how does javascript !function() { } work

This question already has an answer here:

Why do you need to invoke an anonymous function on the same line? 19 answers

JavaScript plus sign in front of function name 3 answers

Javascript anonymous function call [duplicate] 4 answers

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With a return value.. you negate that with !

var x=!function(){return false}();
console.log(x);
// true

double negation

var pizza='pizza';
var x=!!function(){return pizza}();
console.log(x);
// true

// returns true if pizza is defined, not 'pizza'
// returns false if pizza is ''.

demo

http://jsfiddle.net/9shzF/1/

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Pretty much as with any other thing. The anonymous function is autoexecuted, therefore returns a value, and the value is negated.

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