应对React的JavaScript知识（附代码实例）

与我使用过的其他框架相比，我最喜欢React的一点是，当你使用它时，你是如何暴露于JavaScript的。没有模板DSL（JJSX编译为合理的JavaScript），组件API随着React Hooks的增加而变得更加简单，而且该框架在它要解决的核心UI问题之外为你提供了很少的抽象性。

正因为如此，要想有效地使用React构建应用程序，学习JavaScript特性是非常明智的。因此，我建议你花一些时间学习一些JavaScript特性，这样你就可以尽可能有效地使用React。

在我们讨论一些语法问题之前，另一个对React非常有用的东西是函数 "闭包 "的概念。这里有一个关于这个概念的好文章：MDN.io/closure。

好了，让我们来谈谈你想知道的React的JS特性。

模板字元

模板字元就像具有超能力的普通字符串：

const greeting = 'Hello'
const subject = 'World'
console.log(`${greeting} ${subject}!`) // Hello World!

// this is the same as:
console.log(greeting + ' ' + subject + '!')

// in React:
function Box({className, ...props}) {
  return <div className={`box ${className}`} {...props} />
}

简明的属性名称

这是很常见和有用的，以至于我现在不假思索地就这么做了：

const a = 'hello'
const b = 42
const c = {d: [true, false]}
console.log({a, b, c})

// this is the same as:
console.log({a: a, b: b, c: c})

// in React:
function Counter({initialCount, step}) {
  const [count, setCount] = useCounter({initialCount, step})
  return <button onClick={setCount}>{count}</button>
}

箭头函数

箭头函数是另一种在JavaScript中编写函数的方式，但它们确实有一些语义上的差异。幸运的是，在React领域，如果我们在项目中使用钩子（而不是类），我们就不必担心this ，但箭头函数允许更多的匿名函数和隐式返回，所以你会看到并希望大量使用箭头函数。

const getFive = () => 5
const addFive = a => a + 5
const divide = (a, b) => a / b

// this is the same as:
function getFive() {
  return 5
}
function addFive(a) {
  return a + 5
}
function divide(a, b) {
  return a / b
}

// in React:
function TeddyBearList({teddyBears}) {
  return (
    <ul>
      {teddyBears.map(teddyBear => (
        <li key={teddyBear.id}>
          <span>{teddyBear.name}</span>
        </li>
      ))}
    </ul>
  )
}

上面的例子有一点需要注意，就是开头和结尾的小括号( 。这是在使用JSX时利用箭头函数的隐式返回功能的一种常见方式。

MDN: 箭头函数

解构

解构可能是我最喜欢的JavaScript功能。我经常对对象和数组进行重组（如果你正在使用useState ，你可能也是这样）。我喜欢它的声明性。

// const obj = {x: 3.6, y: 7.8}
// makeCalculation(obj)

function makeCalculation({x, y: d, z = 4}) {
  return Math.floor((x + d + z) / 3)
}

// this is the same as
function makeCalculation(obj) {
  const {x, y: d, z = 4} = obj
  return Math.floor((x + d + z) / 3)
}

// which is the same as
function makeCalculation(obj) {
  const x = obj.x
  const d = obj.y
  const z = obj.z === undefined ? 4 : obj.z
  return Math.floor((x + d + z) / 3)
}

// in React:
function UserGitHubImg({username = 'ghost', ...props}) {
  return <img src={`https://github.com/${username}.png`} {...props} />
}

一定要读读MDN的那篇文章。你肯定会学到一些新东西。当你完成后，试着重构这个，使用一行destructuring。

function nestedArrayAndObject() {
  // refactor this to a single line of destructuring...
  const info = {
    title: 'Once Upon a Time',
    protagonist: {
      name: 'Emma Swan',
      enemies: [
        {name: 'Regina Mills', title: 'Evil Queen'},
        {name: 'Cora Mills', title: 'Queen of Hearts'},
        {name: 'Peter Pan', title: `The boy who wouldn't grow up`},
        {name: 'Zelena', title: 'The Wicked Witch'},
      ],
    },
  }
  // const {} = info // <-- replace the next few `const` lines with this
  const title = info.title
  const protagonistName = info.protagonist.name
  const enemy = info.protagonist.enemies[3]
  const enemyTitle = enemy.title
  const enemyName = enemy.name
  return `${enemyName} (${enemyTitle}) is an enemy to ${protagonistName} in "${title}"`
}

