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Question

Why does a parameter of a JavaScript function return 'undefined'?

Answer 1

+1 A:

change

setTimeout("changeBgImage(num, el)", 1000);

to

setTimeout(function() { changeBgImage(num, el) }, 1000);

Daniel A. White 2010-01-11 13:06:17

Answer 2

+1 A:

This is a guess but firstly because your invoking the function in quotes it invokes it in the global context, and num and el are not defined in global context, but within your function. Try using a function literal eg

setTimeout( function() { changeBgImage( num, el ) }, 1000 );

PS, you should be using != null not the string null.

meder 2010-01-11 13:08:28

Answer 3

+5 A:

There's a bunch of things that ain't right about your code. For one thing, your null check only checks that the variable does not have a string value that is exactly 'null'. Where you're setting backgroundImage property, the string you're assigning has unmatching parentheses.

But most of all, setTimeout executes on the window context, where your variables are not available (nor should you want them to be). Instead, change your call to

setTimeout(function() { changeBgImage(num, el) }, 1000);

So a complete fix would be

if(el){
    el.style.backgroundImage='url(images/bg_image'+randomImageNum+'.jpg);';
    var timer = setTimeout(function() { changeBgImage(num, el) }, 1000);
}

David Hedlund 2010-01-11 13:08:45

This causes every second a new closure being generated, which leads to a memory leak in IE

alemjerus 2010-01-11 13:14:36

@alemjerus can you post a screeny of your memory usage graphs?

Breton 2010-01-11 13:34:05

Answer 4

A:

You get it undefined because you transfer to the function "el" in quoted form, as a variable name, which does not exist in called context. To fix this, you need to do it like this:

<script type="text/javascript" charset="utf-8">
function changeBgImage(num, el){
    var imageNum = num;
    var randomImageNum = Math.ceil(Math.random()*imageNum);
document.getElementById(el).style.backgroundImage='url(images/bg_image'+randomImageNum+'.jpg';
            var timer = setTimeout("changeBgImage(num, '" + el + "')", 1000);
}

</script>  
</head>
<title>Dynamic Backgroundimage changer</title>
<body onload="changeBgImage(4, 'mycontainer')

alemjerus 2010-01-11 13:09:40

Answer 5

A:

When you call a function with setTimeout you cannot pass objects as parameters. So I recommend you to approach the problem in other way. You can have shared variables to pass the arguments.

Xim 2010-01-11 21:30:49

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Why does a parameter of a JavaScript function return 'undefined'?

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