easier to keep track of changes over time and debug or inspect the application.
- State is read-only: The only way to change the state is to emit an action, an object describing what happened. This ensures that neither the views nor the network callbacks will ever write directly to the state.
- Changes are made with pure functions: To specify how the state tree is transformed by actions, you write reducers. Reducers are just pure functions that take the previous state and an action as parameters, and return the next state.
Redux相比flux有哪些缺点?
Instead of saying downsides we can say that there are few compromises of using Redux over Flux. Those are as follows:
- You will need to learn to avoid mutations: Flux is un-opinionated about mutating data, but Redux doesn’t like mutations and many packages complementary to Redux assume you never mutate the state. You can enforce this with dev-only packages like
redux-immutable-state-invariant, Immutable.js, or instructing your team to write non-mutating code. - You’re going to have to carefully pick your packages: While Flux explicitly doesn’t try to solve problems such as undo/redo, persistence, or forms, Redux has extension points such as middleware and store enhancers, and it has spawned a rich ecosystem.
- There is no nice Flow integration yet: Flux currently lets you do very impressive static type checks which Redux doesn’t support yet.
- You will need to learn to avoid mutations: Flux is un-opinionated about mutating data, but Redux doesn’t like mutations and many packages complementary to Redux assume you never mutate the state. You can enforce this with dev-only packages like
mapStateToProps和mapDispatchToProps有啥不同?
mapStateToProps()is a utility which helps your component get updated state (which is updated by some other components):1
2
3
4
5const mapStateToProps = (state) => {
return {
todos: getVisibleTodos(state.todos, state.visibilityFilter)
}
}mapDispatchToProps()is a utility which will help your component to fire an action event (dispatching action which may cause change of application state):1
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5
6
7const mapDispatchToProps = (dispatch) => {
return {
onTodoClick: (id) => {
dispatch(toggleTodo(id))
}
}
}可以在reducer中dispatch一个action吗?
Dispatching an action within a reducer is an anti-pattern. Your reducer should be without side effects, simply digesting the action payload and returning a new state object. Adding listeners and dispatching actions within the reducer can lead to chained actions and other side effects.
在组件外如何获取store?
Yes. You just need to export the store from the module where it created with
createStore(). Also, it shouldn’t pollute the global window object.1
2
3store = createStore(myReducer)
export default storeMVW模式有哪些缺点?
- The DOM manipulation is very expensive which causes applications behaves slowly and inefficient.
- Due to circular dependencies, a complicated model was created around models and views.
- Lot of data changes happens for collaborative applications(like Google Docs).
- No way to do undo (travel back in time) easily without adding so much extra code.
Redux和RxJS有哪些相似之处?
These libraries are very different for very different purposes, but there are some vague similarities.
Redux is a tool for managing state throughout the application. It is usually used as an architecture for UIs. Think of it as an alternative to (half of) Angular. RxJS is a reactive programming library. It is usually used as a tool to accomplish asynchronous tasks in JavaScript. Think of it as an alternative to Promises. Redux uses the Reactive paradigm because the Store is reactive. The Store observes actions from a distance, and changes itself. RxJS also uses the Reactive paradigm, but instead of being an architecture, it gives you basic building blocks, Observables, to accomplish this pattern.
页面的加载中咋dispatch一个action?
You can dispatch an action in
componentDidMount()method and inrender()method you can verify the data.1
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19class App extends Component {
componentDidMount() {
this.props.fetchData()
}
render() {
return this.props.isLoaded
? <div>{'Loaded'}</div>
: <div>{'Not Loaded'}</div>
}
}
const mapStateToProps = (state) => ({
isLoaded: state.isLoaded
})
const mapDispatchToProps = { fetchData }
export default connect(mapStateToProps, mapDispatchToProps)(App)咋使用connect?
You need to follow two steps to use your store in your container:
Use
mapStateToProps(): It maps the state variables from your store to the props that you specify.Connect the above props to your container: The object returned by the
mapStateToPropsfunction is connected to the container. You can importconnect()fromreact-redux.1
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14import React from 'react'
import { connect } from 'react-redux'
class App extends React.Component {
render() {
return <div>{this.props.containerData}</div>
}
}
function mapStateToProps(state) {
return { containerData: state.data }
}
export default connect(mapStateToProps)(App)
咋重置redux中的state?
