javascript - Difference between calling function and referencing function?

Question

Look at the following code:

window.onload = someFunction;

Many times I see the use of this kind of code and even I use the same. But, there are still few things unclear.

1. The above is a call to someFunction as soon as the page loads, right. But, aren't we supposed to call a function with brackets ()?
2. From my understanding, we are just assigning someFunction to window.onload, therefore can we or shouldn't we call window.onload();
3. Are there such cases, when functionName() and functionName becames interchangeable. Because at many places, and in various API, I have seen calling the function without brackets.

Answer 1

You're not calling the function, you're telling the browser which function to call, onload.

Referencing a function and calling a function are never interchangeable: they're completely different things.

You may, however, call a function that returns a function:

function getFunction() {
    return function() {
        alert("I'm the real onload function.");
    };
}

window.onload = getFunction();