个人对js理解

个人对js理解吧
1.作用域

var a = 1
(function(){
	a = 2
	console.log(a) // 2
})()

(function(){
	var a = 3
	console.log(a) // 3
	console.log(window.a) // 1
})()

2.函数直接挂载变量

var b = 10;
(function b(){
    b = 20;
    console.log(b);  
})();

由于b在b函数中，所以打印出来是b的函数
3.变量提升

var a = 10;
(function () {
    console.log(a)
    a = 5
    console.log(window.a)
    var a = 20;
    console.log(a)
})()

输出应该为undefined，10，20
4.对对象的理解

// example 1
var a={}, b='123', c=123;  
a[b]='b';
a[c]='c';  
console.log(a[b]);

---------------------
// example 2
var a={}, b=Symbol('123'), c=Symbol('123');  
a[b]='b';
a[c]='c';  
console.log(a[b]);

---------------------
// example 3
var a={}, b={key:'123'}, c={key:'456'};  
a[b]='b';
a[c]='c';  
console.log(a[b]);

由于对象的键名只能是字符串，如果键名是对象或者数组的时候，就会转成字符串
5.深拷贝，浅拷贝

function changeObjProperty(o) {
  o.siteUrl = "http://www.baidu.com"
  o = new Object()
  o.siteUrl = "http://www.google.com"
} 
let webSite = new Object();
changeObjProperty(webSite);
console.log(webSite.siteUrl);

6.对==，===，Object.is(),理解

let obj = {}, arr = [],obj1 = {}
let obj2 = obj
console.log(obj == obj1) // false
console.log(obj === obj1) // false
console.log(obj === obj2) // true
console.log(-0 === +0) // true
console.log(NaN === NaN) // false
console.log(Object.is(NaN,NaN)) // true
console.log(Object.is(-0,+0)) // false
console.log('' == null) // false
console.log(''== undefined) // false
console.log(''=== undefined) // false
console.log(null == undefined) // true
console.log(null === undefined) // false

7.js连续赋值

var a = {n: 1};
var b = a;  
a.x = a = {n: 2};
console.log(a.x);  
console.log(b.x);