You need to write a root reducer in your application which delegate handling the action to the reducer generated by
combineReducers().For example, let us take
rootReducer()to return the initial state afterUSER_LOGOUTaction. As we know, reducers are supposed to return the initial state when they are called withundefinedas the first argument, no matter the action.1
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11const appReducer = combineReducers({
/* your app's top-level reducers */
})
const rootReducer = (state, action) => {
if (action.type === 'USER_LOGOUT') {
state = undefined
}
return appReducer(state, action)
}In case of using
redux-persist, you may also need to clean your storage.redux-persistkeeps a copy of your state in a storage engine. First, you need to import the appropriate storage engine and then, to parse the state before setting it to undefined and clean each storage state key.1
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15const appReducer = combineReducers({
/* your app's top-level reducers */
})
const rootReducer = (state, action) => {
if (action.type === 'USER_LOGOUT') {
Object.keys(state).forEach(key => {
storage.removeItem(`persist:${key}`)
})
state = undefined
}
return appReducer(state, action)
}at在connect装饰器的作用是啥?
The @ symbol is in fact a JavaScript expression used to signify decorators. Decorators make it possible to annotate and modify classes and properties at design time.
Let’s take an example setting up Redux without and with a decorator.
Without decorator:
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18import React from 'react'
import * as actionCreators from './actionCreators'
import { bindActionCreators } from 'redux'
import { connect } from 'react-redux'
function mapStateToProps(state) {
return { todos: state.todos }
}
function mapDispatchToProps(dispatch) {
return { actions: bindActionCreators(actionCreators, dispatch) }
}
class MyApp extends React.Component {
// ...define your main app here
}
export default connect(mapStateToProps, mapDispatchToProps)(MyApp)With decorator:
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17import React from 'react'
import * as actionCreators from './actionCreators'
import { bindActionCreators } from 'redux'
import { connect } from 'react-redux'
function mapStateToProps(state) {
return { todos: state.todos }
}
function mapDispatchToProps(dispatch) {
return { actions: bindActionCreators(actionCreators, dispatch) }
}
@connect(mapStateToProps, mapDispatchToProps)
export default class MyApp extends React.Component {
// ...define your main app here
}
The above examples are almost similar except the usage of decorator. The decorator syntax isn’t built into any JavaScript runtimes yet, and is still experimental and subject to change. You can use babel for the decorators support.
React的context和Redux有啥区别?
You can use Context in your application directly and is going to be great for passing down data to deeply nested components which what it was designed for. Whereas Redux is much more powerful and provides a large number of features that the Context API doesn’t provide. Also, React Redux uses context internally but it doesn’t expose this fact in the public API.
为啥Redux中的state函数叫reducers?
Reducers always return the accumulation of the state (based on all previous and current actions). Therefore, they act as a reducer of state. Each time a Redux reducer is called, the state and action are passed as parameters. This state is then reduced (or accumulated) based on the action, and then the next state is returned. You could reduce a collection of actions and an initial state (of the store) on which to perform these actions to get the resulting final state.
在Redux中咋用AJAX?
You can use
redux-thunkmiddleware which allows you to define async actions.Let’s take an example of fetching specific account as an AJAX call using fetch API:
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17export function fetchAccount(id) {
return dispatch => {
dispatch(setLoadingAccountState()) // Show a loading spinner
fetch(`/account/${id}`, (response) => {
dispatch(doneFetchingAccount()) // Hide loading spinner
if (response.status === 200) {
dispatch(setAccount(response.json)) // Use a normal function to set the received state
} else {
dispatch(someError)
}
})
}
}
function setAccount(data) {
return { type: 'SET_Account', data: data }
}应该把所有state都放在Redux的store中吗?
Keep your data in the Redux store, and the UI related state internally in the component.
访问Redux中store的正确方式是啥?
