proxys.js

/************************************************************
 *
 * 💪🏼💪🏼💪🏼 Javascript Proxy 💪🏼💪🏼💪🏼
 * 
 ************************************************************/

// 1️⃣ Performing an action when accesing a property

const person = { name: "Gerardo", email: "gerardo@latteandcode.com" };
const personProxy = new Proxy(person, {
  get: function (item, property, itemProxy) {
    console.log(`'${property}' property has been acccesed`);
    return item[property];
  },
});
personProxy.name; // "'name' property has been acccesed"


// 2️⃣ Performing an action when setting the value of a property

const person = { name: "Gerardo", email: "gerardo@latteandcode.com" };
const personProxySetter = new Proxy(person, {
  set: function (item, property, value, itemProxy) {
    console.log(`'${property}' property has been set`);
    item[property] = value; // 🆒
  },
});
personProxySetter.name = 'Gerardo Fernández'; // "'name' property has been set"


// 3️⃣ Private properties

const person = { name: "Gerardo", _email: "gerardo@latteandcode.com" };
const personWithPrivateProperties = new Proxy(person, {
  has: (obj, prop) => {
    if (typeof prop === "string" && prop.startsWith("_")) {
      return false;
    }
    return prop in obj;
  },
  ownKeys: (obj) => {
    return Reflect.ownKeys(obj).filter(
      (prop) => typeof prop !== "string" || !prop.startsWith("_")
    );
  },
  get: (obj, prop) => {
    if (typeof prop === "string" && prop.startsWith("_")) {
      return undefined;
    }
    return obj[prop];
  },
});
console.log(personWithPrivateProperties._email); // undefined
for (let property in personWithPrivateProperties) {
  console.log(property); // Only 'name'
}


// 4️⃣ Validating data

const person = { name: "Gerardo", age: 33 };
const personValidator = new Proxy(
  person,
  {
    set(item, property, value) {
      if (property === "age") {
        if (!Number.isInteger(value) || value < 0 || value > 110) {
          throw new TypeError("Invalid age");
        }
      }
      item[property] = value;
    },
  }
);
personValidator.age = "33"; // Type Error: invalid age


// 5️⃣ Negative index for arrays

const array = ["foo", "bar", "zeta"];
const arrayWithNegativeIndex = new Proxy(array, {
  get: function (target, propKey) {
    const propKeyNumber = Number(propKey);
    if (Number.isInteger(propKeyNumber) && propKeyNumber < 0) {
      propKey = String(target.length + propKeyNumber);
    }
    return target[propKey];
  },
});

console.log(arrayWithNegativeIndex[-2]); // 'bar'