Copy es6 analytical variables

ES6 allowed according to a certain pattern, the array and object to extract values, variable assignment, which is referred to deconstructed (Destructuring).

If deconstruction is not successful, the value of the variable is equal undefined.

let [x, y] = [1, 2, 3];
x // 1
y // 2

let [a, [b], d] = [1, [2, 3], 4];
a // 1
b // 2
d // 4

　　If the right side of the equal sign is not an array (or strictly speaking, can not traverse the structure, see "Iterator" chapter), it would be an error.

// 报错
let [foo] = 1;
let [foo] = false;
let [foo] = NaN;
let [foo] = undefined;
let [foo] = null;
let [foo] = {};