Question

Sorry for posting this but !function is not google-able and I did not find it in my JavaScript code.

Here is how Twitter uses it:

<a href="https://twitter.com/share" class="twitter-share-button" data-url="http://google.com" data-text="Google says">Tweet</a>
<script>!function(d,s,id){var js,fjs=d.getElementsByTagName(s)[0];if(!d.getElementById(id)){js=d.createElement(s);js.id=id;js.src="//platform.twitter.com/widgets.js";fjs.parentNode.insertBefore(js,fjs);}}(document,"script","twitter-wjs");</script>

from https://twitter.com/about/resources/buttons#

Answer 1

It is short-hand or alternative of self-invoking anonymous function:

(function(){
  // code
})();

Can be written:

!function(){
  // code
}();

You can also use + instead of !.

If you simply did:

function(){
  // code
}();

That will create problems, that’s why you need to add ! before it which turns the function declaration into function expression.

Quoting docs, section 12.4:

An ExpressionStatement cannot start with the function keyword because
that might make it ambiguous with a FunctionDeclaration.

To better understand the concept, you should check out:

• Function Declarations vs. Function Expressions