Your First JavaScript Proxy

January 06, 2019

Your First JavaScript Proxy

A Proxy is a fancy word for intercepting how you interact with objects in JavaScript. Want to hi-jack what happens when you set a value? Care to log out every time you call a function? All this and much more can be done by wrapping your object in a Proxy and "handling" those interactions.

console.log When They're Changed with handler.set

First we'll create a simple object:

let person = { name: 'John' }

If I change the name to Mindy, if I want to capture the change to see the value before and after it changes, I have to do it around setting the value.

console.log( = 'Mindy'

A Proxy will let me intercept when the value is set by using a handler with a set function:

let handler = {
set(target, property, value) {
//target is the original `person`
//property is the `name` property
//value would be "Mindy"

set is called a "trap". A "trap" is a function that intercepts the original behavior and allows you to re-write any behavior you would like.

The implementation of set is simply logging out the old target[property] then logging out the new value

let handler = {
set(target, prop, value) {
//log out what's changing
console.log(`Changing from ${target[prop]} to ${value}`)
//actually change it
return (target[prop] = value)

So now we can drop our person and handler into a new Proxy and everything will be wired up:

person = new Proxy(person, handler)

Now, when we change the name, the console will log out the change: = 'Mindy'
//console logs out "Changing from John to Mindy"

Prove it!

Here's a codesandbox for you to prove you understand how the set trap works. Implement set in the handler so that it logs out "Changing from John to Mindy" when []( is set to "Mindy".


Reflect.set is the proper tool for setting values inside of Proxy set traps, but makes no noticeable difference in this demo and was left out to avoid any confusion. We'll cover more about Reflect in future posts.