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JavaScript Proxy

JavaScript Proxy

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Suprabha Supi
·Mar 16, 2022·

3 min read

A JavaScript Proxy is an object that wraps another object and intercepts the fundamental operations of the target object.

syntax:

let proxy = new Proxy(target, handler);
  • target – is an object to wrap, can be anything, including functions.
  • handler – proxy configuration: an object with “traps”, methods that intercept operations. – e.g. get trap for reading a property of target, set trap for writing a property into target, and so on.

Let's look a quick example by defining an object called user:

const user = {
    firstName: 'suprabha',
    lastName: 'supi',
}

// defining a handler function
const handler = {}

// now, create a proxy
const userProxy = new Proxy(user, handler);

userProxy object uses the user object to store data. The userProxy can access all properties of the user object.

Let’s see the output:

console.log(userProxy.firstName); // suprabha
console.log(userProxy.lastName); // supi

If you modify the original object user, the change is reflected in the userProxy

user.firstName = 'sam';
console.log(userProxy.firstName); // sam

Similarly, a change in the userProxy object will be reflected in the original object user:

proxyUser.lastName = 's';
console.log(user.lastName); // s

There are methods in proxy, here we will cover most important methods:

  1. get
  2. set
  3. apply

1️⃣ get:

The handler.get() method is a trap for getting a property value.

Also you can make the changes using get :

const user = {
    firstName: 'suprabha',
    lastName: 'supi',
}

// defining a handler function
const handler = {
    get(target, prop, receiver) {
    return "sumi";
  }
}

// now, create a proxy
const userProxy = new Proxy(user, handler);

console.log(userProxy.firstName) // sumi
console.log(userProxy.lastName)  // sumi

As of now we don’t have fullUserName in user object, so let’s create it in proxy using get trap:

const user = {
    firstName: 'suprabha',
    lastName: 'supi',
}

const handler = {
    get(target, property) {
        return property === 'fullUserName' ?
            `${target.firstName} ${target.lastName}` :
            target[property]
    }
};

const userProxy = new Proxy(user, handler)

console.log(userProxy.fullUserName) // suprabha supi

2️⃣ set:

set trap controls behaviour when a property of the target object is set.

So, let’s say you have to add some condition, so you can do it in set trap.

const user = {
    firstName: 'suprabha',
    lastName: 'supi',
        age: 15
}

const handler = {
    set(target, property, value) {
        if (property === 'age') {
            if (typeof value !== 'number') {
                throw new Error('Age should be in number!');
            }
            if (value < 18) {
                throw new Error('The user must be 18 or older!')
            }
        }
        target[property] = value;
    }
};

const userProxy = new Proxy(user, handler);

// if you try to set age to bool, you will get error
userProxy.age = true;
// Error: Age must be a number.

userProxy.age = '16';
// The user must be 18 or older.

userProxy.age = '20'
// no errors would be found

3️⃣ apply

The handler.apply() method is a trap for a function call. Here is the syntax:

let proxy = new Proxy(target, {
    apply: function(target, thisArg, args) {
        //...
    }
});

Now, lets follow the above example by captialzing the first and last name.

const user = {
    firstName: 'suprabha',
    lastName: 'supi'
}

const getFullName = function (user) {
    return `${user.firstName} ${user.lastName}`;
}

const getFullNameProxy = new Proxy(getFullName, {
    apply(target, thisArg, args) {
        return target(...args).toUpperCase();
    }
});

console.log(getFullNameProxy(user)); // SUPRABHA SUPI

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