使用 React 要懂的 JavaScript 特性[每日前端夜话0xB3]

与我使用的其他框架相比，我最喜欢 React 的原因之一就是它对 JavaScript 的暴露程度。没有模板DSL（ JSX 编译为合理的 JavaScript），组件 API 只是通过添加 React Hooks 变得更简单，并且该框架为解决的核心 UI 问题提供非常少的抽象概念。

因此，学习 JavaScript 对于使用 React 有效构建应用程序是非常可取的。所以这里有一些 JavaScript 功能，我建议你花一些时间学习，这样你就可以尽可能有效地使用 React。

模板文字

模板文字就像具有超能力的字符串：

 1const greeting = 'Hello' 2const subject = 'World' 3console.log(`${greeting} ${subject}!`) // Hello World! 4 5// this is the same as: 6console.log(greeting + ' ' + subject + '!') 7 8// in React: 9function Box({className, ...props}) {10  return <div className={`box ${className}`} {...props} />11}12

MDN：模板文字（https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals）

简写属性名

很常见并且有用，我直到现在都没有想到可以这样做。

 1const a = 'hello' 2const b = 42 3const c = {d: [true, false]} 4console.log({a, b, c}) 5 6// this is the same as: 7console.log({a: a, b: b, c: c}) 8 9// in React:10function Counter({initialCount, step}) {11  const [count, setCount] = useCounter({initialCount, step})12  return <button onClick={setCount}>{count}</button>13}

MDN：ECMAScript 2015中对象初始化的新表示法（https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer#New_notations_in_ECMAScript_2015）

箭头函数

箭头函数是在 JavaScript 中另一种编写函数的方法，但它们确实存在一些语义差异。幸运的是我们在 React 的土地上，如果在项目中使用hook（而不是类）就不必担心 this，但是箭头函数允许更复杂的匿名函数和隐式返回，所以你会看到并想要充分利用箭头的功能。

 1const getFive = () => 5 2const addFive = a => a + 5 3const divide = (a, b) => a / b 4 5// this is the same as: 6function getFive() { 7  return 5 8} 9function addFive(a) {10  return a + 511}12function divide(a, b) {13  return a / b14}1516// in React:17function TeddyBearList({teddyBears}) {18  return (19    <ul>20      {teddyBears.map(teddyBear => (21        <li key={teddyBear.id}>22          <span>{teddyBear.name}</span>23        </li>24      ))}25    </ul>26  )27}

MDN：箭头函数（https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions）

解构

解构可能是我最喜欢的 JavaScript 功能。我一直在构造对象和数组（如果你使用 useState，可能也是如此，就像这样【https://kentcdodds.com/blog/react-hooks-array-destructuring-fundamentals】）。我喜欢它的陈述性。

 1// const obj = {x: 3.6, y: 7.8} 2// makeCalculation(obj) 3 4function makeCalculation({x, y: d, z = 4}) { 5  return Math.floor((x + d + z) / 3) 6} 7 8/ this is the same as 9function makeCalculation(obj) {10  const {x, y: d, z = 4} = obj11  return Math.floor((x + d + z) / 3)12}1314// which is the same as15function makeCalculation(obj) {16  const x = obj.x17  const d = obj.y18  const z = obj.z === undefined ? 4 : obj.z19  return Math.floor((x + d + z) / 3)20}2122// in React:23function UserGitHubImg({username = 'ghost', ...props}) {24  return <img src={`https://github.com/${username}.png`} {...props} />25}26

MDN：解构分配（https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment）

 一等要去阅读 MDN 文章，你肯定能够学到新东西。当你完成后，尝试用单行解构：

1function nestedArrayAndObject() { 2  // refactor this to a single line of destructuring... 3  const info = { 4    title: 'Once Upon a Time', 5    protagonist: { 6      name: 'Emma Swan', 7      enemies: [ 8        {name: 'Regina Mills', title: 'Evil Queen'}, 9        {name: 'Cora Mills', title: 'Queen of Hearts'},10        {name: 'Peter Pan', title: `The boy who wouldn't grow up`},11        {name: 'Zelena', title: 'The Wicked Witch'},12      ],13    },14  }15  // const {} = info // <-- replace the next few `const` lines with this16  const title = info.title17  const protagonistName = info.protagonist.name18  const enemy = info.protagonist.enemies[3]19  const enemyTitle = enemy.title20  const enemyName = enemy.name21  return `${enemyName} (${enemyTitle}) is an enemy to ${protagonistName} in "${title}"`22}

