Need parenthesis () when calling functions sometimes and not other times?

Question

Why is it that when we call functions we usually need to put parenthesis (i.e. var randomQuote = getRandomQuote()) however in other cases we don't.

For example in calling window.setInterval(printQuote, 30000) I noticed that if I put parenthesis after printQuote() I received some wierd error somewhere else in my script file however without them the code works just fine.

Answer 1 · 2016-11-16T03:09:50Z

November 16, 2016 3:09am

If you use parentheses, you are calling the function.

If you don't use parentheses, you are just referring to that function and not really calling the function.

The cool thing about functions is that you can have functions as arguments:

function callFunc(func) {
    func();
}

function hello() {
    document.write("<h1>Hello!</h1>");
}

With those functions in mind we can call the callFunc function with the hello function as an argument.

Note that you shouldn't use parentheses since we are not calling the function (in the callFunc function we are) but referring to that specific function.

callFunc(hello);

And when we want to call functions we use the parentheses:

hello();

I hope you understand more :)

Good luck! ~alex

Answer 2 · 2016-11-16T03:14:53Z

November 16, 2016 3:14am

Thanks Alex! Yes, that was a great explanation to my question and the concept.

I really appreciate the quick response as well.

Thanks! Kirk

Welcome to the Treehouse Community

Looking to learn something new?

Kirk Rohani

Kirk Rohani

Need parenthesis () when calling functions sometimes and not other times?

2 Answers

Alexander Davison

Alexander Davison

Kirk Rohani

Kirk Rohani

Alexander Davison

Alexander Davison