摘要：模板字符串?dāng)U展操作符操作符，有兩個(gè)主要用處復(fù)制一個(gè)新的數(shù)組或?qū)ο蟀讯鄠€(gè)參數(shù)賦值給一個(gè)數(shù)組變量把一個(gè)數(shù)組變量賦值給多個(gè)參數(shù)是一個(gè)新的數(shù)組，內(nèi)容和一樣合并對(duì)象屬性，后邊的屬性會(huì)覆蓋前邊的，可用于修改對(duì)象的某個(gè)屬性值輸出默認(rèn)參數(shù)給方法添加默認(rèn)參

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 }.`;

… 操作符，有兩個(gè)主要用處：

復(fù)制一個(gè)新的數(shù)組或?qū)ο?/p>
把多個(gè)參數(shù)賦值給一個(gè)數(shù)組變量
把一個(gè)數(shù)組變量賦值給多個(gè)參數(shù)

let a = [1, 2, 3]
let b = [...a] // b是一個(gè)新的數(shù)組，內(nèi)容和a一樣
let c = [...a, 4, 5, 6]

let car = { type: "vehicle ", wheels: 4};
let newCar = {...car}
console.log(newCar); // { type: "vehicle ", wheels: 4}

// 合并對(duì)象屬性，后邊的屬性會(huì)覆蓋前邊的，可用于修改對(duì)象的某個(gè)屬性值
let car2 = {...car, type: "vehicle2", wheels: 2} // {type: "vehicle2", wheels: 2}

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

// 給方法添加默認(rèn)參數(shù)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

// 默認(rèn)參數(shù)值也可以是表達(dá)式或者函數(shù)
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 ]

// 對(duì)象參數(shù)默認(rèn)值，如果參數(shù)為空，則會(huì)拋出異常
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

// 解決辦法：
function show({ title = "title", width = 100, height = 200 } = {}) {
  console.log( `${title} ${width} ${height}` );
}

show(); // title 100 200
show({width: 200}) // title 200 200

// key變量重命名， first --> firstName
const person = {
  first: "foo",
  last: "tom",
};

const { first: firstName } = person;
console.log(firstName); // foo

// 默認(rèn)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"

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

// 如果嵌套的屬性不存在
const user = {
  id: 339,
  name: "Fred",
  age: 42
};
const {education: {degree}} = user;  // TypeError: Cannot match against "undefined" or "null".

// 解決辦法：
const user = {
  id: 339,
  name: "Fred",
  age: 42
};
const {education: {degree} = {}} = user;
console.log(degree); //prints: undefined

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]

const years = [2016, 2017, 2017, 2018, 2018, 2019]

// set構(gòu)造函數(shù)的參數(shù)是一個(gè)array
const distinctYears = [...new Set(years)] // [2016, 2017, 2018, 2019]

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"