The best way to access your store in a component is to use the
connect()function, that creates a new component that wraps around your existing one. This pattern is called Higher-Order Components, and is generally the preferred way of extending a component’s functionality in React. This allows you to map state and action creators to your component, and have them passed in automatically as your store updates.Let’s take an example of
<FilterLink>component using connect:1
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18import { connect } from 'react-redux'
import { setVisibilityFilter } from '../actions'
import Link from '../components/Link'
const mapStateToProps = (state, ownProps) => ({
active: ownProps.filter === state.visibilityFilter
})
const mapDispatchToProps = (dispatch, ownProps) => ({
onClick: () => dispatch(setVisibilityFilter(ownProps.filter))
})
const FilterLink = connect(
mapStateToProps,
mapDispatchToProps
)(Link)
export default FilterLinkDue to it having quite a few performance optimizations and generally being less likely to cause bugs, the Redux developers almost always recommend using
connect()over accessing the store directly (using context API).1
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5class MyComponent {
someMethod() {
doSomethingWith(this.context.store)
}
}在Redux中component和container有啥区别?
Component is a class or function component that describes the presentational part of your application.
Container is an informal term for a component that is connected to a Redux store. Containers subscribe to Redux state updates and dispatch actions, and they usually don’t render DOM elements; they delegate rendering to presentational child components.
在Redux中的constants有啥作用?
Constants allows you to easily find all usages of that specific functionality across the project when you use an IDE. It also prevents you from introducing silly bugs caused by typos – in which case, you will get a
ReferenceErrorimmediately.Normally we will save them in a single file (
constants.jsoractionTypes.js).1
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6export const ADD_TODO = 'ADD_TODO'
export const DELETE_TODO = 'DELETE_TODO'
export const EDIT_TODO = 'EDIT_TODO'
export const COMPLETE_TODO = 'COMPLETE_TODO'
export const COMPLETE_ALL = 'COMPLETE_ALL'
export const CLEAR_COMPLETED = 'CLEAR_COMPLETED'In Redux you use them in two places:
During action creation:
Let’s take
actions.js:1
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5import { ADD_TODO } from './actionTypes';
export function addTodo(text) {
return { type: ADD_TODO, text }
}In reducers:
Let’s create
reducer.js:1
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16import { ADD_TODO } from './actionTypes'
export default (state = [], action) => {
switch (action.type) {
case ADD_TODO:
return [
...state,
{
text: action.text,
completed: false
}
];
default:
return state
}
}
mapDispatchToProps有哪几种方式绑定action?
There are a few ways of binding action creators to
dispatch()inmapDispatchToProps(). Below are the possible options:1
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3const mapDispatchToProps = (dispatch) => ({
action: () => dispatch(action())
})1
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3const mapDispatchToProps = (dispatch) => ({
action: bindActionCreators(action, dispatch)
})1
const mapDispatchToProps = { action }
The third option is just a shorthand for the first one.
ownProps在mapStateToProps和mapDispatchToProps中的作用是啥?
If the
ownPropsparameter is specified, React Redux will pass the props that were passed to the component into your connect functions. So, if you use a connected component:1
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3import ConnectedComponent from './containers/ConnectedComponent';
<ConnectedComponent user={'john'} />The
ownPropsinside yourmapStateToProps()andmapDispatchToProps()functions will be an object:1
{ user: 'john' }
You can use this object to decide what to return from those functions.
咋构建Redux的项目目录?
Most of the applications has several top-level directories as below:
- Components: Used for dumb components unaware of Redux.
- Containers: Used for smart components connected to Redux.
- Actions: Used for all action creators, where file names correspond to part of the app.
- Reducers: Used for all reducers, where files name correspond to state key.
- Store: Used for store initialization.
This structure works well for small and medium size apps.
redux-saga是啥?
redux-sagais a library that aims to make side effects (asynchronous things like data fetching and impure things like accessing the browser cache) in React/Redux applications easier and better.It is available in NPM:
1
npm install --save redux-saga
redux-saga的核心模型是啥?
Saga is like a separate thread in your application, that’s solely responsible for side effects.
redux-sagais a redux middleware, which means this thread can be started, paused and cancelled from the main application with normal Redux actions, it has access to the full Redux application state and it can dispatch Redux actions as well.在redux-saga中call和put有啥区别?
Both
call()andput()are effect creator functions.call()function is used to create effect description, which instructs middleware to call the promise.put()function creates an effect, which instructs middleware to dispatch an action to the store.Let’s take example of how these effects work for fetching particular user data.
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11function* fetchUserSaga(action) {
// `call` function accepts rest arguments, which will be passed to `api.fetchUser` function.
