es6新特性中...的用法

1.展開運算符

能夠將對象字面量展開為多個元素

const books = ["Don Quixote", "The Hobbit", "Alice in Wonderland", "Tale of Two Cities"];
console.log(...books);

輸出：Don Quixote The Hobbit Alice in Wonderland Tale of Two Cities

還可以將數組結合

沒有...之前，使用的是concat

const fruits = ["apples", "bananas", "pears"];
const vegetables = ["corn", "potatoes", "carrots"];
const produce = fruits.concat(vegetables);
console.log(produce);

　輸出：["apples", "bananas", "pears", "corn", "potatoes", "carrots"]

使用...之后

const fruits = ["apples", "bananas", "pears"];
const vegetables = ["corn", "potatoes", "carrots"];
const produce = [...fruits,...vegetables];
console.log(produce);

　　輸出結果一樣

2.剩余參數

可以將剩余不定數的參數保存到一個數組中

const order = [20.17, 18.67, 1.50, "cheese", "eggs", "milk", "bread"];
const [total, subtotal, tax, ...items] = order;
console.log(total, subtotal, tax, items);

　　輸出：20.17, 18.67, 1.50,["cheese", "eggs", "milk", "bread"]

3.代替argumens

對於參數不固定的函數，es6之前的處理方法是使用arguments，

function sum() {
  let total = 0;  
  for(const argument of arguments) {
    total += argument;
  }
  return total;
}

　　而現在直接使用剩余參數運算符就可以

function sum(...nums){
  let total = 0;
  for(const num of nums ){
      total += num; 
   }
  return total;  
}