参数默认值

这是我经常使用的另一个功能。它是一种非常强大的方式，可以为你的函数声明性地表达默认值。

// add(1)
// add(1, 2)
function add(a, b = 0) {
  return a + b
}

// is the same as
const add = (a, b = 0) => a + b

// is the same as
function add(a, b) {
  b = b === undefined ? 0 : b
  return a + b
}

// in React:
function useLocalStorageState({
  key,
  initialValue,
  serialize = v => v,
  deserialize = v => v,
}) {
  const [state, setState] = React.useState(
    () => deserialize(window.localStorage.getItem(key)) || initialValue,
  )

  const serializedState = serialize(state)
  React.useEffect(() => {
    window.localStorage.setItem(key, serializedState)
  }, [key, serializedState])

  return [state, setState]
}

休息/传播

... 语法可以被认为是一种 "集合 "语法，它对一个值的集合进行操作。我一直在使用它，并强烈建议你学习它的使用方法和使用场合。实际上，它在不同的情况下有不同的含义，所以学习其中的细微差别将有助于你。

const arr = [5, 6, 8, 4, 9]
Math.max(...arr)
// is the same as
Math.max.apply(null, arr)

const obj1 = {
  a: 'a from obj1',
  b: 'b from obj1',
  c: 'c from obj1',
  d: {
    e: 'e from obj1',
    f: 'f from obj1',
  },
}
const obj2 = {
  b: 'b from obj2',
  c: 'c from obj2',
  d: {
    g: 'g from obj2',
    h: 'g from obj2',
  },
}
console.log({...obj1, ...obj2})
// is the same as
console.log(Object.assign({}, obj1, obj2))

function add(first, ...rest) {
  return rest.reduce((sum, next) => sum + next, first)
}
// is the same as
function add() {
  const first = arguments[0]
  const rest = Array.from(arguments).slice(1)
  return rest.reduce((sum, next) => sum + next, first)
}

// in React:
function Box({className, ...restOfTheProps}) {
  const defaultProps = {
    className: `box ${className}`,
    children: 'Empty box',
  }
  return <div {...defaultProps} {...restOfTheProps} />
}

ESModules

如果你正在用现代工具构建一个应用程序，那么它有可能支持模块，学习该语法是一个好主意，因为任何应用程序，即使是微不足道的规模，也可能需要利用模块来进行代码重用和组织。

export default function add(a, b) {
  return a + b
}

/*
 * import add from './add'
 * console.assert(add(3, 2) === 5)
 */

export const foo = 'bar'

/*
 * import {foo} from './foo'
 * console.assert(foo === 'bar')
 */

export function subtract(a, b) {
  return a - b
}

export const now = new Date()

/*
 * import {subtract, now} from './stuff'
 * console.assert(subtract(4, 2) === 2)
 * console.assert(now instanceof Date)
 */

// dynamic imports
import('./some-module').then(
  allModuleExports => {
    // the allModuleExports object will be the same object you get if you had
    // used: import * as allModuleExports from './some-module'
    // the only difference is this will be loaded asynchronously which can
    // have performance benefits in some cases
  },
  error => {
    // handle the error
    // this will happen if there's an error loading or running the module
  },
)

// in React:
import React, {Suspense, Fragment} from 'react'

// dynamic import of a React component
const BigComponent = React.lazy(() => import('./big-component'))
// big-component.js would need to "export default BigComponent" for this to work

作为另一个资源，我做了一个关于这种语法的讲座，你可以在这里观看该讲座。

三元组

我喜欢三元组。它们是漂亮的声明性的。特别是在JSX中。

const message = bottle.fullOfSoda
  ? 'The bottle has soda!'
  : 'The bottle may not have soda :-('

