Lazy Object Initialization

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The Firefox DevTools underlying code, which is written with JavaScript and HTML, is a complex application. Due to the complexity and amount of work going on, the DevTools team has done everything they can to load as little as possible. Furthermore the team has a system of lazily importing and initializing objects when they’re needed. I’ve taken a bit of time to reduce the initializer, let’s take a look!

The system relies on taking advantage of Object.defineProperty‘s get function to initialize an object when needed:

// Lazily initializes an object's property until it's used
function lazyGet(hostObj, name, initializer) {
    let defined = false;
    Object.defineProperty(hostObj, name, {
        get: function () {
            // If not already defined, define it by executing
            // its initializer and setting it as value
            if (!defined) {
                defined = true;
                // Overrides the original property definition
                // which is the initializer
                Object.defineProperty(hostObj, name, {
                    configurable: true,
                    enumerable: true,
                    value: initializer.apply(hostObj),
                    writable: true,
                });
                return hostObj[name];
            }
        },
        configurable: true,
        enumerable: true
    });
}

With the lazyGet function, the property you want is only initialized and processing down when its getter is called:

// Don't define window.myProp until someone tries to use it
// Thus, if it's never used, it's never initialized
lazyGet(window, "myProp", () => {
    return { message: "Hello!" };
});

// window.myProp is now undefined, since it hasn't been requested yet

// Use it for something, which triggers initialization and returns its value
console.log(window.myProp.message);

// Using it again doesn't initialize again, since it was already created
console.log(window.myProp.message);

// And it can be reassigned later on:
window.myProp = null;

Mozilla’s initializer is much more complex as it also acts as a loader, but you get the idea. We always think about lazy loading resources but it’s also good to think about initializing properties as they may not be needed! Keep a tiny footprint if you can!

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