I am trying to work with ES6 Proxies but am noticing something super weird, it is getting called twice

ES6 Proxies! One of the most powerful and mind-bending features of modern JavaScript. But, oh, the frustration when they don’t behave as expected. You’re not alone, friend! Today, we’re going to tackle the mysterious case of the double-called Proxy.

What’s a Proxy, you ask?

A Proxy is an object that acts as a middleman between your target object and the outside world. It allows you to intercept and modify the behavior of the target object, adding a layer of indirection that can be incredibly useful for things like validation, caching, or logging.

const target = { foo: 'bar' };
const proxy = new Proxy(target, {
  get: (target, prop) => {
    console.log(`Getting property ${prop}`);
    return target[prop];
  }
});

console.log(proxy.foo); // Output: Getting property foo
                     // Output: bar

The Double-Call Conundrum

But wait, what’s this? You’re setting up a simple Proxy, and suddenly it’s being called twice for every operation. You’re left scratching your head, wondering what dark magic is at play.

const target = { foo: 'bar' };
const proxy = new Proxy(target, {
  get: (target, prop) => {
    console.log(`Getting property ${prop}`);
    return target[prop];
  }
});

console.log(proxy.foo); // Output: Getting property foo
                     // Output: Getting property foo
                     // Output: bar

Fear not, dear developer! This is not a bug, nor is it a feature (although it might seem like it). It’s simply a quirk of how Proxies work in JavaScript.

The Reason Behind the Double-Call

When you create a Proxy, it gets wrapped around the target object. However, the Proxy itself is also an object, which means it needs to be initialized. This initialization process involves calling the Proxy’s internal `getOwnPropertyDescriptor` method to retrieve the property descriptor for the target object’s properties.

This is where the magic happens – or, rather, where the magic goes awry. The `getOwnPropertyDescriptor` method is called not only for the property you’re trying to access (in this case, `foo`) but also for the `__proto__` property. Yes, you guessed it: the internal `getOwnPropertyDescriptor` method is called twice, once for `foo` and once for `__proto__`.

Solution 1: The `has` Trap

One way to avoid the double-call is to implement the `has` trap in your Proxy. The `has` trap is called when the `in` operator is used, and it allows you to specify whether a property exists on the target object or not.

const target = { foo: 'bar' };
const proxy = new Proxy(target, {
  get: (target, prop) => {
    console.log(`Getting property ${prop}`);
    return target[prop];
  },
  has: (target, prop) => {
    return prop in target;
  }
});

console.log(proxy.foo); // Output: Getting property foo
                     // Output: bar

By implementing the `has` trap, you’re telling the Proxy to only call the `get` trap when the property actually exists on the target object. This eliminates the unnecessary call for the `__proto__` property.

Solution 2: The `getOwnPropertyDescriptor` Method

Another approach is to implement the `getOwnPropertyDescriptor` method in your Proxy. This method is called when the Proxy needs to retrieve the property descriptor for a given property.

const target = { foo: 'bar' };
const proxy = new Proxy(target, {
  get: (target, prop) => {
    console.log(`Getting property ${prop}`);
    return target[prop];
  },
  getOwnPropertyDescriptor: (target, prop) => {
    return Object.getOwnPropertyDescriptor(target, prop);
  }
});

console.log(proxy.foo); // Output: Getting property foo
                     // Output: bar

By implementing the `getOwnPropertyDescriptor` method, you’re providing the Proxy with the necessary information to determine whether the property exists on the target object or not. This, in turn, prevents the double-call.

Conclusion

ES6 Proxies can be a powerful tool in your JavaScript toolkit, but they require a deep understanding of their inner workings. By understanding the reason behind the double-call and implementing the `has` trap or `getOwnPropertyDescriptor` method, you can avoid this common pitfall and create more robust and efficient Proxies.

Remember, when working with Proxies, it’s essential to keep in mind the subtleties of JavaScript’s object model and the internal mechanisms of Proxies themselves. With practice and patience, you’ll become a master of Proxy manipulation and be able to tackle even the most complex challenges.

Solution	Description
Solution 1: The `has` Trap	Implement the `has` trap to specify whether a property exists on the target object or not.
Solution 2: The `getOwnPropertyDescriptor` Method	Implement the `getOwnPropertyDescriptor` method to provide the Proxy with the necessary information to determine whether the property exists on the target object or not.

Further Reading

If you’re interested in diving deeper into the world of ES6 Proxies, I recommend checking out the following resources:

• MDN Web Docs: Proxy
• JavaScript.info: Proxy
• Tutorials Point: ES6 Proxies

Remember to always keep practicing and experimenting with Proxies to solidify your understanding of this powerful feature.

Final Thoughts

In conclusion, the double-call phenomenon is not a bug, but rather a quirk of how Proxies work in JavaScript. By understanding the reason behind it and implementing the `has` trap or `getOwnPropertyDescriptor` method, you can avoid this common pitfall and create more robust and efficient Proxies.

Happy coding, and may the Proxy be with you!

Frequently Asked Question

Let’s dive into the mysterious world of ES6 Proxies and unravel the enigma of the double call!