参数默认值

这是另一个我一直在用的功能：一种以声明方式表达函数默认值的非常强大的方法。

 1// add(1) 2// add(1, 2) 3function add(a, b = 0) { 4  return a + b 5} 6 7// is the same as 8const add = (a, b = 0) => a + b 910// is the same as11function add(a, b) {12  b = b === undefined ? 0 : b13  return a + b14}1516// in React:17function useLocalStorageState({18  key,19  initialValue,20  serialize = v => v,21  deserialize = v => v,22}) {23  const [state, setState] = React.useState(24    () => deserialize(window.localStorage.getItem(key)) || initialValue,25  )2627  const serializedState = serialize(state)28  React.useEffect(() => {29    window.localStorage.setItem(key, serializedState)30  }, [key, serializedState])3132  return [state, setState]33}

MDN：默认参数（https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters）

Rest/Spread

...语法可以被认为是一种“集合”语法，它在一组值上运行。我一直都在使用，强烈建议你也学习。它实际上在不同的环境中有不同的含义，因此学习那些细微差别会对你有所帮助。

1const arr = [5, 6, 8, 4, 9] 2Math.max(...arr) 3// is the same as 4Math.max.apply(null, arr) 5 6const obj1 = { 7  a: 'a from obj1', 8  b: 'b from obj1', 9  c: 'c from obj1',10  d: {11    e: 'e from obj1',12    f: 'f from obj1',13  },14}15const obj2 = {16  b: 'b from obj2',17  c: 'c from obj2',18  d: {19    g: 'g from obj2',20    h: 'g from obj2',21  },22}23console.log({...obj1, ...obj2})24// is the same as25console.log(Object.assign({}, obj1, obj2))2627function add(first, ...rest) {28  return rest.reduce((sum, next) => sum + next, first)29}30// is the same as31function add() {32  const first = arguments[0]33  const rest = Array.from(arguments).slice(1)34  return rest.reduce((sum, next) => sum + next, first)35}3637// in React:38function Box({className, ...restOfTheProps}) {39  const defaultProps = {40    className: `box ${className}`,41    children: 'Empty box',42  }43  return <div {...defaultProps} {...restOfTheProps} />44}45

MDN：Spread语法（https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax）

MDN：Rest 参数（https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters）

ESModules

如果你正在使用现代工具构建自己的程序，它应该能够支持模块，了解语法怎样工作是个好主意，因为所有的甚至微不足道的程序都可能需要使用模块来重用代码。

1export default function add(a, b) { 2  return a + b 3} 4 5/* 6 * import add from './add' 7 * console.assert(add(3, 2) === 5) 8 */ 910export const foo = 'bar'1112/*13 * import {foo} from './foo'14 * console.assert(foo === 'bar')15 */1617export function subtract(a, b) {18  return a - b19}2021export const now = new Date()2223/*24 * import {subtract, now} from './stuff'25 * console.assert(subtract(4, 2) === 2)26 * console.assert(now instanceof Date)27 */2829// in React:30import React, {Suspense, Fragment} from 'react'

MDN：import（https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import）

MDN：export（https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/export）

三元表达式

我喜欢三元表达式。他们的声明很漂亮。特别是在 JSX 中。

1const message = bottle.fullOfSoda 2  ? 'The bottle has soda!' 3  : 'The bottle may not have soda :-(' 4 5// is the same as 6let message 7if (bottle.fullOfSoda) { 8  message = 'The bottle has soda!' 9} else {10  message = 'The bottle may not have soda :-('11}1213// in React:14function TeddyBearList({teddyBears}) {15  return (16    <React.Fragment>17      {teddyBears.length ? (18        <ul>19          {teddyBears.map(teddyBear => (20            <li key={teddyBear.id}>21              <span>{teddyBear.name}</span>22            </li>23          ))}24        </ul>25      ) : (26        <div>There are no teddy bears. The sadness.</div>27      )}28    </React.Fragment>29  )30}31

   我意识到，在 prettier 出现并清理我们的代码之前，一些人不得不花时间弄清楚三元运算符是怎么回事，这让三元表达式变得令人反感。如果你还没有使用 prettier，我强烈建议你这样做。prettier 将使你的三元表达式更容易阅读。

MDN：条件（三元）运算符（https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Conditional_Operator）

