Object.prototype.watch has been deprecated

Published: | Categories: JavaScript


The non-standard Object.prototype.watch and unwatch methods are now considered deprecated and will be removed in the near future. Firefox 57 and later shows a warning in the Console for these methods that are not supported by any other browsers. Use the standard Proxy or Reflect object instead.

The following example is a simple replacement of Object.prototype.watch by Proxy:

const o = { a: 1 };

// Don't do this
o.watch('a', (prop, oldval, newval) => {
  console.log(`o.${prop} has been changed from ${oldval} to ${newval}`);
  return newval;

o.a = 2; // o.a has been changed from 1 to 2

// Do this
const p = new Proxy(o, {
  set: (obj, prop, newval) => {
    const oldval = obj[prop];
    obj[prop] = newval;
    console.log(`p.${prop} has been changed from ${oldval} to ${newval}`);
    return true;

p.a = 3; // p.a has been changed from 2 to 3

Update: These methods have been removed with Firefox 58.