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ES2019 新特性

ES2019

Object.fromEntries

在ES2017中,引入了Object.entries来把Object转成数组,如下:

let students = {
  amelia: 20,
  beatrice: 22,
  cece: 20,
  deirdre: 19,
  eloise: 21
}

Object.entries(students) 
// [
//  [ 'amelia', 20 ],
//  [ 'beatrice', 22 ],
//  [ 'cece', 20 ],
//  [ 'deirdre', 19 ],
//  [ 'eloise', 21 ]
// ]
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引入Object.entries这个方法的目的是为了使Object也可以使用数组的mapfilterreduce等原生方法,但是使用完之后必须要自己手动还原回去,例如:

let students = {
  amelia: 20,
  beatrice: 22,
  cece: 20,
  deirdre: 19,
  eloise: 21
}

// convert to array in order to make use of .filter() function
let overTwentyOne = Object.entries(students).filter(([name, age]) => {
  return age >= 21
}) // [ [ 'beatrice', 22 ], [ 'eloise', 21 ] ]

// turn multidimensional array back into an object
let DrinkingAgeStudents = {}
for (let [name, age] of overTwentyOne) {
    DrinkingAgeStudents[name] = age;
}
// { beatrice: 22, eloise: 21 }
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上述代码使用了Object.entries及数组的filter方法过滤了年龄大于等于21的成员,然后又使用了for...of把数字转变回了ObjectObject.fromEntries就是用来替代上面代码中的for...of循环的,使得Object调用数组的原生方法更简洁方便,代码如下:

let students = {
  amelia: 20,
  beatrice: 22,
  cece: 20,
  deirdre: 19,
  eloise: 21
}

// convert to array in order to make use of .filter() function
let overTwentyOne = Object.entries(students).filter(([name, age]) => {
  return age >= 21
}) // [ [ 'beatrice', 22 ], [ 'eloise', 21 ] ]

// turn multidimensional array back into an object
let DrinkingAgeStudents = Object.fromEntries(overTwentyOne); 
// { beatrice: 22, eloise: 21 }
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需要注意的一点是,使用Object.fromEntries会把Object中的重复key覆盖掉,只保存最后一个,如下:

let students = [
  [ 'amelia', 22 ], 
  [ 'beatrice', 22 ], 
  [ 'eloise', 21], 
  [ 'beatrice', 20 ]
]

let studentObj = Object.fromEntries(students); 
// { amelia: 22, beatrice: 20, eloise: 21 }
// dropped first beatrice!
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Array.prototype.flat

通常,数组会出现二维或者以上的情况,这个时候如果我们需要降维的话,常用的一种方法时使用concat方法,如下:

let courses = [
  {
    subject: "math",
    numberOfStudents: 3,
    waitlistStudents: 2,
    students: ['Janet', 'Martha', 'Bob', ['Phil', 'Candace']]
  },
  {
    subject: "english",
    numberOfStudents: 2,
    students: ['Wilson', 'Taylor']
  },
  {
    subject: "history",
    numberOfStudents: 4,
    students: ['Edith', 'Jacob', 'Peter', 'Betty']
  }
]

let courseStudents = courses.map(course => course.students)
// [
//   [ 'Janet', 'Martha', 'Bob', [ 'Phil', 'Candace' ] ],
//   [ 'Wilson', 'Taylor' ],
//   [ 'Edith', 'Jacob', 'Peter', 'Betty' ]
// ]

[].concat.apply([], courseStudents) // we're stuck doing something like this
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但是在ES2019中引入了Array.prototype.flat,其接受一个可选参数表示需要展开的深度,默认值为1,可以帮助我们快速实现数组降维,如下:

let courseStudents = [
  [ 'Janet', 'Martha', 'Bob', [ 'Phil', 'Candace' ] ],
  [ 'Wilson', 'Taylor' ],
  [ 'Edith', 'Jacob', 'Peter', 'Betty' ]
]

let flattenOneLevel = courseStudents.flat(1)
console.log(flattenOneLevel)
// [
//   'Janet',
//   'Martha',
//   'Bob',
//   [ 'Phil', 'Candace' ],
//   'Wilson',
//   'Taylor',
//   'Edith',
//   'Jacob',
//   'Peter',
//   'Betty'
// ]

let flattenTwoLevels = courseStudents.flat(2)
console.log(flattenTwoLevels)
// [
//   'Janet',   'Martha',
//   'Bob',     'Phil',
//   'Candace', 'Wilson',
//   'Taylor',  'Edith',
//   'Jacob',   'Peter',
//   'Betty'
// ]
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不指定参数与参数值等于1的时候一样:

