JS data type

Boolean null undefined number String Symbol (ECMAScript 2015)

The type can be detected by TypeOf.

var x = 1;        Console.log (typeof x);//number                 var a = undefined;        Console.log (typeof a);//undefined                  var b = null;        Console.log (typeof b);//object, (null is a null object reference/or pointer).                   var c = new Object ();        Console.log (typeof c);//object                    var e = [n/a];        Console.log (typeof e);//object                      var d = function () {                  //... Statement block        }        Console.log (typeof D);//function        var e = {};        Console.log (typeof e); Object        var F;        Console.log (typeof f);//undefined        var g = "HelloWorld";        Console.log (typeof g);//string        var h = true;        Console.log (typeof h);//Boolean

1.Number Type

Number type contains integers and floating-point numbers

NaN: Non-numeric type. Features: ① any operation related to Nan, will return Nan②nan not equal to itself

var ab = "A1"; Console.log (AB/10);//NaNconsole.log (Nan = = Nan);//false;

The IsNaN () function, which is used to determine whether a non-numeric type. Returns true if the passed parameter is a non-numeric type, otherwise false;

        var = num1;        Console.log (IsNaN (NUM1));    False        var num2 = "Test";         Console.log (IsNaN (num2));    True        var num3 = "666";        Console.log (IsNaN (num3));  False

The Infinity property is used to hold numeric values that represent positive infinity.

var num4 = 66/0;console.log (NUM4);//infinityconsole.log (typeof (Num4));//number    

Numeric conversions:

Number () transformation function, which can be used for any data type;

parseint (), convert the value to integer type, use more;

Parsefloat (); Converts a value to a floating-point type.

2.String type

String conversion: The transform function, string (), applies to any data type (null,undefined converted to null and undefined), and the ToString () method (null,defined does not have the ToString () method).

var BC = "Test"; var bd = Null;var be = Undefined;var ob = {};console.log (bc.tostring ()); Testconsole.log (Ob.tostring ()); [Object Object]console.log (typeof ob.tostring ()); String//console.log (Bd.tostring ()),//error error//console.log (be.tostring ()),//error error Console.log ("------"); Console.log (String (BC)); Testconsole.log (String (BD)); Nullconsole.log (BE); Undefinedconsole.log (String (ob));    

3.Null type

The null type is considered a null object pointer, and the null type is also an empty object reference. There is only one value, the null value, so when you use the TypeOf operator to detect the value of a null type, the result is the object type.

var test = Null;console.log (typeof test); Object

4.Undefined Type

There is only one value, that is, the undefined value. The variable is declared with Var, but the value of the variable is not initialized to the value of undefined.

var name = "Test"; var age;console.log (name);//testconsole.log (age);//undefined//console.log (height);//error, Variable height does not declare console.log (typeof name);//stringconsole.log (typeof age);//undefinedconsole.log (typeof height);// Undefined, variable height is not declared

5.Boolean type

True or False

var times = new Date (""); Console.log (times); Invalid Dateconsole.log (times = = null);  Falseconsole.log (Times = = "Invalid Date"); Trueconsole.log (Times = = = "Invalid Date");//Falseconsole.log (typeof times);//objectconsole.log (times.tostring () = = = "Invalid Date"); Truevar Newobj1;/console.log (NEWOBJ1);//undefinedconsole.log (typeof newObj1);//undefinedconsole.log (newObj1 = = NULL);//trueconsole.log (newObj1 = = undefined);//trueconsole.log (newObj1 = = null);//falseconsole.log (newObj1 = = = undefined);//true var newObj2 = null; Console.log (NEWOBJ2);//nullconsole.log (typeof newObj2);//objectconsole.log ( NewObj2 = = null);//trueconsole.log (newObj2 = = undefined);//trueconsole.log (newObj2 = = = null);//trueconsole.log ( NewObj2 = = = undefined);//false//0/0 5/0var tnum = 5/0; Console.log (Tnum); Infinityconsole.log (typeof Tnum);//numberconsole.log (Tnum = True);//falseconsole.log (IsNaN (Tnum));//false

JS Data type