What does !(function()) mean in JavaScript/jQuery? -

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• javascript function leading bang ! syntax 6 answers

i saw in parsleyjs library folowing:

what expression !(function(f){...}) mean?
negation?

edit:
after explanations, observed code looks

!( f(y){}( f(x){} ) ); 

or can written

!( f(z) ); 

or

!(z); 

where z = f(z), z = f(y){}, , y = f(x){}...
not clear function executes expression !(z);

usually use either

!function(f){...}() 

or

(function(f){...})() 

or

+function(f){...}() 

the developers here combined first two, redundant.