ES6变量赋值和基本数据类型详解

var a = [];
for (var i = 0; i < 10; i  ) {
  a[i] = function () {
    console.log(i);
  };
}
a[6](); // 10

var a = [];
for (let i = 0; i < 10; i  ) {
  a[i] = function () {
    console.log(i);
  };
}
a[6](); // 6

var tmp = 123;
if (true) {
    tmp = 'abc'; // ReferenceError
    let tmp;
}

let a = 10;
var a = 1; //Uncaught SyntaxError: Identifier 'a' has already been declared

let a = 10;
let a = 1; //Uncaught SyntaxError: Identifier 'a' has already been declared

const foo = {};
foo.prop = 123;

console.log(foo.prop); // 123

foo = {}; // TypeError: "foo" is read-only

const foo = Object.freeze({});

// 常规模式时，下面一行不起作用；
// 严格模式时，该行会报错
foo.prop = 123; // Can't add property prop, object is not extensible

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

var [x=1]=[,3];
console.log(x) // x = 1

var {foo,bar} = {foo:'aaa',bar:'bbb'}
等价于
var {foo:foo,bar:bar} = {foo:'aaa',bar:'bbb'}

var x;
{x} = {x: 1}; // 会报错,因为JavaScript引擎会将{x}理解成一个代码块

var x;
({x} = {x:1}); // 正确写法

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]

注意对比与下面代码的区别
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]
此处是给{x,y}这个对象设置了默认值,而不是给x,y分别设置了默认值

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

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

function f([x, y, z]) { ... }
f([1, 2, 3]);

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

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

jQuery.ajax = function (url, {
  async = true,
  beforeSend = function () {},
  cache = true,
  complete = function () {},
  crossDomain = false,
  global = true,
  // ... more config
}) {
  // ... do stuff
};
// 指定参数的默认值，就避免了在函数体内部再写var foo = config.foo || 'default foo';这样的语句。

var 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

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

es5:
"\uD842\uDFB7"
// "𠮷"

"\u20BB7"
// " 7"

es6:
"\u{20BB7}"
// "𠮷"

let hello = 123;
hell\u{6F} // 123

function is32Bit(c) {
  return c.codePointAt(0) > 0xFFFF;
}

is32Bit("𠮷") // true
is32Bit("a") // false