摘要：模板字符串擴展操作符操作符，有兩個主要用處復制一個新的數組或對象把多個參數賦值給一個數組變量把一個數組變量賦值給多個參數是一個新的數組，內容和一樣合并對象屬性，后邊的屬性會覆蓋前邊的，可用于修改對象的某個屬性值輸出默認參數給方法添加默認參

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，有兩個主要用處：

復制一個新的數組或對象
把多個參數賦值給一個數組變量
把一個數組變量賦值給多個參數

let a = [1, 2, 3]
let b = [...a] // b是一個新的數組，內容和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}

// 合并對象屬性，后邊的屬性會覆蓋前邊的，可用于修改對象的某個屬性值
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" ] 

// 給方法添加默認參數值
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

// 默認參數值也可以是表達式或者函數
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 ]

// 對象參數默認值，如果參數為空，則會拋出異常
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構造函數的參數是一個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"