Question

suppose I have some javascript which says:

window.onunload=some_jsfunction();

how can I add another function to this handler? something like this:

window.onunload=some_jsfunction() + another_jsfunction();

Answer 1

The first and foremost way: addEventListener:

window.addEventListener('unload', function () { /* Do things. */ });
window.addEventListener('unload', function () { /* Do other things. */ });

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Another way is too create a new event handler with a call to the old handler from it, then overwrite the old handler. I can imagine a situation where it could be useful, but not with DOM events:

var onUnload = function () {
    /* Do things. */
};

var oldOnUnload = onUnload;

onUnload = function () {
    oldOnUnload();
    /* Do new things. */
};

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The most advanced way is creating your own observer on top of the DOM’s observer (this is what frameworks like Mootools do internally). It can be helpful in the long term; for example, to add event namespaces.

var handlers = [];

window.onunload = function () {
    handlers.forEach(function (fn) { fn(); });
};

handlers.push(function () { /* Do things. */ });
handlers.push(function () { /* Do other things. */ });