Immer is an NPM package that allows modification of complex Javascript objects and maintains immutability with your application.


When dealing with objects in Javascript, you are often dealing with references to points in memory.

When you need to make a copy of that object, it's easy enough to use const copy = JSON.stringify(JSON.parse(obj)), which makes a deep copy. But it kind of sucks and doesn't handle anything outside of strings, numbers, and booleans and will null everything else aside from Date objects, or have weird behavior like serializing keys of Maps or Sets.

Another alternative is using the spread operator: const copy = { ...obj };. Unfortunately, this only performs a shallow copy, meaning it's still pointing to the same memory references and modifying something in copy may modify the values in obj.

Immer gives the best of both. It allows you to manage this by using a produce function that lets you modify a draft and will return a new object that:

This behavior is particularly useful with React, where modifying state without using the proper hooks or setState call can result in missed updates or faulty recording of the data you want to modify.



const nextState = baseState.slice() // shallow clone the array
nextState[1] = {
    // replace element 1...
    ...nextState[1], // with a shallow clone of element 1
    done: true // ...combined with the desired update
// since nextState was freshly cloned, using push is safe here,
// but doing the same thing at any arbitrary time in the future would
// violate the immutability principles and introduce a bug!
nextState.push({title: "Tweet about it"})


import produce from "immer"

const nextState = produce(baseState, draft => {
    draft[1].done = true
    draft.push({title: "Tweet about it"})


If you want to log something to the console, it is recommended to use the current method from the immer package. Without this, you may end up with a Proxy object, which is not helpful.

import { current, produce } from 'immer';

const updated_ds = produce(ds, working_ds => { = 'new value';
  console.log(working_ds); // returns a `Proxy` object, which gives no useful info
  console.log(current(working_ds)); // returns the actual data



Last modified: 202112060311