// is the same as
let message
if (bottle.fullOfSoda) {
  message = 'The bottle has soda!'
} else {
  message = 'The bottle may not have soda :-('
}

// in React:
function TeddyBearList({teddyBears}) {
  return (
    <React.Fragment>
      {teddyBears.length ? (
        <ul>
          {teddyBears.map(teddyBear => (
            <li key={teddyBear.id}>
              <span>{teddyBear.name}</span>
            </li>
          ))}
        </ul>
      ) : (
        <div>There are no teddy bears. The sadness.</div>
      )}
    </React.Fragment>
  )
}

我意识到，在prettier出现并清理我们的代码之前，一些人不得不忍受试图理解三元组的痛苦，所以三元组会引起他们的反感。如果你还没有使用prettier，我强烈建议你使用。Prettier将使你的三元组更容易阅读。

数组方法

数组是很奇妙的，我一直都在使用数组方法！我最常使用的是以下方法。我可能最常使用的是以下方法：

• 查找
• 一些
• 每一个
• 包括
• 地图
• 过滤
• 减少

这里有一些例子

const dogs = [
  {
    id: 'dog-1',
    name: 'Poodle',
    temperament: [
      'Intelligent',
      'Active',
      'Alert',
      'Faithful',
      'Trainable',
      'Instinctual',
    ],
  },
  {
    id: 'dog-2',
    name: 'Bernese Mountain Dog',
    temperament: ['Affectionate', 'Intelligent', 'Loyal', 'Faithful'],
  },
  {
    id: 'dog-3',
    name: 'Labrador Retriever',
    temperament: [
      'Intelligent',
      'Even Tempered',
      'Kind',
      'Agile',
      'Outgoing',
      'Trusting',
      'Gentle',
    ],
  },
]

dogs.find(dog => dog.name === 'Bernese Mountain Dog')
// {id: 'dog-2', name: 'Bernese Mountain Dog', ...etc}

dogs.some(dog => dog.temperament.includes('Aggressive'))
// false

dogs.some(dog => dog.temperament.includes('Trusting'))
// true

dogs.every(dog => dog.temperament.includes('Trusting'))
// false

dogs.every(dog => dog.temperament.includes('Intelligent'))
// true

dogs.map(dog => dog.name)
// ['Poodle', 'Bernese Mountain Dog', 'Labrador Retriever']

dogs.filter(dog => dog.temperament.includes('Faithful'))
// [{id: 'dog-1', ..etc}, {id: 'dog-2', ...etc}]

dogs.reduce((allTemperaments, dog) => {
  return [...allTemperaments, ...dog.temperament]
}, [])
// [ 'Intelligent', 'Active', 'Alert', ...etc ]

// in React:
function RepositoryList({repositories, owner}) {
  return (
    <ul>
      {repositories
        .filter(repo => repo.owner === owner)
        .map(repo => (
          <li key={repo.id}>{repo.name}</li>
        ))}
    </ul>
  )
}

MDN: Array

虚值凝聚运算符

如果一个值是null 或undefined ，那么你可能想回退到一些默认值。

// here's what we often did for this:
x = x || 'some default'

// but this was problematic for numbers or booleans where "0" or "false" are valid values

// So, if we wanted to support this:
add(null, 3)

// here's what we had to do before:
function add(a, b) {
  a = a == null ? 0 : a
  b = b == null ? 0 : b
  return a + b
}

// here's what we can do now
function add(a, b) {
  a = a ?? 0
  b = b ?? 0
  return a + b
}

// in React:
function DisplayContactName({contact}) {
  return <div>{contact.name ?? 'Unknown'}</div>
}

MDN: Nullish coalescing operator

可选链式

也被称为 "猫王操作符"，它允许你安全地访问属性和调用可能存在或不存在的函数。在可选链之前，我们使用的是一种依赖于falsy/truthy-ness的 hacky-workaround。

// what we did before optional chaining:
const streetName = user && user.address && user.address.street.name

