ES6数组与对象的解构赋值详解

// 以前为变量赋值，只能直接指定值
var a = 1;
var b = 2;
var c = 3;
// ES6允许写成这样
var [a,b,c] = [1,2,3];

let [foo,[[bar],baz]] = [1,[[2],3]];
foo // 1
bar // 2
baz // 3
let [,,third] = ["foo","bar","baz"];
third // "baz"
let [head,...tail] = [1,2,3,4];
head // 1
tail // [2,3,4]
let [x,y,...z] = ['a'];
x // "a"
y // undefined
z // []

var [foo = true] = [];
foo // true
[x,y='b'] = ['a'];
// x='a', y='b'

var [x=1] = [undefined];
x //1
var [x=1] = [null];
x // null

function f(){
 console.log('aaa');
}
let [x=f()] = [1];

let x;
if([1][0] === undefined){
 x = f();
}else{
 x = [1][0];
}

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

var {foo,bar} = {foo:"aaa",bar:"bbb"};
foo // "aaa"
bar // "bbb"

var {bar,foo} = {foo:"aaa",bar:"bbb"};
foo // "aaa"
bar // "bbb"
var {baz} = {foo:"aaa",bar:"bbb"};
baz // undefined

var {foo:baz} = {foo:"aaa",bar:"bbb"};
baz // "aaa"
let obj = {first:"hello",last:"world"};
let {first:f,lats:l} = obj;
f // "hello"
l // "world"

var {foo:baz} = {foo:"aaa",bar:"bbb"};
baz // "aaa"
foo // error: foo is not defined

var obj = {
 p:["hello",{y:"world"}]
};
var {p:[x,{y}]} = obj;
x // "hello"
y // "world"

var {x=3} = {};
x // 3
var {x,y=5} = {x:1};
x // 1
y // 5
var {x: y=3} = {x: 5};
y // 5

var {foo} = {bar:"baz"};
foo // undefined

// 报错
var {foo: {bar}} = {baz:"baz"};

var arr = [1,2,3];
var {0:first, [arr.length-1]:last} = arr;
first // 1
last // 3