What does "javascript:void(0)" mean?

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I've seen such hrefs many times, but I don't know what exactly that means.

Answer

The void operator evaluates the given
expression and then returns undefined.

The void operator is often used merely
to obtain the undefined primitive
value, usually using “void(0)” (which
is equivalent to “void 0”). In these
cases, the global variable undefined

can be used instead (assuming it has
not been assigned to a non-default
value).

An explanation is provided here: void operator.

The reason you’d want to do this with the href of a link is that normally, a javascript: URL will redirect the browser to a plain text version of the result of evaluating that JavaScript. But if the result is undefined, then the browser stays on the same page. void(0) is just a short and simple script that evaluates to undefined.