Mutate a copy of data without changing the original source

This is a drop in replacement for react-addons-update:

// import update from 'react-addons-update';
import update from 'immutability-helper';

const state1 = ['x'];
const state2 = update(state1, {$push: ['y']}); // ['x', 'y']

The main difference this has with react-addons-update is that you can extend this to give it more functionality:

update.extend('$addtax', function(tax, original) {
  return original + (tax * original);
});
const state = { price: 123 };
const withTax = update(state, {
  price: {$addtax: 0.8},
});
assert(JSON.stringify(withTax) === JSON.stringify({ price: 221.4 });

Note that original in the function above is the original object, so if you plan making a mutation, you must first shallow clone the object. Another option is to use update to make the change return update(original, { foo: {$set: 'bar'} })

If you don't want to mess around with the globally exported update function you can make a copy and work with that copy:

import { newContext } from 'immutability-helper';
const myUpdate = newContext();
myUpdate.extend('$foo', function(value, original) {
  return 'foo!';
});

react-addons-update has react as a peer dependency. Also Facebook is deprecating react-addons-update 😦