javascript function leading bang ! syntax

In Javascript, a line beginning with function is expected to be a function statement and is supposed to look like

function doSomething() {
}

A self-invoking function like

function(){
  // do stuff
}();

doesn't fit that form (and will cause a syntax error at the first opening paren because there is no function name), so the brackets are used to delineate an anonymous function expression.

(function(){
  // do stuff
})();

But anything that creates an expression (as opposed to a function statement) will do, so hence the !. It's telling the interpreter that this is not a function statement. Other than that, operator precedence dictates that the function is invoked before the negation.

I wasn't aware of this convention, but if it becomes common it may contribute to readability. What I mean is that anybody reading the !function at the top of a large block of code will expect a self-invocation, the way we are conditioned already to expect the same when we see (function. Except that we will lose those annoying parentheses. I would expect that's the reason, as opposed to any savings in speed or character count.

For one thing, jsPerf shows that using ! (UnaryExpression's) are usually faster. Sometimes they come out to be equal, but when they aren't, I haven't seen the non-banged one triumph too much over the others yet: http://jsperf.com/bang-function

This was tested on the latest Ubuntu with the oldest (per say..) Chrome, version 8. So results may differ of course.

Edit: How about something crazy like delete?

delete function() {
   alert("Hi!"); 
}();

or void?

void function() {
   alert("Hi!"); 
}();

Besides the things that were already said, the syntax with the ! is useful if you write javascript without semicolons:

var i = 1
!function(){
  console.log('ham')
}()

i = 2
(function(){
  console.log('cheese')
})()

The first example outputs 'ham' as expected, but the second will throw an error because the i = 2 statement isn't terminated due to the following parenthesis.

Also in concatenated javascript files you don't have to worry if the preceding code has missing semicolons. So no need for the common ;(function(){})(); to make sure your own won't break.

I know my answer is kind of late but i think it haven't been mentioned yet:)

Ideally you should be able to do all this simply as:

function(){
  // do stuff
}(); 

That means declare anonymous function and execute it. But that will not work due to specifics of JS grammar.

So shortest form of achieving this is to use some expression e.g. UnaryExpression (and so CallExpression):

!function(){
  // do stuff
}(); 

Or for the fun:

-function(){
  // do stuff
}(); 

Or:

+function(){
  // do stuff
}(); 

Or even:

~function(){
  // do stuff
  return 0;
}( );