"undefined" in javascript is not working

Question

I am using javascript function where i need to check undefined value in if condition.

var a = getParameterByName('a');
// if condition comes true only then below line should be executed. 
// but everytime either condition is true or false, below statement is executed.
if (typeof (a) !== "undefined") { 
    document.getElementById("img1").src = getParameterByName('a');
}
if (typeof getParameterByName('d') !== "undefined") {
    document.getElementById("video").src = getParameterByName('d');
}

is there any problem in using 'undefined' ?? please help me as soon as posssible. Thanks

possible duplicate of [How to check for "undefined" in JavaScript?](http://stackoverflow.com/questions/3390396/how-to-check-for-undefined-in-javascript) — Bob Gilmore, Feb 19 '14 at 05:26
You're probably just looking for if(a). No need for typeof undefined check. — Brandon, Feb 19 '14 at 05:26
already answered here: http://stackoverflow.com/questions/2703102/typeof-undefined-vs-null — erapert, Feb 19 '14 at 05:26

score 0 · Answer 1 · answered Feb 19 '14 at 05:30

0

it might look like var a = getParameterByName('a'); is getting value a = '';.

In javascript '' != undefined so check type of a.

answered Feb 19 '14 at 05:30

Sohil Desai

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mplungjan · Answer 2 · 2014-02-19T05:43:18.563

0

Very inefficient code.

Try this. Src will be changed if a is none of false, empty string, null, undefined or 0 It will not work as expected if a can be 1, true or some string not a known image type. Pragmatically I assume the getParameterByName will return "image.gif" or "image.jpg" or one of the falsy values I described

var a = getParameterByName('a'); 
if (a) document.getElementById("img1").src = a;

edited Feb 19 '14 at 05:43

answered Feb 19 '14 at 05:37

mplungjan

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What if the value of `a` is `false`? – jasonscript Feb 19 '14 at 05:38
Then there is still no reason to change an image source to false – mplungjan Feb 19 '14 at 05:39

"undefined" in javascript is not working

2 Answers2