JavaScript's Proxy object is a powerful feature that enables you to intercept and customize operations performed on objects. This versatile tool allows developers to create more efficient and flexible code while also improving code maintainability. In this article, we will dive into the world of JavaScript proxies, explore their use cases, and provide examples for expert developers.

A Brief Introduction to Proxies in JavaScript

A Proxy object in JavaScript is used to define custom behaviour for fundamental operations such as property lookup, assignment, enumeration, and function invocation. It acts as a wrapper around another object, called the target, and allows you to intercept and manipulate operations performed on the target object.

Creating a JavaScript Proxy

To create a JavaScript Proxy, you need two things: a target object and a handler object. The target object is the object you want to wrap with the Proxy, while the handler object defines the custom behaviour for the proxy.

Here's a simple example of creating a Proxy:

const target = { name: "John", surname: "Doe", fullName: "John Doe" };
const handler = {
  get(_target, prop) {
    return _target[prop] ? _target[prop].toUpperCase() : "Property not found";
  },
  set(_target, prop, value) {
    if (prop === 'name') {
      _target.fullName = value + ' ' + _target.surname;
    }
    if (prop === 'surname') {
      _target.fullName = _target.name + ' ' + value;
    }
    if (prop === 'fullName') {
      throw new Error('fullName is not writable')
    }
    _target[prop] = value;
    return true;
  }
};

const proxy = new Proxy(target, handler);
console.log(proxy.fullName); // Output: "JOHN DOE"
console.log(proxy.nonexistent); // Output: "Property not found"
proxy.name = 'Jane';
console.log(proxy.fullName); // Output: "Jane Doe"
proxy.surname = 'Smith';
console.log(proxy.fullName); // Output: "Jane Smith"
proxy.fullName = 'Tom Smith'; // Error: fullName is not writable

In this example, we defined a get trap in the handler object, which intercepts the property lookup operation on the target object. When a property is accessed on the proxy object, the get trap is invoked, and it returns the property value in uppercase if the property exists or a custom message if it doesn't.

Then, we defined a set trap, which intercepts a property's set. If the property name is name or surname calculates the fullName and sets the target object with the right value, but if you try to set the fullName property the code raises an error.

Use Cases for Proxies in JavaScript

Validation and Constraints

Proxies can be used to enforce data validation and constraints when setting properties on an object. For example, you can ensure that a property value is a certain type or within a specific range.

const target = { age: 25 };
const handler = {
  set(_target, prop, value) {
    if (prop === "age" && (typeof value !== "number" || value < 0 || value > 150)) {
      throw new TypeError("Invalid age value");
    }
    _target[prop] = value;
    return true;
  },
};

const proxy = new Proxy(target, handler);
proxy.age = 30; // Works fine
proxy.age = "thirty"; // Throws TypeError: "Invalid age value"

Logging and Profiling

Proxies can be used to log and profile operations performed on an object, which can be helpful during debugging and performance analysis.

const target = { message: "Hello, World!" };
const handler = {
  get(_target, prop) {
    console.log(`Accessing property "${prop}"`);
    return _target[prop];
  },
  set(_target, prop, value) {
    console.log(`Setting property "${prop}" to "${value}"`);
    _target[prop] = value;
    return true;
  },
};

const proxy = new Proxy(target, handler);
console.log(proxy.message); // Logs: "Accessing property "message"" and outputs: "Hello, World!"
proxy.message = "Goodbye, World!"; // Logs: "Setting property "message" to "Goodbye, World!""

To learn more don't miss my YouTube video about JavaScript Proxy on my YouTube channel

Conclusion

JavaScript's Proxy object is a powerful and versatile feature that allows developers to create more efficient and flexible code. By understanding and utilizing proxies, you can unlock their full potential and become a true JavaScript superhero.

You can find the code of this article here.

Luca Del PuppoFollow

@Microsoft MVP, @Google Developer Expert, @GitKraken Ambassador, Senior Software Developer and JavaScript enthusiastic Youtube Channel https://youtube.com/@Puppo_92