What is "!functionname()"?
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! is the boolean not operator. !function() converts the return value of function() to boolean and returns its opposite value
If you are substituting the word "function" for the name of a function, it simply means "negate the result of the function". The ! means not. So
!true == false
functionname is an expression (which presumably evaluates to a function-object) and the result of this evaluation (a function-object) is invoked with () which invokes the function and evaluates to the return value.
Now, this return value (which was the result of an expression) is then negated with the unary ! (not) operator. The rules for ! are !truthy -> false and !falsy -> true , where truthy and falsy are concepts covered "truthy and falsy" in JavaScript.
The example could be written as: !((functioname)()) , but that's just silly