javascript null and undefined</title> ; <BR></head> <br><body> <br><script type="text/javascript"> <br>/**<br>******************null keyword************************ <br>JavaScript The keyword null is a special value that means "no value". Null is often regarded as a special value of the object type, that is, a value that represents "no object". <br>null is a unique value that is different from all other values. If the value of a variable is null, it means that its value is not a valid object, array, number, string <br> and Boolean value. <br>When null is used in a boolean environment, it is automatically converted to false. When it is used in a numeric environment, it is automatically converted to the number 0. When it is used in a string environment, it is <br> Automatically converted to "null". <br>*/ <br>var $null = null ; <br>//For Boolean environment <br>if($null){ <br>document.write("When null is used in Boolean environment, no conversion is performed"); <br>}else{ <br>document .write("When null is used in a Boolean environment, it is automatically converted to false");//This sentence will be output<br>} <br>//Used in a numeric environment<br>if(0 $null = = 0){ <br>document.write("<br/>When null is used in a numeric environment, it is automatically converted to 0");//This sentence will be output<br>}else{ <br>Document.write("<br/>When null is used in a numeric environment, no conversion is performed"); <br>} <br>//Used in a string environment<br>document.write("<br />For string environment: "$null);//null <br>//typeof of null: object <br>document.write("<br/>The data type of $null is: " typeof $null);//object <br>/**<br>**********************undefined****************** <br>undefined is a special value, it Not a keyword in javascript. The following situations need to be noted: <br>1. When using a variable that has been declared but not yet assigned a value <br> 2. When using an object attribute that does not exist <br> In the above two cases, what is returned is This value is undefined. The undefined value is different from null. <br>When an undefined value is used in a Boolean environment, it is automatically converted to false; <br>When it is used in a numeric environment, it is automatically converted to NaN; (Interpretation of NaN :The NaN attribute is a special value that represents a non-numeric value. This attribute is used to indicate that a value is not a number. <br>When it is used in a string context, it is automatically converted to "undefined"; <br>*/ <br>var $aaa;//Declared but not initialized. <br>var $bbb = "Test";//Declared and initialized. <br>var $obj = new Object(); <br>document.write("<br/>$aaa's type: " typeof $aaa ", $aaa's value: " $aaa);//undefined , undefined <br>document.write("<br/>Type of $bbb: " typeof $bbb ", Value of $bbb: " $bbb);//string, test <br>document.write(" <br/>The type of a non-existent property myProp of $obj: " typeof $obj.myProp ", the value of $obj.myProp: " $obj.myProp);//undefined, undefined <br>// document.write("<br/>Output the value of an undeclared variable: " $abc);//Run error<br>/**<br>The connection between undefined and null <br>Although undefined and null values are different, the == operator treats them as equal. <br>var obj = new Objec(); <br>obj.prop == null //return true <br>If you want to strictly distinguish between undefiend and null, you need to use the ==== operator or typeof <br> var obj = new Objec(); <br>obj.prop === null //return false <br>typeof obj.prop //output undefined <br>typeof null //output object <br>*/ <br></script> <br></body> <br></html> <br>
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