js总结概述2

二. 题

1. typeof判断类型（识别所有值类型，识别函数，判断是否是引用类型）,instanceof,constructor,toString,

1. typeof undefined //undefined

   typeof null //object

   typeof 3.4 //number

   typeof 'abc' //string

   typeof true//boolean

   typeof f //function

   typeof arr //object

   typeof {} //object

2. instanceof只能判断出来引用类型，判断不出来简单类型

   [] instanceof Array//true
   let obj = {}
   obj instanceof Objecet//true
   function f (){}
   f instanceof Function//true
   

3. constructor，不能识别undefined,null

   ''.constructor//ƒ String() { [native code] }
   false.constructor//ƒ Boolean() { [native code] }
   let num = 1
   num.constructor//ƒ Number() { [native code] }
   [].constructor//ƒ Array() { [native code] }
   f.constructor//ƒ Function() { [native code] }
   let obj = {name:1}
   obj.constructor//ƒ Object() { [native code] }
   

4. toString,都能识别出来

   Object.prototype.toString.call(null)//"[object Null]"
   Object.prototype.toString.call(undefined)//"[object Undefined]"
   Object.prototype.toString.call('')//"[object String]"
   Object.prototype.toString.call(false)//"[object Boolean]"
   Object.prototype.toString.call(1)//"[object Number]"
   Object.prototype.toString.call([])//"[object Array]"
   Object.prototype.toString.call({})//"[object Object]"
   Object.prototype.toString.call(f)//"[object Function]"
   

2. 0.1 + 0.2 === 0.3//false

2. 值类型与引用类型的区别

1. 值类型赋值是值，引用类型赋值是地址
2. 值类型是栈，引用类型是栈中存放地址，堆中存放数据，指向栈也就是地址

2. 字符串拼接

const a = 100 + '10'//'10010'
const b = true + '10'//'true10'

3. 强制类型转换和隐式类型转换

1. 强制： parseInt,parseFloat ,toString 等
2. 隐式：if，逻辑运算， ==，+字符串拼接

3. ==和===，只有==null时用==，其他的都用===，===数据类型也得一样，==会将数据类型自动转换

100 == '100'//true
null == undefined//true
0 == false//true
0 == ''//true
'' == false//true

if(obj.a == null)//相当于obj.a === null || obj.a === undefined，所以用==简便

100 === '100'//false
null === undefined//false
0 === false//false
0 === ''//false
'' === false//false

4. truly和falsely变量,通过!! index=== true就是truly变量，!! index === false就是falsely变量

1. 以下都是falsely变量，除此以外都是truly变量

   !!0 === false
   !!false === false
   !!'' === false
   !!undefined === false
   !!null === false
   !!NaN === false
   

   if内容判断，&&，||，！都是用truly,falsely变量