Question

I just noticed this strange effect

window.onload = undefined;
console.log(window.onload); // print 'null', instead of 'undefined'

While it works as expected for other objects, including built-in, objects, e.g.

Array.prototype.slice = undefined;
console.log(Array.prototype.slice); // print 'undefined'

Why is that the case?

Answer 1

This behavior is like this because .onload is a setter and it works something like this:

window = {
    // Other window properties and methods

    get onload() {
        // returns null if no function was added or returns the last function added
    },

    set onload(value) {
        if (typeof value === 'function') {
            loadListener = value; // loadListener is the function called when load event is triggered
        }
    }

    // Other window properties and methods
}