// Instructing middleware to call promise, it resolved value will be assigned to `userData` variable
const userData = yield call(api.fetchUser, action.userId)
// Instructing middleware to dispatch corresponding action.
yield put({
type: 'FETCH_USER_SUCCESS',
userData
})
}redux-thunk是啥?
Redux Thunk middleware allows you to write action creators that return a function instead of an action. The thunk can be used to delay the dispatch of an action, or to dispatch only if a certain condition is met. The inner function receives the store methods
dispatch()andgetState()as parameters.redux-saga和redux-thunk有啥区别?
Both Redux Thunk and Redux Saga take care of dealing with side effects. In most of the scenarios, Thunk uses Promises to deal with them, whereas Saga uses Generators. Thunk is simple to use and Promises are familiar to many developers, Sagas/Generators are more powerful but you will need to learn them. But both middleware can coexist, so you can start with Thunks and introduce Sagas when/if you need them.
了解Redux调试工具吗?
Redux DevTools is a live-editing time travel environment for Redux with hot reloading, action replay, and customizable UI. If you don’t want to bother with installing Redux DevTools and integrating it into your project, consider using Redux DevTools Extension for Chrome and Firefox.
Redux调试工具有哪些特性?
- Lets you inspect every state and action payload.
- Lets you go back in time by cancelling actions.
- If you change the reducer code, each staged action will be re-evaluated.
- If the reducers throw, you will see during which action this happened, and what the error was.
- With
persistState()store enhancer, you can persist debug sessions across page reloads.
Redux选择器是啥?
Selectors are functions that take Redux state as an argument and return some data to pass to the component.
For example, to get user details from the state:
1
const getUserData = state => state.user.data
Redux表单是啥?
Redux Form works with React and Redux to enable a form in React to use Redux to store all of its state. Redux Form can be used with raw HTML5 inputs, but it also works very well with common UI frameworks like Material UI, React Widgets and React Bootstrap.
Redux表单有哪些主要特性?
- Field values persistence via Redux store.
- Validation (sync/async) and submission.
- Formatting, parsing and normalization of field values.
Redux中咋添加多个中间件?
You can use
applyMiddleware().For example, you can add
redux-thunkandloggerpassing them as arguments toapplyMiddleware():1
2import { createStore, applyMiddleware } from 'redux'
const createStoreWithMiddleware = applyMiddleware(ReduxThunk, logger)(createStore)Redux中咋设置初始的state?
You need to pass initial state as second argument to createStore:
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13const rootReducer = combineReducers({
todos: todos,
visibilityFilter: visibilityFilter
})
const initialState = {
todos: [{ id: 123, name: 'example', completed: false }]
}
const store = createStore(
rootReducer,
initialState
)Redux和Relay有啥不同?
Relay is similar to Redux in that they both use a single store. The main difference is that relay only manages state originated from the server, and all access to the state is used via GraphQL queries (for reading data) and mutations (for changing data). Relay caches the data for you and optimizes data fetching for you, by fetching only changed data and nothing more.
React supported libraries & Integration
reselect是啥?
Reselect is a selector library (for Redux) which uses memoization concept. It was originally written to compute derived data from Redux-like applications state, but it can’t be tied to any architecture or library.
Reselect keeps a copy of the last inputs/outputs of the last call, and recomputes the result only if one of the inputs changes. If the the same inputs are provided twice in a row, Reselect returns the cached output. It’s memoization and cache are fully customizable.
了解Flow吗?
Flow is a static type checker designed to find type errors in JavaScript. Flow types can express much more fine-grained distinctions than traditional type systems. For example, Flow helps you catch errors involving
null, unlike most type systems.Flow和PropTypes之间有啥区别?
Flow is a static analysis tool (static checker) which uses a superset of the language, allowing you to add type annotations to all of your code and catch an entire class of bugs at compile time. PropTypes is a basic type checker (runtime checker) which has been patched onto React. It can’t check anything other than the types of the props being passed to a given component. If you want more flexible typechecking for your entire project Flow/TypeScript are appropriate choices.
在React中咋使用Font字体的icons?
The below steps followed to include Font Awesome in React:
- Install
font-awesome:
1
npm install --save font-awesome
- Import
font-awesomein yourindex.jsfile:
1
import 'font-awesome/css/font-awesome.min.css'
- Add Font Awesome classes in
className:
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3render() {
return <div><i className={'fa fa-spinner'} /></div>
}- Install
简单介绍下React调试工具?
