what does !function in Javascript mean?

 

This question already has an answer here:

  • What does the exclamation mark do before the function? 9 answers

    • 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

    • they're negating the result, not the function itself: 

    !function( x ){ return x }( true );
!true
false

    In reality, it's a slightly compressed form of: 

    (function(){}())

    since it requires 1 less character. The reason it's needed is that you can't call a function declaration directly, eg this is invalid: 

    function(){}()

    but adding the ! at the beginning turns it into a function expression and makes it work.

    It's usually used to work around a quirk in the JavaScript syntax. This gives a syntax error: 

    function() {
}();

    It's read as a function declaration (like function foo () {} ), rather than a function expression. So adding the ! before it, or wrapping it in parentheses, forces the parser to understand that it's an expression.

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