数组的解构赋值（基本语法和定义）

在ES6中按照一定模式，从数组和对象中提取值对变量进行赋值，这被称为解构（Destructuring）

以前，为变量赋值，只能直接指定值

  let a = 'a';
  let b = 'b';
  let c = 'c';

ES6 允许写成下面这样

  let [a, b, c] = ['a', 'b', 'c']
  console.log(a); //a
  console.log(c); //c

这表示，可以从数组中提取值，按照对应位置，对变量赋值

举几个🌰

  let [a,[[b],c]] = [1,[[2],3]]
  console.log(a); //1
  console.log(b); //2
  console.log(c); //3

  let [, , thrid] = ['first', 'second', 'thrid']
  console.log(thrid);//thrid

  let [a, , c] = [1, 2, 3];
  console.log(a); //1
  console.log(c); //3

  let [a, ...b] = [1, 2, 3, 4]
  console.log(a); // 1
  console.log(b); // [2,3,4]

  const [a, b, ...c] = ['a'];
  console.log(a); // a
  console.log(b); // undefined
  console.log(c); // []

如果解构不成功，变量值就是undefined

    let [a] = [];
    let [b, c] = [1];
    console.log(a); // undefined
    console.log(b); // 1
    console.log(c); // undefined

或者是不完全解构，就是等号左边的模式，只匹配一部分等号右边的数组，这种情况是可以解构成功

    let [x, y] = [1, 2, 3];
    console.log(x); //1
    console.log(y); //2
    let [a, [b], d] = [1, [2, 3], 4]
    console.log(a); //1
    console.log(b); //2
    console.log(d); //4

如果等号右边不是可以遍历的解构（不具有Iterator接口），就会报错

    //以下全部报错
    let [a] = 1;
    let [a] = false;
    let [a] = NaN;
    let [a] = undefined;
    let [a] = null;
    let [a] = {};

Set结构，也可以使用数组的解构赋值

    let [a, b, c] = new Set(['a', 'b', 'c'])
    console.log(b); // b

事实上，只要数据结构具有Iterator接口，都可以采用数组形式进行结构赋值.

    //fibs是一个Generator函数，原生具有Iterator接口
    function* fibs() {
        let [a, b] = [0, 1];
        while (true) {
            yield a;
            [a, b] = [b, a + b]
        }
    }
    let [first, second, thrid, fourth, fifth, sixth] = fibs();
    console.log(first); // 0
    console.log(sixth); // 5