ES6十大特性(转载CSDN)

1. const和let关键字

const用于定义常量。 let用于定义变量。但是JavaScript中不是已经有变量了吗？ 是的，这很正确，但用var声明的变量具有函数作用域，并会被提升到顶部。 这意味着在声明之前就可以使用这个变量。 let变量和常量具有块作用域（用{}包围），在声明之前不能被使用。

function f() {
  var x = 1
  let y = 2
  const z = 3
  {
    var x = 100
    let y = 200
    const z = 300
    console.log('x in block scope is', x)
    console.log('y in block scope is', y)
    console.log('z in block scope is', z)
  }
  console.log('x outside of block scope is', x)
  console.log('y outside of block scope is', y)
  console.log('z outside of block scope is', z)
}

f()

x in block scope is 100
y in block scope is 200
z in block scope is 300
x outside of block scope is 100
y outside of block scope is 2
z outside of block scope is 3 

2. 数组助手函数

非常酷的助手函数出现了。像这样的逻辑你实现了多少次了：过滤，检查是否有元素满足条件，然后进行元素转换。也许有无数次吧。现在，有一些非常强大的功能可以代替你做这些事情。在我看来，这些是最有价值的功能：

forEach

对数组的每个元素执行所提供的函数，将数组元素作为参数传递进入。

var colors = ['red', 'green', 'blue']

function print(val) {
  console.log(val)
}

colors.forEach(print)

red
green
blue 

map

创建一个包含相同数量元素的新数组，但是由提供的函数创建输出元素。它只是将数组中的每个元素做了转换。

var colors = ['red', 'green', 'blue']

function capitalize(val) {
    return val.toUpperCase()
}

var capitalizedColors = colors.map(capitalize)

console.log(capitalizedColors)

["RED","GREEN","BLUE"] 

filter

创建一个包含原始数组子集的新数组。新数组的元素则是那些通过了所提供函数测试的元素，测试函数应返回true或false。

var values = [1, 60, 34, 30, 20, 5]

function lessThan20(val) {
    return val < 20
}

var valuesLessThan20 = values.filter(lessThan20)

console.log(valuesLessThan20)

[1,5] 

find

找到通过所提供函数测试的第一个元素，测试函数应返回true或false。

var people = [
  {name: 'Jack', age: 50},
  {name: 'Michael', age: 9},
  {name: 'John', age: 40},
  {name: 'Ann', age: 19},
  {name: 'Elisabeth', age: 16}
]

function teenager(person) {
    return person.age > 10 && person.age < 20
}

var firstTeenager = people.find(teenager)

console.log('First found teenager:', firstTeenager.name)

First found teenager: Ann 

every

检查数组的每个元素是否通过所提供函数的测试，测试函数应返回true或false。

var people = [
  {name: 'Jack', age: 50},
  {name: 'Michael', age: 9},
  {name: 'John', age: 40},
  {name: 'Ann', age: 19},
  {name: 'Elisabeth', age: 16}
]

function teenager(person) {
    return person.age > 10 && person.age < 20
}

var everyoneIsTeenager = people.every(teenager)

console.log('Everyone is teenager: ', everyoneIsTeenager)

Everyone is teenager:  false 

some

检查数组中是否有某个元素能够通过所提供函数的测试，测试函数应返回true或false。

var people = [
  {name: 'Jack', age: 50},
  {name: 'Michael', age: 9},
  {name: 'John', age: 40},
  {name: 'Ann', age: 19},
  {name: 'Elisabeth', age: 16}
]

function teenager(person) {
    return person.age > 10 && person.age < 20
}

var thereAreTeenagers = people.some(teenager)

console.log('There are teenagers:', thereAreTeenagers)

There are teenagers: true 

reduce

第一个参数是一个函数，该函数的参数为一个累加器以及数组中的每个元素（从左到右），并最终输出单个值。累加器的初始值则由reduce函数的第二个参数提供。

var array = [1, 2, 3, 4]

function sum(acc, value) {
  return acc + value
}

function product(acc, value) {
  return acc * value
}

var sumOfArrayElements = array.reduce(sum, 0)
var productOfArrayElements = array.reduce(product, 1)

console.log('Sum of', array, 'is', sumOfArrayElements)
console.log('Product of', array, 'is', productOfArrayElements)

Sum of [1,2,3,4] is 10
Product of [1,2,3,4] is 24 

3. 箭号函数

有时候，实现一些简单的功能也需要编写大量的重复代码。有什么补救办法吗？有，请尝试一下箭头函数！

var array = [1, 2, 3, 4]

const sum = (acc, value) => acc + value
const product = (acc, value) => acc * value

var sumOfArrayElements = array.reduce(sum, 0)
var productOfArrayElements = array.reduce(product, 1)

箭头函数也可以内联。它真的让代码更简单了：

var array = [1, 2, 3, 4]

var sumOfArrayElements = array.reduce((acc, value) => acc + value, 0)
var productOfArrayElements = array.reduce((acc, value) => acc * value, 1)

箭头函数也可以更复杂：

var array = [1, 2, 3, 4]

const sum = (acc, value) => {
  const result = acc + value
  console.log(acc, ' plus ', value, ' is ', result)
  return result
}

var sumOfArrayElements = array.reduce(sum, 0)

