变量的解构赋值

// ES5

let a = 1;
let b = 2;
let c = 3;

// ES6

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

let [head, ...tail] = [1, 2, 3, 4];
head // 1
tail // [2, 3, 4]

let [x, y, ...z] = ['a'];
x // "a"
y // undefined
z // []

let [foo] = [];
let [bar, foo] = [1];

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

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

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

let [x, y, z] = new Set(['a', 'b', 'c']);
x // "a"

function* fibs() {
  let a = 0;
  let b = 1;
  while (true) {
    yield a;
    [a, b] = [b, a + b];
  }
}

let [first, second, third, fourth, fifth, sixth] = fibs();
sixth // 5

let [foo = true] = [];
foo // true

let [x, y = 'b'] = ['a']; // x='a', y='b'
let [x, y = 'b'] = ['a', undefined]; // x='a', y='b'

let [x = 1] = [undefined];
x // 1

let [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: y is not defined

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

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

let { baz } = { foo: 'aaa', bar: 'bbb' };
baz // undefined

let {foo} = {bar: 'baz'};
foo // undefined

// 例一
let { log, sin, cos } = Math;

// 例二
const { log } = console;
log('hello') // hello

let { foo: baz } = { foo: 'aaa', bar: 'bbb' };
baz // "aaa"

let obj = { first: 'hello', last: 'world' };
let { first: f, last: l } = obj;
f // 'hello'
l // 'world'

let { foo: foo, bar: bar } = { foo: 'aaa', bar: 'bbb' };

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

let obj = {
  p: [
    'Hello',
    { y: 'World' }
  ]
};

let { p: [x, { y }] } = obj;
x // "Hello"
y // "World"

let obj = {
  p: [
    'Hello',
    { y: 'World' }
  ]
};

let { p, p: [x, { y }] } = obj;
x // "Hello"
y // "World"
p // ["Hello", {y: "World"}]

const node = {
  loc: {
    start: {
      line: 1,
      column: 5
    }
  }
};

let { loc, loc: { start }, loc: { start: { line }} } = node;
line // 1
loc  // Object {start: Object}
start // Object {line: 1, column: 5}

let obj = {};
let arr = [];

({ foo: obj.prop, bar: arr[0] } = { foo: 123, bar: true });

obj // {prop:123}
arr // [true]

// 报错
let {foo: {bar}} = {baz: 'baz'};

const obj1 = {};
const obj2 = { foo: 'bar' };
Object.setPrototypeOf(obj1, obj2);

const { foo } = obj1;
foo // "bar"

var {x = 3} = {};
x // 3

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

var {x: y = 3} = {};
y // 3

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

var { message: msg = 'Something went wrong' } = {};
msg // "Something went wrong"

var {x = 3} = {x: undefined};
x // 3

var {x = 3} = {x: null};
x // null

// 错误的写法
let x;
{x} = {x: 1};
// SyntaxError: syntax error

// 正确的写法
let x;
({x} = {x: 1});

// 表达式虽然毫无意义，但是语法是合法的，可以执行。
({} = [true, false]);
({} = 'abc');
({} = []);

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

const [a, b, c, d, e] = 'hello';
a // "h"
b // "e"
c // "l"
d // "l"
e // "o"

let {length : len} = 'hello';
len // 5

let {toString: s} = 123;
s === Number.prototype.toString // true

let {toString: s} = true;
s === Boolean.prototype.toString // true

let { prop: x } = undefined; // TypeError
let { prop: y } = null; // TypeError

function add([x, y]){
  return x + y;
}

add([1, 2]); // 3

[[1, 2], [3, 4]].map(([a, b]) => a + b);
// [ 3, 7 ]

function move({x = 0, y = 0} = {}) {
  return [x, y];
}

move({x: 3, y: 8}); // [3, 8]
move({x: 3}); // [3, 0]
move({}); // [0, 0]
move(); // [0, 0]

function move({x, y} = { x: 0, y: 0 }) {
  return [x, y];
}

move({x: 3, y: 8}); // [3, 8]
move({x: 3}); // [3, undefined]
move({}); // [undefined, undefined]
move(); // [0, 0]

[1, undefined, 3].map((x = 'yes') => x);
// [ 1, 'yes', 3 ]

[1, undefined, 3].map((x = 'yes') => x);
// [ 1, 'yes', 3 ]

以下三种解构赋值不得使用圆括号。
变量声明语句
// 全部报错
let [(a)] = [1];

let {x: (c)} = {};
let ({x: c}) = {};
let {(x: c)} = {};
let {(x): c} = {};

let { o: ({ p: p }) } = { o: { p: 2 } };

函数参数

// 报错
function f([(z)]) { return z; }
// 报错
function f([z,(x)]) { return x; }

赋值语句的模式
// 全部报错
({ p: a }) = { p: 42 };
([a]) = [5];

// 报错
[({ p: a }), { x: c }] = [{}, {}];

[(b)] = [3]; // 正确
({ p: (d) } = {}); // 正确
[(parseInt.prop)] = [3]; // 正确

交换变量的值
let x = 1;
let y = 2;

[x, y] = [y, x];

从函数返回多个值

// 返回一个数组

function example() {
  return [1, 2, 3];
}
let [a, b, c] = example();

// 返回一个对象

function example() {
  return {
    foo: 1,
    bar: 2
  };
}
let { foo, bar } = example();

函数参数的定义

// 参数是一组有次序的值
function f([x, y, z]) { ... }
f([1, 2, 3]);

// 参数是一组无次序的值
function f({x, y, z}) { ... }
f({z: 3, y: 2, x: 1});

提取 JSON 数据

let jsonData = {
  id: 42,
  status: "OK",
  data: [867, 5309]
};

let { id, status, data: number } = jsonData;

console.log(id, status, number);
// 42, "OK", [867, 5309]

const { SourceMapConsumer, SourceNode } = require("source-map");

// 获取键名
for (let [key] of map) {
  // ...
}

// 获取键值
for (let [,value] of map) {
  // ...
}

const map = new Map();
map.set('first', 'hello');
map.set('second', 'world');

for (let [key, value] of map) {
  console.log(key + " is " + value);
}
// first is hello
// second is world