Can dynamically loaded JavaScript be unloaded?

Question

I am writing a web application that has a static outer "shell" and a dynamic content section. The dynamic content section has many updates as users navigate the system. When a new content block is loaded, it may also optionally load another JavaScript file. In the name of good housekeeping, I remove script blocks from the DOM that apply to old content blocks, since that JavaScript is no longer needed.

The problem comes next, when I realized that although I have removed the <script> element from the DOM, the JavaScript that was previously evaluated is still available for execution. That makes sense of course, but I'm worried that it may cause a memory leak if the users navigate to a lot of different sections.

The question then, is should I be worried about this situation? If so, is there a way to force the browser to cleanup stale JavaScript?

Answer 1

<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>

Answer 2

(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;
    }
}