javascript check undefined or null

; Capabilities> Capabilityname= "Ecmascriptversion"value= "3.0" /> Capabilityname= "Jscriptversion"value= "5.6" /> Capabilityname= "JavaScript"value= "true" /> Capabilityname= "Javascriptversion"value= "1.5" /> Capabilityname= "Msdomversion"value= "${majorversion}.${minorversion}" /> Capabilityname= "W3cdomversion"value= "1.0" /> Capabilityname= "exchangeomasupported"value= "true" /> Capabilit

. JavaScript undefined attributes Definition and usage The undefined attribute is used to store the undefined value of JavaScript. Syntax Undefined Description You cannot use the for/in loop to enumerate the

"Alert (typeof a5); // displays "object"Alert (typeof a6); // displays "object"Alert (typeof a7); // display "number"Alert (typeof a8); // display "undefined"From the code above, we can see that undefined is undefined for undefined values and definitions. null is a special o

"Object"Alert (typeof A6); Show "Object"Alert (typeof A7); Show "Number"Alert (typeof A8); Show "Undefined" As you can see from the code above, undefined values and undefined undefined,null is a special Object,nan is a special number. 2. Comparison operationvar A1; The va

Most computer languages have and have only one value that represents "none", for example, the Null,java language of the C language, the Null,python language of the None,ruby language of nil.Oddly enough, the JavaScript language actually has two values that represent "none": Undefined and

Most computer languages have and have only one value that represents "none", for example, the Null,java language of the C language, the Null,python language of the None,ruby language of nil.Oddly enough, the JavaScript language actually has two values that represent "none": Undefined and

to null and (2) Nan is unequal to any value and not equal to itself. nbsp; JavaScript undefined properties nbsp; definitions and usages undefined properties are used to store JavaScript undefined values. nbsp; Syntax

"alert (typeof A6);//Display "Object"alert (typeof A7);//display "number"alert (typeof A8);//Show "undefined"you can see from the above code that undefined values and definitions are unassigned for Undefined,null is a special Object,nan is a special number. 2. Comparison Operationsvar A1; The value of A1 is undefinedva

the two are equal. if (!undefined) console.log('undefined is false');// undefined is falseif (!null) console.log('null is false');// null is falseundefined == null// true The code abo

inverse compiled text of the function is displayed. The following example illustrates the use of the caller property: Caller Demo { function Callerdemo () { if (callerdemo.caller) { var a= callerDemo.caller.toString (); alert (a); } else { alert ("This was a top function"); } function Handlecaller () { callerdemo (); } Execution results: C.undefined and Null ---------------------------------------------------------------------------

In javascript, how does one determine whether a variable is null, undefined, Or NaN? Let's take a look at the specific judgment method. It is mainly determined through the typeof method. typeof returns a string and has six possible results: "number", "string", "boolean", "object", "function", and "undefined ". 1. Deter

Reproduced from: Author: one pixel Www.cnblogs.com/onepixel/p/7337248.html In a recent book on "JavaScript Advanced Programming", which mentions the equality operator (= =), you cannot convert null and undefined to any other value until you compare equality, but remember that null = =

) {var a=else {alert ("This I S a top function "); }}function Handlecaller () {Callerdemo ();} Handlecaller (); Execution Result:　　　　C.undefined and Null Most computer languages have and have only one value that represents "none", for example, the Null,java language of the C language, the Null,python language of the None,ruby language of nil. Oddly

following example illustrates the use of the caller property:// Caller Demo {function Callerdemo () {if (callerdemo.caller) {var a= callerDemo.caller.toString (); alert (a); Else { alert ("This is a top function");} } function Handlecaller () { callerdemo ();} Handlecaller ();Execution Result:　　　　C.undefined and Null Most computer languages have and have only one value that represents "none", for example, the

down.The concept of NULL, undefined, and Nan1. In JS, I have always felt that the difference between undefined and null is not very small, usually use the time will not be too concerned. Generally speaking:Undefined generally means "nonexistent" at all, and null means defin

So, after thinking carefully, write down this article, please heroes to shoot bricks. Generally known: Null = = undefined But: Null!== undefined So what's the difference between the two? Null This is an object, but is empty. Because it is an object, typeof

JavaScript has two special data types: undefined and null. The following section describes the null judgment and the undefined judgment. The following is an incorrect usage: varexpundefined; if (expundefined) {alert JavaScript has

a value assigned.Null represents a reference data type that is not assigned a value.Let's look at a piece of code: 12345678 var age; var id = 100;var div02 = document.getElementById("div02");//注:div02是不存在的var div01 = document.getElementById("div01");//注:div01存在alert(id);//100alert(age);//undefinedalert(div02);//nullalert(div01);//object Let's take a look at the memory situation:Solve the first question: Why Undefine inherits from nullIn

Oddly enough, the JavaScript language actually has two values that represent "none": Undefined and null. Why is that? First, similarity In JavaScript, assigning a variable to undefined or null is, to be honest, almost indistin

In JavaScript development, people ask: what is the difference between null and undefined? A bad time to answer, especially undefined, because it involves the implementation of undefined principle.General known: null = = undefinedH