ES6+好用的小技巧，让你的代码更干净，短巧，易读

摘要：模板字符串扩展操作符操作符，有两个主要用处复制一个新的数组或对象把多个参数赋值给一个数组变量把一个数组变量赋值给多个参数是一个新的数组，内容和一样合并对象属性，后边的属性会覆盖前边的，可用于修改对象的某个属性值输出默认参数给方法添加默认参

</>复制代码 
let name = "siri", age = 18, job = "front-end engineer"
let oldStr = "Hi, " + name + ", I"m " + age + " and work as a " + job + ".";
let newStr = `Hi, ${ name }, I"m ${ age } and work as a ${ job }.`;

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… 操作符，有两个主要用处：
复制一个新的数组或对象
把多个参数赋值给一个数组变量
把一个数组变量赋值给多个参数

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let a = [1, 2, 3]
let b = [...a] // b是一个新的数组，内容和a一样
let c = [...a, 4, 5, 6]
let car = { type: "vehicle ", wheels: 4};
let newCar = {...car}
console.log(newCar); // { type: "vehicle ", wheels: 4}
// 合并对象属性，后边的属性会覆盖前边的，可用于修改对象的某个属性值
let car2 = {...car, type: "vehicle2", wheels: 2} // {type: "vehicle2", wheels: 2}

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function foo(...args) {
    console.log(args); 
} 
foo( "car", 54, "tree");  //  console.log 输出 [ "car", 54, "tree" ] 

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// 给方法添加默认参数值
function foo( a = 5, b = 10) {
    console.log( a + b);
} 
foo();  // 15
foo( 7, 12 );  // 19
foo( undefined, 8 ); // 13
foo( 8 ); // 18
foo( null ); // 10 as null is coerced to 0

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// 默认参数值也可以是表达式或者函数
function foo( a ) { return a * 4; }
// y = x + 4, z = foo(x)
function bar( x = 2, y = x + 4, z = foo(x)) {
    console.log([ x, y, z ]);
}
bar();  // [ 2, 6, 8 ]
bar( 1, 2, 3 ); //[ 1, 2, 3 ] 
bar( 10, undefined, 3 );  // [ 10, 14, 3 ]

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// 对象参数默认值，如果参数为空，则会抛出异常
function show({ title = "title", width = 100, height = 200 }) {
  console.log( `${title} ${width} ${height}` );
}
show() // Cannot destructure property `title` of "undefined" or "null".
show({}) // title 100 200
// 解决办法：
function show({ title = "title", width = 100, height = 200 } = {}) {
  console.log( `${title} ${width} ${height}` );
}
show(); // title 100 200
show({width: 200}) // title 200 200

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// key变量重命名， first --> firstName
const person = {
  first: "foo",
  last: "tom",
};
const { first: firstName } = person;
console.log(firstName); // foo

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// 默认值
const settings = {
    speed: 150
}
const { speed = 750, width = 500 } = settings;
console.log(speed); // 150 
console.log(width); // 500
// 可能不存在的key
const { middle: middleName = "midname" } = person;
console.log(middleName); // "midname"

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// 嵌套赋值
const user = {
  id: 339,
  name: "Fred",
  age: 42,
  education: {
    degree: "Masters"
  }
};
const {education: {degree}} = user;
console.log(degree); //prints: Masters

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// 如果嵌套的属性不存在
const user = {
  id: 339,
  name: "Fred",
  age: 42
};
const {education: {degree}} = user;  // TypeError: Cannot match against "undefined" or "null".
// 解决办法：
const user = {
  id: 339,
  name: "Fred",
  age: 42
};
const {education: {degree} = {}} = user;
console.log(degree); //prints: undefined

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const numRange = (start, end) => {
  return Array(end - start + 1).fill().map((item, index) => start + index);
};
const numbers = numRange(0, 5); // [0, 1, 2, 3, 4, 5]
const numbers2 = numRange(1, 5); // [1, 2, 3, 4, 5]

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const years = [2016, 2017, 2017, 2018, 2018, 2019]
// set构造函数的参数是一个array
const distinctYears = [...new Set(years)] // [2016, 2017, 2018, 2019]

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function generateRandom(length) {
    let radom13chars = function () {
        return Math.random().toString(16).substring(2, 15)
    }
    let loops = Math.ceil(length / 13)
    return new Array(loops).fill(radom13chars).reduce((string, func) => {
        return string + func()
    }, "").substring(0, length)
}
generateRandom(8) // "03836a49"