let courseStudents = [
  [ 'Janet', 'Martha', 'Bob', [ 'Phil', 'Candace' ] ],
  [ 'Wilson', 'Taylor' ],
  [ 'Edith', 'Jacob', 'Peter', 'Betty' ]
]

let defaultFlattened = courseStudents.flat()
console.log(defaultFlattened)
// [
//   'Janet',
//   'Martha',
//   'Bob',
//   [ 'Phil', 'Candace' ],
//   'Wilson',
//   'Taylor',
//   'Edith',
//   'Jacob',
//   'Peter',
//   'Betty'
// ]
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  • 贪心操作Infinity 对于未知深度的数组,可以使用Infinity参数来实现完全降维:
let courseStudents = [
  [ 'Janet', 'Martha', 'Bob', [ 'Phil', 'Candace' ] ],
  [ 'Wilson', 'Taylor' ],
  [ 'Edith', 'Jacob', 'Peter', 'Betty' ]
]

let alwaysFlattened = courseStudents.flat(Infinity)
console.log(alwaysFlattened)
// [
//   'Janet',   'Martha',
//   'Bob',     'Phil',
//   'Candace', 'Wilson',
//   'Taylor',  'Edith',
//   'Jacob',   'Peter',
//   'Betty'
// ]
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事实上,Infinity参数在实际情况中应该谨慎使用,而且在真正未知数组维数时并不推荐使用Infinity参数。

Array.prototype.flatMap

flatMap 等价于mapflat不加参数的结合体,所以flatMap 只展开一层数组。

  • 普通半版数组中插入值:
let grades = [78, 62, 80, 64]

let curved = grades.map(grade => [grade, grade + 7])
// [ [ 78, 85 ], [ 62, 69 ], [ 80, 87 ], [ 64, 71 ] ]

let flatMapped = [].concat.apply([], curved) // now flatten, could use flat but that didn't exist before either
// [
//  78, 85, 62, 69,
//  80, 87, 64, 71
// ]
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  • Array.prototype.flat
let grades = [78, 62, 80, 64]

let flatMapped = grades.map(grade => [grade, grade + 7]).flat()
// [
//  78, 85, 62, 69,
//  80, 87, 64, 71
// ]
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  • flatMap进阶版
let grades = [78, 62, 80, 64]

let flatMapped = grades.flatMap(grade => [grade, grade + 7]);
// [
//  78, 85, 62, 69,
//  80, 87, 64, 71
// ]
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由于是mapflat不加参数的结合体,flatMap 只展开一层数组:

let grades = [78, 62, 80, 64]

let flatMapped = grades.flatMap(grade => [grade, [grade + 7]]);
// [
//   78, [ 85 ],
//   62, [ 69 ],
//   80, [ 87 ],
//   64, [ 71 ]
// ]
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String.trimStart 和 String.trimEnd

等价于之前的String.trimRightString.trimLeft

let message = "   Welcome to CS 101    "
message.trimRight()
// '   Welcome to CS 101'
message.trimLeft()
// 'Welcome to CS 101   '
message.trimRight().trimLeft()
// 'Welcome to CS 101'
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等价于

let message = "   Welcome to CS 101    "
message.trimEnd()
// '   Welcome to CS 101'
message.trimStart()
// 'Welcome to CS 101   '
message.trimEnd().trimStart()
// 'Welcome to CS 101'
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可选的catch绑定

在之前的版本中,try...catch中的catch后面必须要跟一个参数,即使catch块里面什么内容都没有,所以在ES2019中,catch块后面的参数是可选的,如下:

try {
  let parsed = JSON.parse(obj)
} catch(e) {
  // ignore e, or use
  console.log(obj)
}
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e可以不写:

try {
  let parsed = JSON.parse(obj)
} catch {
  console.log(obj)
}
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Function.toString() 更新

之前的版本中,Function.toString()直接把空格去掉了:

function greeting() {
  const name = 'CSS Tricks'
  console.log(`hello from ${name}`)
}

greeting.toString()
//'function greeting() {\nconst name = \'CSS Tricks\'\nconsole.log(`hello from ${name} //`)\n}'
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但是,在ES2019中,会完全保留源码格式打印:

function greeting() {
  const name = 'CSS Tricks'
  console.log(`hello from ${name}`)
}

greeting.toString()
// 'function greeting() {\n' +
//  "  const name = 'CSS Tricks'\n" +
//  '  console.log(`hello from ${name}`)\n' +
//  '}'
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参考

css-tricks.com/all-the-new…

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