! preceding function in javascript? [duplicate]

Answer 1

It simply makes the JavaScript parser parse it as an expression, which is necessary to execute it.

Answer 2

The preceding ! takes the un-parseable statement, and allows it to to be parsed by the JS engine, which in turn returns true.

function(){}();
SyntaxError: Unexpected token (

!function(){}();
>>true

Answer 3

I've tried it, it returned true. The function returns undefined, and !undefined is true.

!function(){}();
^          ^ ^
C          A  B

• A. function(){} is an empty anonymous function
• B. () executes the function (A), returning undefined
• C. ! negates undefined, which becomes true

I think they used that trick for a code golf or an obfuscated code. It is a bad practice to practially use that

Try javascript:alert(!function(){}()) in your browser address bar