Function calling in Javascript with double brackets

Question

When I call function hi()() with double brackets the function displays hi output and it will also give error saying, that hi is not functi

Answer 1

Putting () after something that evaluates to a function will call that function. So, hi() calls the function hi. Assuming hi returns a function then hi()() will call that function. Example:

function hi(){
    return function(){return "hello there";};
}

var returnedFunc = hi();  // so returnedFunc equals function(){return "hello there";};
var msg = hi()();         // so msg now has a value of "hello there"

If hi() doesn't return a function, then hi()() will produce an error, similar to having typed something like "not a function"(); or 1232();.