How to know a variable is undefined?

Edward King · #1 (**permalink**) June 14th, 2005, 04:08 AM

How to know if a variable is defined in Javascript? Which function I can use?

Thanks in advance.

interrupt · #2 (**permalink**) June 14th, 2005, 06:19 AM

Alert it by name?

<script language ="javascript">
var 1 = ("1")

alert(1)
</script>

The alert should return 1.

HTH
interrupt

ChrisScott · #3 (**permalink**) June 14th, 2005, 06:40 AM

You can use typeof...

Code:

if(typeof myVar == "undefined"){
    // variable is undefined, do something 
}

However typeof will return "undefined" if a variable has not been set to any value / object / etc., so var myVar; would return "undefined" and var myVar = null; would not.

HTH,

Chris

Imar · #4 (**permalink**) June 14th, 2005, 10:14 AM

Also, 1 is not a valid variable name; it's a value.

var a1 = ("1")

alert(a1)

would work.....

Imar

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