The data types of learning notes in the classroom Web JavaScript

JavaScript data type six data types (five primitive types, one object type)

• Number
• Sttring
• Boolean
• Null
• Undefined
• Object #对象
  • Function
  • Array
  • Date
  • ...

The JavaScript data type is a weak data type, and you do not need to specify a data type when defining a variable.

var num = 32;num = "this is a string";32 + 32  // 64 #加法运算//"+"理解为字符串拼接，"-"理解为减法运算"32" + 32 // "3232" # 字符串拼接"32" - 32 // 0 # 减法运算

Implicit conversion skillfully uses "+"/"-" rule conversion type

var num = "string"；num - 0 //将num对象转换为numbervar num = 123；num + "" //将num对象转换为string

A = = B #严格等于

• First judgment type, different type, return False
• Same type:
• Number//value same
• string//length and content are the same
• NULL = = NULL
• undefined = = = undefined
• Nan! = Nan//nan is not equal to anything, including comparing yourself.
• New Object! = new Object//is a two empty object and is not equal.
• [1, 2]! = [1, 2]//content is the same, the order is not equal, because they are not exactly the same object
• In JavaScript, the comparison of objects is compared by reference.

A = = B #等于

• Same type, same as = = =
• Type is different, try a type conversion and then compare:
• Null = = undefined//true
• Number = = string//try to turn string number,1 = = "1.0" to True
• Boolean = =? Boolean Turn number,true = 1, false = 0
• Object = = Number | string//Attempt to convert object to basic type new String (' hh ') = = ' Hi '//true
• Other: False

Wrapping Object

var str = "string"; //string类型var strObj = new String("string"); //对象类型，string对应的包装类str.length //str为基本类型，没有属性。当str访问length属性时，javascript会把基本类型转换为对应的包装对象

Type detection typeof

{%note danger%} suitable for detecting basic types and function, encountering null invalidation {%endnote%}

typeof 100    "number"typeof true     "boolean"typeof function   "function"typeof(undefined)  "undefined"typeof new Object()   "object"typeof [1, 2]           "object"typeof NaN           "number"typeof null         "object" #返回object而不是null，是由于历史原因

instanceof

The instanceof operator is based on the prototype chain to judge, usage:obj instanceof Object
Note: Object type detection between different window or IFRAME cannot use instanceof!

{%note danger%} can be used to detect custom objects and native objects {%endnote%}

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

Object.prototype.toString.apply ()

{%note%} is suitable for built-in objects and basic types to detect null compatibility issues {%endnote%}

Object.prototype.toString.apply([]); === "[object Array]"Object.prototype.toString.apply(function(){}); === "[object Function]"Object.prototype.toString.apply(null); === "[object Null]"Object.prototype.toString.apply(unfefined); === "[object Undefined]"IE6/7/8 Object.prototype.toString.apply(null); 返回"[object Object]"

The data types of learning notes in the classroom Web JavaScript