javascript基础

 1.

alert(1=='1')  // true
      alert(1==='1') // false

2.

if ('a' in window) {
        var a = 10
      }

      alert(a)  // 10

3.

num = 123
      function fun1() {
        var num = 123
        return num++
      }
      fun1()
      alert(num) // 123

4.

var str = 'xiao.li@gmail.com';
var num=str.indexOf('.')
console.log(num)  //4

5.

var msg = 'hello'
      for (var i = 0;i < 10; i ++) {
        var msg = 'hello' + i * 2 + i
      }
      console.log(msg) //hello189

6.

var a = /123/,
        b = /123/
      console.log(a == b)  //false
      console.log(a === b) //false

7.

function foo (){
        var oldName = foo.name
        foo.name = 'bar'
        console.log(oldName + ',' + foo.name) //foo,foo
      }
      foo()

8.

console.log(undefined == null) // true

9.

var emp = new Array(5)
      emp[1] = 1
      emp[2] = 3
      console.log(emp.length) // 5

10.

num = 123
      function fun1() {
        var num = 123
        return num++
      }
      fun1()
      console.log(num) //123

11.

console.log(0.1 + 0.1 + 0.1) //0.30000000000000004

12.

HTTP协议、TCP协议、IP协议分别在应用层，传输层，​​网络层​​

13.

const obj = { 1: "a", 2: "b", 3: "c" };
    const set = new Set([1, 2, 3, 4, 5]);
    console.log(obj.hasOwnProperty("1")) //true
    console.log(obj.hasOwnProperty(1)) //true
    console.log(set.has("1")) //false
    console.log(set.has(1)) //true

14.

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

 15.

function say() {
      console.log(name) //undefined
      console.log(age)  //报错
      var name = 'TJH'
      let age = 24
    }
    say()

16.

function Foo () {
      getName = function () {
        console.log(1)
      }
      return this
    }

    Foo.getName = function () {
      console.log(2)
    }

    Foo.prototype.getName = function () {
      console.log(3)
    }

    var getName = function() {
      console.log(4)
    }

    function getName() {
      console.log(5)
    }

    Foo.getName()  //2
    getName()  //4
    Foo().getName() //1
    getName()  //1
    new Foo.getName() //2
    new Foo().getName() //3
    new new Foo().getName()  //3

17.

const a = {}
    const b = {key: 'b'}
    const c = { key: 'c'}
    a[b] = 123
    a[c] = 456
    console.log(a[b]) //456

18.

let obj1 = {
      name: 'xu',
      print: function () {
        return () => console.log(this.name)
      }
    }

    let obj2 = {name: "tongbao"}
    obj1.print()()  //xu
    obj1.print().call(obj2) //xu
    obj1.print.call(obj2)() //tongbao