React Developer Tools let you inspect the component hierarchy, including component props and state. It exists both as a browser extension (for Chrome and Firefox), and as a standalone app (works with other environments including Safari, IE, and React Native).
The official extensions available for different browsers or environments.
- Chrome extension
- Firefox extension
- Standalone app (Safari, React Native, etc)
本地文件为啥不会在chrome中加载调试工具?
If you opened a local HTML file in your browser (
file://...) then you must first open Chrome Extensions and checkAllow access to file URLs.React中咋使用Polymer?
Create a Polymer element:
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7<link rel='import' href='../../bower_components/polymer/polymer.html' />
Polymer({
is: 'calender-element',
ready: function() {
this.textContent = 'I am a calender'
}
})Create the Polymer component HTML tag by importing it in a HTML document, e.g. import it in the
index.htmlof your React application:1
<link rel='import' href='./src/polymer-components/calender-element.html'>
Use that element in the JSX file:
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11import React from 'react'
class MyComponent extends React.Component {
render() {
return (
<calender-element />
)
}
}
export default MyComponent
React相对于Vue的优势在哪?
React has the following advantages over Vue.js:
- Gives more flexibility in large apps developing.
- Easier to test.
- Suitable for mobile apps creating.
- More information and solutions available.
React和Angular之间有啥区别?
React Angular React is a library and has only the View layer Angular is a framework and has complete MVC functionality React handles rendering on the server side AngularJS renders only on the client side but Angular 2 and above renders on the server side React uses JSX that looks like HTML in JS which can be confusing Angular follows the template approach for HTML, which makes code shorter and easy to understand React Native, which is a React type to build mobile applications are faster and more stable Ionic, Angular’s mobile native app is relatively less stable and slower In React, data flows only in one way and hence debugging is easy In Angular, data flows both way i.e it has two-way data binding between children and parent and hence debugging is often difficult 在浏览器调试工具为啥没有React标签?
When the page loads, React DevTools sets a global named
__REACT_DEVTOOLS_GLOBAL_HOOK__, then React communicates with that hook during initialization. If the website is not using React or if React fails to communicate with DevTools then it won’t show up the tab.样式型components是啥?
styled-componentsis a JavaScript library for styling React applications. It removes the mapping between styles and components, and lets you write actual CSS augmented with JavaScript.样式型Components长啥样?
Lets create
<Title>and<Wrapper>components with specific styles for each.1
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15import React from 'react'
import styled from 'styled-components'
// Create a <Title> component that renders an <h1> which is centered, red and sized at 1.5em
const Title = styled.h1`
font-size: 1.5em;
text-align: center;
color: palevioletred;
`
// Create a <Wrapper> component that renders a <section> with some padding and a papayawhip background
const Wrapper = styled.section`
padding: 4em;
background: papayawhip;
`These two variables,
TitleandWrapper, are now components that you can render just like any other react component.1
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3<Wrapper>
<Title>{'Lets start first styled component!'}</Title>
</Wrapper>Relay是啥?
Relay is a JavaScript framework for providing a data layer and client-server communication to web applications using the React view layer.
在create-react-app中咋使用Typescript?
When you create a new project supply
--scripts-versionoption asreact-scripts-ts.react-scripts-tsis a set of adjustments to take the standardcreate-react-appproject pipeline and bring TypeScript into the mix.Now the project layout should look like the following:
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12my-app/
├─ .gitignore
├─ images.d.ts
├─ node_modules/
├─ public/
├─ src/
│ └─ ...
├─ package.json
├─ tsconfig.json
├─ tsconfig.prod.json
├─ tsconfig.test.json
└─ tslint.json
Miscellaneous
reselect有哪些主要特性?
- Selectors can compute derived data, allowing Redux to store the minimal possible state.
- Selectors are efficient. A selector is not recomputed unless one of its arguments changes.
- Selectors are composable. They can be used as input to other selectors.
reselect的用法是啥?