4. 类

当转移到JS项目的时候，哪个Java开发人员不会错过类？谁不喜欢像Java语言那样的显式继承？虽然一些JS开发者对此非常抱怨，但ES6实际已经引入了类的概念。它们不改变继承这个概念，它们只是原型继承的语法糖。

class Point {
    constructor(x, y) {
        this.x = x
        this.y = y
    }

    toString() {
        return '[X=' + this.x + ', Y=' + this.y + ']'
    }
}

class ColorPoint extends Point {
    static default() {
        return new ColorPoint(0, 0, 'black')
    }

    constructor(x, y, color) {
        super(x, y)
        this.color = color
    }

    toString() {
        return '[X=' + this.x + ', Y=' + this.y + ', color=' + this.color + ']'
    }
}

console.log('The first point is ' + new Point(2, 10))
console.log('The second point is ' + new ColorPoint(2, 10, 'green'))
console.log('The default color point is ' + ColorPoint.default())

The first point is [X=2, Y=10]
The second point is [X=2, Y=10, color=green]
The default color point is [X=0, Y=0, color=black] 

5. 增强的对象字面量

对象字面量已得到了增强。现在我们可以更容易地： 
- 定义具有相同名称的变量赋值字段 
- 定义函数 
- 定义动态属性

const color = 'red'
const point = {
  x: 5,
  y: 10,
  color,
  toString() {
    return 'X=' + this.x + ', Y=' + this.y + ', color=' + this.color
  },
  [ 'prop_' + 42 ]: 42
}

console.log('The point is ' + point)
console.log('The dynamic property is ' + point.prop_42)

The point is X=5, Y=10, color=red
The dynamic property is 42 

6. 模板字符串

谁喜欢写一连串的字符串与变量的连接？我相信只有极少数人会喜欢。谁讨厌阅读这样的连接？每个人。幸运的是，ES6引入了非常易于使用的带有占位符的字符串模板。

function hello(firstName, lastName) {
  return `Good morning ${firstName} ${lastName}!
How are you?`
}

console.log(hello('Jan', 'Kowalski'))

Good morning Jan Kowalski!
How are you? 

请注意，我们可以将文本写成多行。

重要提示：请使用反引号而不是撇号来将文本包围起来。

7. 默认函数参数

你不喜欢提供所有可能的函数参数？请使用默认值。

function sort(arr = [], direction = 'ascending') {
  console.log('I\'m going to sort the array', arr, direction)
}

sort([1, 2, 3])
sort([1, 2, 3], 'descending')

I'm going to sort the array [1,2,3] ascending
I'm going to sort the array [1,2,3] descending 

8. Rest和Spread操作符

Spread

它可以将数组或对象内容提取为单个元素。 
示例 - 数组的浅拷贝：

var array = ['red', 'blue', 'green']
var copyOfArray = [...array]

console.log('Copy of', array, 'is', copyOfArray)
console.log('Are', array, 'and', copyOfArray, 'same?', array === copyOfArray)

Copy of ["red","blue","green"] is ["red","blue","green"]
Are ["red","blue","green"] and ["red","blue","green"] same? false 

示例 - 合并数组：

var defaultColors = ['red', 'blue', 'green']
var userDefinedColors = ['yellow', 'orange']

var mergedColors = [...defaultColors, ...userDefinedColors]

console.log('Merged colors', mergedColors)

Merged colors ["red","blue","green","yellow","orange"] 

Rest

要将前几个函数参数绑定到指定变量，而将其他参数作为数组绑定到单个变量上吗？现在你可以很容易地做到这一点。

function printColors(first, second, third, ...others) {
  console.log('Top three colors are ' + first + ', ' + second + ' and ' + third + '. Others are: ' + others)
}
printColors('yellow', 'blue', 'orange', 'white', 'black')

Top three colors are yellow, blue and orange. Others are: white,black 

9. 解构

对于数组

从数组中提取指定元素并将其赋值给变量。

function printFirstAndSecondElement([first, second]) {
    console.log('First element is ' + first + ', second is ' + second)
}

function printSecondAndFourthElement([, second, , fourth]) {
    console.log('Second element is ' + second + ', fourth is ' + fourth)
}

var array = [1, 2, 3, 4, 5]

printFirstAndSecondElement(array)
printSecondAndFourthElement(array)

First element is 1, second is 2
Second element is 2, fourth is 4 

对于对象

从对象中提取指定的属性，并将其赋值给具有相同名称的变量。

function printBasicInfo({firstName, secondName, profession}) {
    console.log(firstName + ' ' + secondName + ' - ' + profession)
}

var person = {
  firstName: 'John',
  secondName: 'Smith',
  age: 33,
  children: 3,
  profession: 'teacher'
}

printBasicInfo(person)

John Smith - teacher 

10. Promises

Promise承诺，你将会得到延时任务或者长期运行任务的未来结果。Promise有两个通道：第一个为结果，第二个为潜在的错误。要获取结果，你要将回调函数作为“then”的函数参数。要处理错误，你要将回调函数作为“catch”的函数参数。

请注意，由于函数调用是随机的，所以，示例每次执行的输出可能是不同的。

function asyncFunc() {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
          const result = Math.random();
          result > 0.5 ? resolve(result) : reject('Oppps....I cannot calculate')
        }, 1)
    });
}

for (let i=0; i<10; i++) {
    asyncFunc()
        .then(result => console.log('Result is: ' + result))
        .catch(result => console.log('Error: ' + result))
}

Result is: 0.7930997430022211
Error: Oppps....I cannot calculate
Result is: 0.6412258210597288
Result is: 0.7890325910244533
Error: Oppps....I cannot calculate
Error: Oppps....I cannot calculate
Result is: 0.8619834683310168
Error: Oppps....I cannot calculate
Error: Oppps....I cannot calculate
Result is: 0.8258410427354488