数组方法

数组很棒，我一直使用数组方法！以下是我常用的方法：

find
some
every
includes
map
filter
reduce

这里有些例子：

 1const dogs = [ 2  { 3    id: 'dog-1', 4    name: 'Poodle', 5    temperament: [ 6      'Intelligent', 7      'Active', 8      'Alert', 9      'Faithful',10      'Trainable',11      'Instinctual',12    ],13  },14  {15    id: 'dog-2',16    name: 'Bernese Mountain Dog',17    temperament: ['Affectionate', 'Intelligent', 'Loyal', 'Faithful'],18  },19  {20    id: 'dog-3',21    name: 'Labrador Retriever',22    temperament: [23      'Intelligent',24      'Even Tempered',25      'Kind',26      'Agile',27      'Outgoing',28      'Trusting',29      'Gentle',30    ],31  },32]3334dogs.find(dog => dog.name === 'Bernese Mountain Dog')35// {id: 'dog-2', name: 'Bernese Mountain Dog', ...etc}3637dogs.some(dog => dog.temperament.includes('Aggressive'))38// false3940dogs.some(dog => dog.temperament.includes('Trusting'))41// true4243dogs.every(dog => dog.temperament.includes('Trusting'))44// false4546dogs.every(dog => dog.temperament.includes('Intelligent'))47// true4849dogs.map(dog => dog.name)50// ['Poodle', 'Bernese Mountain Dog', 'Labrador Retriever']5152dogs.filter(dog => dog.temperament.includes('Faithful'))53// [{id: 'dog-1', ..etc}, {id: 'dog-2', ...etc}]5455dogs.reduce((allTemperaments, dog) => {56  return [...allTemperaments, ...dog.temperaments]57}, [])58// [ 'Intelligent', 'Active', 'Alert', ...etc ]5960// in React:61function RepositoryList({repositories, owner}) {62  return (63    <ul>64      {repositories65        .filter(repo => repo.owner === owner)66        .map(repo => (67          <li key={repo.id}>{repo.name}</li>68        ))}69    </ul>70  )71}

MDN：Array（https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array）

Promises 和 async/await

这是一个很大的主题，可以在它们身上多花一些时间。Promises 在 JavaScript 生态中无处不在，并且由于 React在该生态系统中的根深蒂固，它们几乎到处都是（事实上，React 本身在内部也在使用 promises）。

Promises 可帮助你管理异步代码。Async/await 语法是处理 promises 的特殊语法。这两者是相辅相成的。

1function promises() { 2  const successfulPromise = timeout(100).then(result => `success: ${result}`) 3 4  const failingPromise = timeout(200, true).then(null, error => 5    Promise.reject(`failure: ${error}`), 6  ) 7 8  const recoveredPromise = timeout(300, true).then(null, error => 9    Promise.resolve(`failed and recovered: ${error}`),10  )1112  successfulPromise.then(log, logError)13  failingPromise.then(log, logError)14  recoveredPromise.then(log, logError)15}1617function asyncAwaits() {18  async function successfulAsyncAwait() {19    const result = await timeout(100)20    return `success: ${result}`21  }2223  async function failedAsyncAwait() {24    const result = await timeout(200, true)25    return `failed: ${result}`26  }2728  async function recoveredAsyncAwait() {29    let result30    try {31      result = await timeout(300, true)32      return `failed: ${result}` // this would not be executed33    } catch (error) {34      return `failed and recovered: ${error}`35    }36  }3738  successfulAsyncAwait().then(log, logError)39  failedAsyncAwait().then(log, logError)40  recoveredAsyncAwait().then(log, logError)41}4243function log(...args) {44  console.log(...args)45}4647function logError(...args) {48  console.error(...args)49}5051// This is the mothership of all things asynchronous52function timeout(duration = 0, shouldReject = false) {53  return new Promise((resolve, reject) => {54    setTimeout(() => {55      if (shouldReject) {56        reject(`rejected after ${duration}ms`)57      } else {58        resolve(`resolved after ${duration}ms`)59      }60    }, duration)61  })62}6364// in React:65function GetGreetingForSubject({subject}) {66  const [isLoading, setIsLoading] = React.useState(false)67  const [error, setError] = React.useState(null)68  const [greeting, setGreeting] = React.useState(null)6970  React.useEffect(() => {71    async function fetchGreeting() {72      try {73        const response = await window.fetch('https://example.com/api/greeting')74        const data = await response.json()75        setGreeting(data.greeting)76      } catch (error) {77        setError(error)78      } finally {79        setIsLoading(false)80      }81    }82    setIsLoading(true)83    fetchGreeting()84  }, [])8586  return isLoading ? (87    'loading...'88  ) : error ? (89    'ERROR!'90  ) : greeting ? (91    <div>92      {greeting} {subject}93    </div>94  ) : null95}

MDN：Promise（https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise）

MDN：async function（https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function）

MDN：await（https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await）

结论

当然有许多语言功能在构建 React 应用时很有用，这些是我最喜欢的，我发现自己一次又一次地使用它们。希望对你有帮助。

原文：https://kentcdodds.com/blog/javascript-to-know-for-react