Js_ data Type null,undefined,date (), Typeconvert:

Today, the main thing to learn is the data type aspect-the more learning more feel JS this language, really is amazing,very amazing, summed up under

1. First to compare null with undefined:

The NULL:JS keyword, which represents an object, but is empty. Because it is an object, typeof (NULL) returns object,:number-->0,string--> "null" when used under primitive type context, Bool-->false

Undefined: Not the JS keyword, but the global properties of the Window object, (which can be accessed with window.undefined), typeof (undefined) returns undefined, when using undeclared variables in JS Or declare a variable without assigning a value, its value is undefined,:number-->nan,string--> "undefined" when used under primitive type context, Bool-->false

Ps:null==undefined returns True, but null===undefined returns false because typeof () is not the same.

2.Date ()

Date represents the time object, and the way to get the object is: Var t=new Date ();

1 var New Date (); 2 var New Date (one, one, and all); 3 alert (d.tolocalestring ()); // datetime 2015/12/27 PM 12:41:11 4 alert (d2.tolocaledatestring ()); // Date 2015/12/25 Note: Month starts from 0, so 11 is December

3.Error objects

Pre-defined Error object in JS: Evalerror,typeerror,referenceerror,rangeerror,syntaxerror,urierror
var e = new TypeError (); alert (typeof (e)); // Object

Let's get to the bottom.

4.Regular Expression

Regular Expression object, for string matching

var patt1=New RegExp ("E");d ocument.write (patt1.test (/// string contains ' E ', true 

5. Type Conversion Summary

① here to learn a very powerful point of knowledge, that is, for the primitive type, that is, the NUMBER,STRING,BOOLEAN,JS interpreter will have a encapsulated function to encapsulate it into the object type: Number,string,boolean, This encapsulation is automatically implemented when objects need to be used in context: for example:

1 var s1 = "Hello World"; 2 var s2 = s1.substring (2, s1.length); // object calls the method, property of the string 3 alert (S2);
A call to the following object is equivalent:
5 var New String ("Hello World"); 6 var s2 = s1.substring (2, s1.length); 7 alert (S2);

This encapsulation is temporary, that is, after using the object's characteristics, the object does not exist, only string, of course, the most commonly used is a string, although Number,bool has this feature.

Moreover, this conversion is reversible!wow,cool! Isn't it?

1 var New String ("Hello World"); 2 var s2 = s1 + ", wonderful!"; // the S1 object is first converted to primitive string value, and then the character is spliced 3 alert (S2); // Hello world,wonderful!

The primitive value of ②number,string,bool can be converted to its object form by object ():

1 varS3 = "Hello";2Alerttypeof(S3));//string3S3 =Object (S3);4Alerttypeof(S3));//Object5 6 varB1 =true;7Alerttypeof(B1));//Boolean8B1 =Object (B1);9Alerttypeof(B1));//Object

③ when using object in a bool scenario, as long as object is not NULL, its value is true:

1 //internal value is False,but object convert to true!2 varT1 =NewBoolean (false);3 vart2 =NewNumber (0);4 varT3 =NewString ("");5 varT4 =NewArray ();6 7 if(false|| 0 | | "") {8Alert ("Sorry,you can ' t see me ...");9 }Ten if(T1 && T2 && T3 &&T4) { OneAlert ("Yes, object is true!")); A}

First come here, feel, strong!

Js_ data Type null,undefined,date (), Typeconvert: