What is javascript: in a JavaScript event handler?
Such as:
<input onkeydown="javascript:return false;" type="text" name="textfield" />
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Peter Mortensen
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possible duplicate of [Do you ever need to specify 'javascript:' in an onclick?](http://stackoverflow.com/questions/372159/do-you-ever-need-to-specify-javascript-in-an-onclick) – Bergi Sep 08 '13 at 09:44
5 Answers
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It is a mistake. The pseudo-protocol is not needed in event handlers.
On a URL (a element href attribute, for instance), if you enter javascript: and follow that with javascript, the browser will run the javascript code.
For event handler, this is not needed, though the browser will not report an error.
Oded
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3It only doesn't throw an error by coincidence: it's valid syntax because it represents a label in JS. – Tim Down Jun 01 '11 at 09:56
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In this case it will be interpreted as label. You could also write foobar: here, it would have the same effect.
It is not really needed in JavaScript code (I have never seen it used in real code), though it could be useful:
Provides a statement with an identifier that you can refer to using a
breakorcontinuestatement.For example, you can use a label to identify a loop, and then use the
breakorcontinuestatements to indicate whether a program should interrupt the loop or continue its execution.
In your case, the markup should just be:
<input onkeydown="return false;" type="text" name="textfield" />
But if you use it as scheme in an URI, it tells the browser to interpret and execute the URI as JavaScript:
<a href="javascript:alert(1);">Foo</a>
(I'm not saying you should do something like this.)
I assume people less familiar with JavaScript see this and think they have to put javascript: everywhere in front of JavaScript code in HTML, also in event handlers.
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Felix Kling
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+1 for the label statement. There seems to be some long-standing confusion about this and I believe it stems from the use of MS's vbscript alongside JavaScript on the page. Although, even then it still doesn't do what you might expect (event handlers use the first defined language on the page). – Andy E Jun 01 '11 at 09:56
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@Andy: I've always assumed the confusion came from the widespread use of ``, but I have no evidence to back that up. Was/is it possible to write VBScript in an event handler attribute? – Tim Down Jun 01 '11 at 10:06
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@Tim: I don't have any evidence either, but I remember seeing a piece of code that utilized features of VBScript not available to J[ava]Script, and that used `javascript:` and `vbscript:` in events. I don't think it works though, IIRC IE just picks a default to use in event attributes. – Andy E Jun 01 '11 at 11:21
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You can just write return false. At that time the javascript protocol was useful in links. href attribute: <a href="javascript:return false">
Peter Mortensen
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bjornd
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It's something that shouldn't be there.
The javascript: prefix is primarily used for links, as the javascript: protocol in a browser would generally execute the code, for example:
<a href="javascript:alert('test')">Test</a>
In an event handler, though, it's already parsing JavaScript, thus it is not required. It's basically doing nothing.
Peter Mortensen
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Rudi Visser
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It's just markup to tell the browser that what follows is JavaScript code. However, it is not needed, so you don't have to include it.
Peter Mortensen
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Richard H
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2That is not true. It will be interpreted as label and does not tell the browser anything. – Felix Kling Jun 01 '11 at 09:39