Ken Lee, thoughts and et cetera

Javascript Lazy Initialization using JS Proxy

March 12, 20219 mins read

JS Proxy. A mysterious object that seems to do probably great things. Yet unsure of how its magic can be applied.

Whelp. If you’ve stumbled onto ‘tis humble blog ole mine, then I hope ye in fer some treat.

Let’s begin by defining a problem, and our goal.

Problem

We have an object that is computationally heavy, imagine a server. However, we realize that in our some of our control statement, we may not need to create the server!

Goal

The server shall be created/initialized early in the project.

const server = new HeavyServer({
  arguments1: 1,
  arguments2: 2,
});

Then, in some of our control flow, we will not actually do anything with the server, while in some other control flow, we will do something.

// somewhere deep in the callstack
if (someConfig) {
  server.doSomethingHeavily();
} else {
  // do nothing
}

Now as we may want to do such a thing like initializing just before we call it.

// somewhere deep in the callstack
if (someConfig) {
  const server = new HeavyServer({
    arguments1: 1,
    arguments2: 2,
  });
  server.doSomething();
} else {
  // do nothing
}

It may not be a best choice as we are leaking the configuration of the server over to somewhere deep in the callstack.

So with that constraint, how can Proxy help us to achieve it?

Behold.

class LazyInit {
  constructor(args) {
    this.server = null;
    this.initArgs = args;
  }

  get(target, prop, receiver) {
    if (!this.server) {
      this.server = new Server(this.initArgs);
    }
    return Reflect.get(this.server, prop);
  }
}

const server = new Proxy(
  Server,
  new LazyInit({
    arguments1: 1,
    arguments2: 2,
  })
);

Explanation

The new Proxy

const server = new Proxy(
  Server,
  new LazyInit({
    arguments1: 1,
    arguments2: 2,
  })
);

In line 1, we’ve created a new Proxy Object.

In line 2, it is the object (class in this case) that we want to concerned with.

In line 3, it is our “hooker/trap” logic (aka handler) that we want the Proxy Object to execute.

The handler

class LazyInit {
  constructor(args) {
    this.server = null;
    this.initArgs = args;
  }

  get(target, prop, receiver) {
    if (!this.server) {
      this.server = new Server(this.initArgs);
    }
    return Reflect.get(this.server, prop);
  }
}

In line 7, we’ve defined a get trap, this will let the Proxy object know that we want to hook into any methods that is getting a property value.

In line 8, and 9, we will check for the existence of our server before calling it in line 11.

Conclusion

And that’s it! We’ve delayed the initialization of our server to just before any functions/variables were accessed!

Although we technically do not need to declare the Proxy target…

// we can actually proxy an empty object
const server = new Proxy(
  {},
  new LazyInit({
    arguments1: 1,
    arguments2: 2,
  })
);

// instead of our Server class
const server = new Proxy(
  Server,
  new LazyInit({
    arguments1: 1,
    arguments2: 2,
  })
);

I would recommend you to provide it so as Proxy will forward the various function calls over to target for traps that were not implemented.