Ben Borgers

How to detect when an object changes in JavaScript

April 2, 2020

You can use a JavaScript feature called a Proxy to define how properties of an object are read and set.

The Proxy takes a target (the object that the Proxy is wrapping), and a handler (which defines how the Proxy should act).

Here’s an example:

const sourceObject = {}

const handler = {
  get: (target, key) => {
    if(typeof target[key] === "object" && target[key] !== null) {
      return new Proxy(target[key], handler)

    return target[key]
  set: (target, prop, value) => {
      target[prop] = value
      console.log("A change was made!")
    return true

const object = new Proxy(sourceObject, handler)

In the get method of the Proxy’s handler, we check whether what we’re returning is of type object (an array, object, etc). If it is, we wrap it in a new Proxy using the same handler as itself.

This is to ensure that all changes go through this Proxy, and the console.log statement runs for every change.

If we didn’t wrap objects returned from the get method in another Proxy, this kind of “deeper” change would not result in a console.log:

// continuing example from above

object.list = [] // "A change was made!"
object.list.push("foo") // no console.log statement

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