Can dynamically loaded JavaScript be unloaded?

<theory>You could go with a more object-oriented approach, and build the model in a way that each block of javascript blocks come in as their own objects, with their own methods. Upon unloading it, you simply set that object to null.</theory>

(This is fairly off-the-cuff.)

Memory use is indeed an issue you need to be concerned with in the current browser state of the art, although unless we're talking about quite a lot of code, I don't know that code size is the issue (it's usually DOM size, and leftover event handlers).

You could use a pattern for your loadable modules that would make it much easier to unload them en mass -- or at least, to let the browser know it can unload them.

Consider:

window.MyModule = (function() {

    alert('This happens the moment the module is loaded.');

    function MyModule() {

        function foo() {
            bar();
        }

        function bar() {
        }

    }

    return MyModule;
})();

That defines a closure that contains the functions foo and bar, which can call each other in the normal way. Note that code outside functions runs immediately.

Provided you don't pass out any references to what's inside the closure to anything outside it, then window.MyModule will be the only reference to that closure and its execution context. To unload it:

try {
    delete window.MyModule;
}
catch (e) {
    // Work around IE bug that doesn't allow `delete` on `window` properties
    window.MyModule = undefined;
}

That tells the JavaScript environment you're not using that property anymore, and makes anything it references available for garbage collection. When and whether that collection happens is obviously implementation-dependent.

Note that it will be important if you hook event handlers within the module to unhook them before unloading. You could do that by returning a reference to a destructor function instead of the main closure:

window.MyModule = (function() {

    alert('This happens the moment the module is loaded.');

    function foo() {
        bar();
    }

    function bar() {
    }

    function destructor() {
        // Unhook event handlers here
    }

    return destructor;
})();

Unhooking is then:

if (window.MyModule) {
    try {
        window.MyModule();
    }
    catch (e) {
    }
    try {
        delete window.MyModule;
    }
    catch (e) {
        // Work around IE bug that doesn't allow `delete` on `window` properties
        window.MyModule = undefined;
    }
}

If you save the evaluated code in namespaces, such as:

var MYAPP = {
    myFunc: function(a) { ... }
}

"Freeing" the whole thing should be as simple as setting MYPP to some random value, ala

MYAPP = 1

This does depend on there being no other means of referencing the variable, which isn't trivial

How about loading the JS files into an iframe? Then (in theory, never tested it myself) you can remove the iframe from the DOM and remove the "memory" it's using.

I think... or I hope...

If you are worried about memory leaks then you will want to make certain that there is no event handlers in the code you want removed referring to the still existing dom tree.

It may be that you need to keep a list of all event handlers your code added, and before unloading, go through and remove the event handlers.

I have never done it that way, I always worry about when I remove nodes that there is still a reference.

Here is a good article on javascript memory leaks: http://javascript.crockford.com/memory/leak.html

Can dynamically loaded JavaScript be unloaded?

Related

Recent Posts