Let’s take calculations and different amounts of a shipment order with the simplified usage of Reselect:
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35import { createSelector } from 'reselect'
const shopItemsSelector = state => state.shop.items
const taxPercentSelector = state => state.shop.taxPercent
const subtotalSelector = createSelector(
shopItemsSelector,
items => items.reduce((acc, item) => acc + item.value, 0)
)
const taxSelector = createSelector(
subtotalSelector,
taxPercentSelector,
(subtotal, taxPercent) => subtotal * (taxPercent / 100)
)
export const totalSelector = createSelector(
subtotalSelector,
taxSelector,
(subtotal, tax) => ({ total: subtotal + tax })
)
let exampleState = {
shop: {
taxPercent: 8,
items: [
{ name: 'apple', value: 1.20 },
{ name: 'orange', value: 0.95 },
]
}
}
console.log(subtotalSelector(exampleState)) // 2.15
console.log(taxSelector(exampleState)) // 0.172
console.log(totalSelector(exampleState)) // { total: 2.322 }Redux中的action长啥样的?
Actions are plain JavaScript objects or payloads of information that send data from your application to your store. They are the only source of information for the store. Actions must have a type property that indicates the type of action being performed.
For example an example action which represents adding a new todo item:
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4{
type: ADD_TODO,
text: 'Add todo item'
}ES6写的Class组件中能使用静态对象吗?
No,
staticsonly works withReact.createClass():1
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7someComponent= React.createClass({
statics: {
someMethod: function() {
// ..
}
}
})But you can write statics inside ES6+ classes like this:
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9class Component extends React.Component {
static propTypes = {
// ...
}
static someMethod() {
// ...
}
}Redux只能在React中使用吗?
Redux can be used as a data store for any UI layer. The most common usage is with React and React Native, but there are bindings available for Angular, Angular 2, Vue, Mithril, and more. Redux simply provides a subscription mechanism which can be used by any other code.
使用Redux需要特殊的工具构建吗?
Redux is originally written in ES6 and transpiled for production into ES5 with Webpack and Babel. You should be able to use it regardless of your JavaScript build process. Redux also offers a UMD build that can be used directly without any build process at all.
Redux表单咋从state中获取初始值?
You need to add
enableReinitialize : truesetting.1
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4const InitializeFromStateForm = reduxForm({
form: 'initializeFromState',
enableReinitialize : true
})(UserEdit)If your
initialValuesprop gets updated, your form will update too.PropTypes中咋给prop变量设置多个类型?
You can use
oneOfType()method ofPropTypes.For example, the height property can be defined with either
stringornumbertype as below:1
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6Component.PropTypes = {
size: PropTypes.oneOfType([
PropTypes.string,
PropTypes.number
])
}React中能引入SVG文件作为组件吗?
You can import SVG directly as component instead of loading it as a file. This feature is available with
react-scripts@2.0.0and higher.1
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8import { ReactComponent as Logo } from './logo.svg'
const App = () => (
<div>
{/* Logo is an actual react component */}
<Logo />
</div>
)Note: Don’t forget about the curly braces in the import.
为啥不推荐写行列式或者函数式的ref?
If the ref callback is defined as an inline function, it will get called twice during updates, first with null and then again with the DOM element. This is because a new instance of the function is created with each render, so React needs to clear the old ref and set up the new one.
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17class UserForm extends Component {
handleSubmit = () => {
console.log("Input Value is: ", this.input.value)
}
render () {
return (
<form onSubmit={this.handleSubmit}>
<input
type='text'
ref={(input) => this.input = input} /> // Access DOM input in handle submit
<button type='submit'>Submit</button>
</form>
)
}
}But our expectation is for the ref callback to get called once, when the component mounts. One quick fix is to use the ES7 class property syntax to define the function
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20class UserForm extends Component {
handleSubmit = () => {
console.log("Input Value is: ", this.input.value)
}
setSearchInput = (input) => {
this.input = input
}
render () {
return (
<form onSubmit={this.handleSubmit}>
<input
type='text'
ref={this.setSearchInput} /> // Access DOM input in handle submit
<button type='submit'>Submit</button>
</form>
)
}
}React中的渲染劫持是啥?
The concept of render hijacking is the ability to control what a component will output from another component. It actually means that you decorate your component by wrapping it into a Higher-Order component. By wrapping you can inject additional props or make other changes, which can cause changing logic of rendering. It does not actually enables hijacking, but by using HOC you make your component behave in different way.
HOC工厂是啥?
