Sep 28, 2023

See what object properties are accessed in JavaScript

At work I was mocking an object to be passed to a React context provider. I knew the type for the object, but I wanted to learn what properties were being accessed on the object from this point in the component tree.

Thankfully, JavaScript has a native way to “watch” an object and respond whenever any of its properties are accessed or reassigned: a Proxy.

First start with the object that you want to watch:

const user = {
  name: "",
  email: "",
  username: "",
  createdAt: "",
};

Then pass your object to the Proxy constructor along with a handler object that determines what you want to do you when a property is get or set:

const userProxy = new Proxy(user, {
  get(target, property) {
    console.log(`Getting ${property} on ${target}`);
    return target[property];
  },
  set(target, property, value) {
    console.log(`Setting ${property} on ${target} to ${value}`);
    target[property] = value;
    return true;
  },
});

Now you can use userProxy as if it were the original user object. I passed the proxy into the context provider and then watched then checked the console to see what properties for that object were accessed.

Nice and easy! Let me know if you have any other use cases for a proxy in JavaScript.

---