! preceding function in javascript?

2018-05-29 23:02:15

Possible Duplicate:
What does the exclamation mark do before the function?

I saw a function formatted like this today for the first time:

!function(){}();

What is the preceding exclamation mark for? I assume it functions the same as:

(function(){})();

But... what's going on here?

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

它只是使JavaScript解析器将其解析为一个表达式，这对执行它是必需的。

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

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