为什么在函数名称之前需要写“ javascript：”？

Question

If I want to use an external javascipt function like "functionname()", see the example below:

<from onSubmit="javascript: funtionname()" >
</form>

why do I need sometimes to write javascript: before the function name?

Answer 1

In handlers ( onclick , onchange , etc.), you don't have to use the prefix. For these, Javascript is already expected and the prefix is interpreted as a label and has no effect.

If you want to execute Javascript in an href attribute for example, the prefix "javascript:" is needed. Otherwise the value would be interpreted as a URL.

In general, though, this practice should be avoided as it contradicts unobtrusive Javascript policy.

Answer 2

The javascript: notation is a URL scheme, and may be used wherever a URL is expected. As far as I am aware, passing a URL to an event handler is incorrect (at least according to the HTML5 spec) though browsers appear to support it. You are expected to specify a valid function or null as an event handler. The browser may be interpreting this javascript prefix as a label, though it might be technically incorrect to allow this.

Answer 3

You don't.

You will need to write javascript: when you using an url type tag only (like: href and src ), to specify that the string need to eval a javascript code.

Example:

<a href="javascript: doSomething();">
    My link is a javascript function doSomething()
</a>

You can:

<form onSubmit="doSomething();">
    When submit the form, will ran doSomething, 
    if returns false will cancel the form submission
</form>

Answer 4

No need to do it in handlers. It is used only as protocol in URL-like method, eg:

<a href="javascript:funtionname()">Click here</a>