There are two main ways of implementing HOCs in React. 1. Props Proxy (PP) and 2. Inheritance Inversion (II). They follow different approaches for manipulating the WrappedComponent.
Props ProxyIn this approach, the render method of the HOC returns a React Element of the type of the WrappedComponent. We also pass through the props that the HOC receives, hence the name Props Proxy.
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function ppHOC(WrappedComponent) {
return class PP extends React.Component {
render() {
return <WrappedComponent {...this.props}/>
}
}
}Inheritance Inversion
In this approach, the returned HOC class (Enhancer) extends the WrappedComponent. It is called Inheritance Inversion because instead of the WrappedComponent extending some Enhancer class, it is passively extended by the Enhancer. In this way the relationship between them seems inverse.1
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7function iiHOC(WrappedComponent) {
return class Enhancer extends WrappedComponent {
render() {
return super.render()
}
}
}React组件咋传递数字?
You should be passing the numbers via curly braces({}) where as strings inn quotes
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React.render(<User age={30} department={"IT"} />, document.getElementById('container'));
我应该将所有state都放进Redux中吗，何时用组件内部的state?
It is up to developer decision. i.e, It is developer job to determine what kinds of state make up your application, and where each piece of state should liveSome users prefer to keep every single piece of data in Redux, to maintain a fully serializable and controlled version of their application at all times. Others prefer to keep non-critical or UI state, such as “is this dropdown currently open”, inside a component’s internal state.
Below are the thumb rules to determine what kind of data should be put into Redux
- Do other parts of the application care about this data?
- Do you need to be able to create further derived data based on this original data?
- Is the same data being used to drive multiple components?
- Is there value to you in being able to restore this state to a given point in time (ie, time travel debugging)?
- Do you want to cache the data (ie, use what’s in state if it’s already there instead of re-requesting it)?
React中的registerServerWorker是干啥的?
React creates a service worker for you without any configuration by default. The service worker is a web API that helps you cache your assets and other files so that when the user is offline or on slow network, he/she can still see results on the screen, as such, it helps you build a better user experience, that’s what you should know about service worker’s for now. It’s all about adding offline capabilities to your site.
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7import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';
import registerServiceWorker from './registerServiceWorker';
ReactDOM.render(<App />, document.getElementById('root'));
registerServiceWorker();React16中的memo是啥?
Class components can be restricted from rendering when their input props are the same using PureComponent or shouldComponentUpdate. Now you can do the same with function components by wrapping them in React.memo.
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3const MyComponent = React.memo(function MyComponent(props) {
/* only rerenders if props change */
});React16中的lazy是啥?
The React.lazy function lets you render an dynamic import as a regular component. It will automatically load the bundle containing the OtherComponent when the component gets rendered. This must return a Promise which resolves to a module with a default export containing a React component.
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9const OtherComponent = React.lazy(() => import('./OtherComponent'));
function MyComponent() {
return (
<div>
<OtherComponent />
</div>
);
}Note:
React.lazy and Suspense is not yet available for server-side rendering. If you want to do code-splitting in a server rendered app, we still recommend React Loadable.通过setState咋避免不必要的更新?
You can compare current value of the state with an existing state value and decide whether to rerender the page or not. If the values are same then you need to return null to stop rerendering otherwise return the latest state value. For example, the user profile information is conditionally rendered as follows,
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10getUserProfile = user => {
const latestAddress = user.address;
this.setState(state => {
if (state.address === latestAddress) {
return null;
} else {
return { title: latestAddress };
}
});
};在React16中咋渲染数组、字符和数字?
Arrays: Unlike older releases, you don’t need to make sure render method return a single element in React16. You are able to return multiple sibling elements without a wrapping element by returning an array. For example, let us take the below list of developers,
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7const ReactJSDevs = () => {
return [
<li key="1">John</li>,
<li key="2">Jackie</li>,
<li key="3">Jordan</li>
];
}You can also merge this array of items in another array component
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10const JSDevs = () => {
return (
<ul>
<li>Brad</li>
<li>Brodge</li>
<ReactJSDevs/>
<li>Brandon</li>
</ul>
);
}Strings and Numbers: You can also return string and number type from the render method
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7render() {
return 'Welcome to ReactJS questions';
}
// Number
render() {
return 2018;
}