// what we can do now:
const streetName = user?.address?.street?.name

// this will run even if options is undefined (in which case, onSuccess would be undefined as well)
// however, it will still fail if options was never declared,
// since optional chaining cannot be used on a non-existent root object.
// optional chaining does not replace checks like if (typeof options == "undefined")
const onSuccess = options?.onSuccess

// this will run without error even if onSuccess is undefined (in which case, no function will be called)
onSuccess?.({data: 'yay'})

// and we can combine those things into a single line:
options?.onSuccess?.({data: 'yay'})

// and if you are 100% certain that onSuccess is a function if options exists
// then you don't need the extra ?. before calling it. Only use ?. in situations
// where the thing on the left might not exist.
options?.onSuccess({data: 'yay'})

// in React:
function UserProfile({user}) {
  return (
    <div>
      <h1>{user.name}</h1>
      <strong>{user.bio?.short ?? 'No bio provided'}</strong>
    </div>
  )
}

围绕这一点需要注意的是，如果你发现自己在代码中经常做?. ，你可能要考虑这些值的来源地，并确保它们一致地返回它们应该的值。

许诺和async/await

这是个很大的问题，需要花点时间和实践才能熟练掌握。诺言在JavaScript生态系统中随处可见，由于React在该生态系统中的地位非常稳固，它们在那里也随处可见（事实上，React本身也在内部使用诺言）。

许诺帮助你管理异步代码，并从许多DOM API以及第三方库中返回。Async/await 语法是处理承诺的特殊语法。这两者是相辅相成的。

function promises() {
  const successfulPromise = timeout(100).then(result => `success: ${result}`)

  const failingPromise = timeout(200, true).then(null, error =>
    Promise.reject(`failure: ${error}`),
  )

  const recoveredPromise = timeout(300, true).then(null, error =>
    Promise.resolve(`failed and recovered: ${error}`),
  )

  successfulPromise.then(log, logError)
  failingPromise.then(log, logError)
  recoveredPromise.then(log, logError)
}

function asyncAwaits() {
  async function successfulAsyncAwait() {
    const result = await timeout(100)
    return `success: ${result}`
  }

  async function failedAsyncAwait() {
    const result = await timeout(200, true)
    return `failed: ${result}` // this would not be executed
  }

  async function recoveredAsyncAwait() {
    try {
      const result = await timeout(300, true)
      return `failed: ${result}` // this would not be executed
    } catch (error) {
      return `failed and recovered: ${error}`
    }
  }

  successfulAsyncAwait().then(log, logError)
  failedAsyncAwait().then(log, logError)
  recoveredAsyncAwait().then(log, logError)
}

function log(...args) {
  console.log(...args)
}

function logError(...args) {
  console.error(...args)
}

// This is the mothership of all things asynchronous
function timeout(duration = 0, shouldReject = false) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (shouldReject) {
        reject(`rejected after ${duration}ms`)
      } else {
        resolve(`resolved after ${duration}ms`)
      }
    }, duration)
  })
}

// in React:
function GetGreetingForSubject({subject}) {
  const [isLoading, setIsLoading] = React.useState(false)
  const [error, setError] = React.useState(null)
  const [greeting, setGreeting] = React.useState(null)

  React.useEffect(() => {
    async function fetchGreeting() {
      try {
        const response = await window.fetch('https://example.com/api/greeting')
        const data = await response.json()
        setGreeting(data.greeting)
      } catch (error) {
        setError(error)
      } finally {
        setIsLoading(false)
      }
    }
    setIsLoading(true)
    fetchGreeting()
  }, [])

  return isLoading ? (
    'loading...'
  ) : error ? (
    'ERROR!'
  ) : greeting ? (
    <div>
      {greeting} {subject}
    </div>
  ) : null
}

总结

当然，在构建React应用程序时，还有许多语言功能是有用的，但这些是我最喜欢的一些，我发现自己在不断地使用。我希望你觉得这很有用。

如果你想进一步了解这些特性，我有一个JavaScript研讨会，是我在PayPal工作时举办和录制的，你可能会觉得很有帮助：PayPal的ES6和超越研讨会

祝您好运!