What is "javascript:" in a JavaScript event handler?

What is javascript: 开发者_运维技巧in a JavaScript event handler?

Such as:

<input onkeydown="javascript:return false;" type="text" name="textfield" />

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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.

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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 break or continue statement.

For example, you can use a label to identify a loop, and then use the break or continue statements 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" />

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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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You can just write return false. At that time the javascript protocol was useful in links. href attribute: <a href="javascript:return false">

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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.

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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.