Question

I’ve noticed that you can reference a function with or without the parentheses. Why?, what’s the difference?

As a slight aside, I’ve noticed this works:
window.onload = functionName;

Whereas this doesn’t:
window.onload = functionName();

Could anyone explain why the top line of code works and the bottom doesn’t?

Answer 1

The () version doesn’t reference the function. It invokes the function and references its return value.

Given this function:

function functionName() {
    return "I'm a function";
}

This references the function:

 // window.onload will reference the function, and invoked it when the page loads
window.onload = functionName;

…but this references the string that was returned, which isn’t very useful to window.onload:

 // the function is invoked immediately, so now window.onload references the
 //    string "I'm a function" that was returned
window.onload = functionName();