JavaScript I learn a variable type

This article is NetEase Cloud classroom Jin Xu-liang Teacher's course note, records down, to prepare for forgetting.

Variable Type

There are only 6 kinds: four raw data Types Boolean, number, String, undefine, and other object,function are objects

Typeof,instanceof

See examples directly:

        var NULL ;        Console.info (typeof obj);       // Object        var arr = [];         instanceof Object);   // true        instanceof Array);    // true

wrapper objects for raw data types (Wapper object)    String,number,boolean all correspond to specific packaging objects Data type Conversions    Convert to numeric type with Parseint,parsetfolat

//  the //  the // NaN // 3

JavaScript is Dynamic TypeProgramming language, the same variable, what type of data to grab what kind of variable type

//  Number var value = +//stringvalue = "Qiu"//Objectvalue = [1 , ' both ', 3];

various patterns of the "=" Number:

               = =          Assignment                = = =        , etc.                = = =      Strict judgment such as var x =; var y = "x"//true; // false

undefined vs nullUdefine: Refers to the variable is undefined, there is no valid value (not yet variable, what from the talk value)    Null:nothing, a variable does not refer to any object. Null is an object, type object (with variable, but no reference value)

var NULL ; if NULL {alert (// This sentence will be executed }else  {alert ("Obj!=null");} Alert (typeof//Object

Undefine and null judgments, etc.

varMyVar;//trueConsole.log (typeofMyVar = = = "undefined"); Console.log (MyVar===undefined); varMYVAR2 =NULL; Console.log (typeofMYVAR2);//Object//true;Console.log (MyVar2 = =NULL); Console.log (MyVar2===NULL); //trueConsole.info (MyVar = = myVar2);//undefine = = null; is True//falseConsole.info (MyVar = = = MyVar2);//Undefine = = = NULL; is False

true and False    Undefined, Null,nan, "", 0
    In addition to these values, the other values are true; operator:!! with | |!! Convert the following expression to a Boolean value that returns TRUE or FALSE! " Qiu "True!! Null false | |    var ns = NS | | {} If NS is not defined to return {}, the NS note is returned: define variables must be VAR, otherwise drop pits! If you do not write Var, it will become a global variable .

JavaScript I learn a variable type