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Answer 1

+1 A:

if (document.getElementById('theElement')) // do whatever after this

For undefined things that throw errors, test the property name of the parent object instead of just the variable name - so instead of:

if (blah) ...

do:

if (window.blah) ...

Delan Azabani 2010-05-06 06:37:27

no I don't think because you get a null when you compare a xhtml element with javascript always you can do element == null or what you do... I mean when you compara VARIABLE that is not in the cpu memory... for examplefunction(){ if(variableNeverDeclared == undefined) //do something //this is gonna throw an erro cause variableNeverDeclared does not exist}

nahum 2010-05-06 06:46:48

I been doing try catch but I dont think is the best option

nahum 2010-05-06 06:47:40

Updated the answer to accurately answer your question.

Delan Azabani 2010-05-06 07:07:25

Answer 2

A:

if (!obj) {
    // object (not class!) doesn't exist yet
}
else ...

Thevs 2010-05-06 06:48:37

Answer 3

+6 A:

The best way is to check the type, because undefined/null/false are a tricky thing in JS. So:

if(typeof obj != "undefined") {
    // obj is a valid variable, do something here.
}

Note that typeof always returns a string, and doesn't generate an error if the variable doesn't exist at all.

Makram Saleh 2010-05-06 07:00:00

ooook .. that's cool thanks =)

nahum 2010-05-06 07:05:04

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