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href javascript ; void 0

The void operator is often used to obtain the undefined primitive value. 

<a href="javascript:void(0);">
  Click Me
</a>
In this case, it is used to avoid # in the URI bar. The hrefs value will be undefined. Hence, when user clicks on the Click Me link, the URI will remain the same in the address bar.

Alternatives:

<a href="#" onclick="return false;">
  Click Me
</a>

<a href="#" onmousedown="return false;">
  Click Me
</a>
The return false prevents the page from being navigated and unwanted scrolling of a window.