How to work with Proxy in JavaScript - Part 2

Published July 9, 2020

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Photo bySiarhei Horbach

In the last blog on How to work with Proxy in JavaScript - Part 1, we were discussing how to make a proxy, handler, and set a trap function called set.

Let's now discuss another trap function called get.

If you recall from our last example, we have john object with a set trap function defined on its Proxy.

// john object
const john = {
  name: "",
  mob: null,
};

// proxy
const johnProxy = new Proxy(john, {
  set: function (obj, property, value) {
    // check if mob length is equal to 10 numbers
    // otherwise throw an error
    if (property === "mob" && value.toString().length !== 10) {
      throw new Error("Mobile Number Invalid 🚫");
    }
    // If everything's okay then add the value to john object
    obj[property] = value;
  },
  get: function (obj, property) {
    // custom logic
  },
});

// adding properties to the proxy object

johnProxy.mob = 8075457642; // no error
johnProxy.name = "John Doe"; // no error

console.log(john);

The get trap function

The get trap functions are invoked or called whenever the user is trying to access a property from the johnProxy object.

Let's add functionality to the get trap function where we will add an extra string Hello to the name property value when accessed by the user.

// john object
const john = {
  name: "",
  mob: null,
};

// proxy
const johnProxy = new Proxy(john, {
  set: function (obj, property, value) {
    // check if mob length is equal to 10 numbers
    // otherwise throw an error
    if (property === "mob" && value.toString().length !== 10) {
      throw new Error("Mobile Number Invalid 🚫");
    }
    // If everything's okay then add the value to john object
    obj[property] = value;
  },
  get: function (obj, property) {
    // adding extra Hello to name property
    if (property !== "name") {
      return obj[property];
    }
    // adding extra Hello to name
    return `Hello ${obj[property]}`;
  },
});

// adding properties to the proxy object

johnProxy.mob = 8075457642; // no error
johnProxy.name = "John Doe"; // no error

In the get function, it checks to see if the property accessed is the name or not, if it the name property it will add Hello string to it when accessed.

// adding extra Hello to name property
if (obj[property] !== "name") {
  return obj[property];
}
// adding extra Hello to name
return `Hello ${obj[property]}`;

Now if you try to access the name property from johnProxy object, you will see Hello John Doe.

console.log(johnProxy.name);

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