Data types in JavaScript

JavaScript contains five basic data types (raw data types), namely: Undefined, NULL, number, Boolean, string; and a reference type Object, the reference type contains a special class: the function type. Number, Boolean, String three basic types for a wrapper type number,boolean,string, these three wrapper types are object types.

For these types, the following are explained separately:

1. typeof operator

You can get the type of the variable by this operator, the operation return value is a string, the following 7 kinds: "Number", "string", "boolean", "null", "undefined", "function", "Object".

2. Basic types

Undefined type: There is only one value undefined, when you declare a variable with VAR but do not initialize it, the value of this variable is undefined

var sum;    // The default value of undefinedalert (sum)  //  "undefined"alert (age)    is not initialized after declaration Variable not declared, error  = = undefined)       //  truealert (typeof SUM)            //  "undefined"alert (typeof Age)            //  "undefined" the     typeof of undeclared variables here also returns "undefined", which is logically justified, indicating that the variable cannot be manipulated

Null type: is also a data type with only one value, and a value of NULL, from a logical point of view, NULL represents an empty object pointer.

var NULL ; alert (typeof title);    // "Object" // because performing a typeof operation on a variable that is assigned null returns an object, //  //  This only needs to detect if it is null, to know if it has saved a reference to an object ifnull) {      // do something}

Number, Boolean, String type, and its wrapper type:

varnum = 5;varNUM =NewNumber (5);//Create a Number object hereAlerttypeofNUM);//"Number"AlerttypeofNUM);//"Object"varNUMF = Number (5); alert (typeofnumber);//"function" here number is a wrapper functionAlerttypeofNUMF);//"Number"//Boolean and string types are similar to numbervarBOOL =false;varBOOL =NewBoolean (false); alert (typeof(bool); alert (typeofBOOL);varstr = ' Hello ';varSTR =NewString (' Hello '); alert (typeofstr); alert (typeofSTR);

3. NaN: is a special value that indicates that a value should be returned without returning a numeric value, for example: except 0, Number ("a"), etc.

//1. Any operation that involves Nan will return NanAlert (1+nan)//NaNAlert (NAN/10)//NaN//2.NaN is not equal to any valueAlert (nan = = Nan)//falseAlert (2 = = NaN)//falseAlert ("abc" = = NaN)//false//3. IsNaN () function: values that cannot be converted to numbers return trueAlert (IsNaN (5))//falseAlert (IsNaN (true))//falseAlert (IsNaN ("a"))//true

4. Reference type: A set of data and features. An object is an instance of a reference type.

[[Class]] is an intrinsic property that can be used to classify an object, which has the following values:

"Arguments", "Array", "Boolean", "Date", "Error", "Function", "JSON", "Math", "number", "Object", RegExp "," String "

JavaScript can only get this intrinsic property through the ToString () method (Object.prototype.toString ())

Object.prototype.toString.call (undefined)//"[Object Undefined]"Object.prototype.toString.call (NULL)//"[Object Null]"Object.prototype.toString.call (function(){})//"[Object Function]"Object.prototype.toString.call (Math)//"[Object Math]"Object.prototype.toString.call (Array)//"[Object Function]"Object.prototype.toString.call ([])//"[Object Array]"Object.prototype.toString.call ({})//"[Object Object]"//the underlying type is converted to the wrapper type according to the corresponding rule processingObject.prototype.toString.call ("abc")//"[Object String]"Object.prototype.toString.call (200)//"[Object number]"Object.prototype.toString.call (true)//"[Object Boolean]"

You can get the object type by using the following function:

functiongetclass (x) {varstr =Object.prototype.toString.call (x); return/^\[object (. *) \]$/.exec (str) [1]; }getclass (NULL);//"Null"GetClass ({})//"Object"GetClass ([])//"Array"GetClass (JSON)//"JSON"GetClass (function(){})//"Function"functionFoo () {}getclass (NewFoo ())//"Object"

Data types in JavaScript