Share comparisons between JavaScript types

Types of JavaScript

Original type:

• number

• string

• boolean

• null

• ##undefined

• Object

• function

• Array

• Date

• ...

+/- Operation

• "37" + 7 = "377"

• "37" - 7 = 30

== Operation

• "1.23" == 1.23

• 0 == false

• null == undefined

===Strictly equal to

• Different types, return false

• Same type, the following is true:

• null === null

• undefine === null

• NaN != NaN

• new Object != new Obejct

== is equal to

• and has the same type as

  ===

• Different types, try type conversion comparison

• null == undefined

• number == string to number

• boolean == ? Convert number

• Object == number | string Try to convert the object to a basic type

• Others: false

var a = "string";
alert(a.length);    //6

a.t = 3;
alert(a.t);         //undefined

typeof 100 === “number”
typeof true === “boolean”
typeof function () {} === “function”
typeof(undefined) ) === “undefined”
typeof(new Object() ) === “object”
typeof( [1， 2] ) === “object”
typeof(NaN ) === “number”   //NaN也为number
typeof(null) === “object”

obj instanceof Object Use the prototype chain for judgment, suitable for judgment between objects. It expects an object on the left and a function object or function constructor on the right.

[1, 2] instanceof Array === true
new Object() instanceof Array === false

Object.prototype.toString.apply([]); === “[object Array]”;
Object.prototype.toString.apply(function(){}); === “[object Function]”;
Object.prototype.toString.apply(null); === “[object Null]”
Object.prototype.toString.apply(undefined); === “[object Undefined]”

// IE6/7/8 Object.prototype.toString.apply(null) 返回”[object Object]”

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• typeof

  Suitable for basic type and function detection, and will fail when encountering null.
  

• [[Class]]

  Get it through {}.toString, suitable for built-in objects and primitive types. When encountering null and undefined, it will fail (IE678, etc. return [object Object] ).
  

• instanceof

  is suitable for custom objects and can also be used to detect native objects. It will fail when detected between different iframes and windows.
  

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