Question

Please ignore that this can be re-written without variables. It’s just to have a simple example.

window.onload = function() {
    var a = document.body, b = function() {console.log(1)};
    a.onkeydown = b;
};

I know what happens: it works. But how?

If b was a global variable, the interpreter would store a reference to it. In this example, does the interpreter store a reference to the local variable, only to replace it, with what I assume is a copy of the function, when the local variable is destroyed? Or is the reference to the local variable still stored somewhere behind the scenes, and is then re-purposed?

Answer 1

Functions (and other objects) are always passed by reference. b does not contain the function, rather it points to it. When you assign a.onkeydown = b, you are making a.onkeydown point to the same function object. Then the function ends, so the local b variable is destroyed but the function it points to is still there – it would only be removed by the garbage collector if there were nothing else pointing it to it.