Various programming stuff

Easy immutable objects in Javascript

With the rise of Redux and other similar Javascript frameworks (e.g Hyperapp) that try to be a little more functional (functional as in functional programming), a new problem was introduced to Javascript programmers (at least to those that weren’t familair with functional programming): How to keep their application’s state “immutable”.

Immutable means that the state should be an object that does not change (mutates) - instead of changing it, Redux needs the state object to be created from the beginning.

So when something happens in your application you need to discard the existing state object and create a new one from scratch by modifying and copying the previous state values. This is easy in most toy-apps that are used when introducing the concept, for example if your state is {counter: 0} then you could just define the reducer for the ADD action (i.e when the user clicks the + button) like this:

let reducer = (state={counter: 0}, action) => {
  switch (action.type) {
    case 'ADD': return { 'counter': state.counter+1 }
    default: return state

Unfortunately, your application will definitely have a much more complex state than this!

In the following, I’ll do a quick introduction on how to keep your state objects immutable using modern Javascript techniques, I’ll present how complex it is to modify non-trivial immutable objects and finally I’ll give you a quick recipe for modifying your non-trivial immutable objects. If you want to play with the concepts I’ll introduce you can do it at a playground I’ve created on repl.it.

Please keep in mind that this article has been written for ES6 - take a look at my browserify with ES6 article to see how you can also use it in your projects with Browserify.

Also, if you’d like to see a non-toy React + Redux application or you’d like a gentle introduction to the concepts I talked about (state, reducers, actions etc) you can follow along my React Redux tutorial. This is a rather old article (considering how quickly the Javascript framework state change) but the basic concepts introduced there are true today.

Immutable objects

Let’s start our descent into avoiding mutations by supposing that you had something a little more complex than the initial example, for example your state was like this:

let state = {
    'counter': 0,
    'name': 'John',
    'age': 36

If you continued the same example then your ADD reducer would need to return something like this

return {
    'counter': state.counter+1,
    'name': state.name,
    'age': state.age

This gets difficult and error prone very soon - and what happens if you later need to add another attribute to your state?

The correct way to implement this would be to enumerate all properties of state except ‘counter’, copy them to a new object, and then assign counter+1 to the new object’s counter attribute. You could implement this by hand however, thankfully, there’s the Object.assign method! This method will copy all attributes from a list of objects to an object which will return as a result and is defined like this:

Object.assign(target, ...sources)

The target parameter is the object that will retrieve all attributes from sources (which is a variadic argument - you can have as many sources as you want - even 0; in this case the target will be returned). For a quick example, running:

let o = {'a': 1}
let oo = Object.assign(o, {'b': 2, 'c': 1}, {'c': 3})
console.log(oo, o===oo)

will return { a: 1, b: 2, c: 3 } true i.e the attributes ‘b’ and ‘c’ were copied to o and it was assigned to oo — notice that o and oo are the same object (thus o is modified now). Also, notice that the the attributes of objects to the right have priority over the attributes of the objects to the left ('c': 1 was overriden by 'c': 3).

As you should have guessed by now, you should never pass the state as the target but instead you should create a new object, thus the ADD reducer should return the following:

return Object.assign({}, state, {'counter': state.counter+1)

This means that it will create a new object which will copy all current attributes of state and increase the existing counter attribute.

I’d like to also add here that instead of using the Object.assign method you could use the spread syntax to more or less do the same. The spread syntax on an object takes this object’s attributes and outputs them as key-value dictionary pairs (for them to be used to initialize other objects). Thus, you can use the spread syntax to create an new object that has the same attributes of another object like this:

let newState = {...state}
// which is similar to
newState = Object.assign({}, state)

Of course you usually need to override some attributes, which can be passed directly to the newly created object, for example for the ADD reducer:

return {...state, 'counter': state.counter+1 }

Like Object.assign, you can have as many sources as you want in your spread syntax thus nothing stops you from using ... multiple times to copy the attributes of multiple objects for example you could define ADD like this:

return {...state, ...{'counter': state.counter+1 } }

The order is similar to Object.assign, i.e the attributes that follow will override the previous ones.

One final comment is that both Object.assign and copying objects with the spread syntax will do a “shallow” copy i.e it will copy only the outer object, not the objects its keys refer to. An example of this behavior is that if you run the following:

let a = {'val': 3 }
let x = {a }
let y = {...x}
console.log(x, y)
x['val2'] = 4
y['val2'] = 5
a['val'] = 33
console.log(x, y)

you’ll get:

{ a: { val: 3 } } { a: { val: 3 } }
{ a: { val: 33 }, val2: 4 } { a: { val: 33 }, val2: 5 }

i.e x and y got a different val2 attribute since they not the same object, however both x and y have a reference to the same a thus when it’s val attribute was changed this change appears to both x and y!

What the above means is that if you have a state object containing other objects (or arrays) you will also need to copy these children objects to keep your state immutable. We’ll see examples on this later.

Immutable arrays

One thing we haven’t talked about yet is what happens if there’s an array in the state, for example your state is let state=[] and you have and APPEND reducer that puts something in the end of that array. The naive (and wrong) way to do it is to call push directly to the state - this will mutate your state and is not be allowed!

You need to copy the array elements and the tool for this job is Array.slice. This methods takes two optional arguments (begin and end) that define the range of elements that will be copied; if you call it without arguments then it will copy the whole array. Using slice, your APPEND reducer can be like this:

let newState = state.slice()
newState.push('new element')
return newState

Also, you could use the Array.concat method which will return a new array by copying all the elements of its arguments

return state.concat(['new element'])

This will append new element to a new object that will have the elements of state (it won’t modify the existing state) and is easier if you have this exact requirement. The advantage of slice is that you can use it to add/remove/modify elements from any place in the original array. For example, here’s how you can add an element after the first element of an array:

let x = ['a', 'b', 'c' ]
let y = x.slice(0,1).concat(['second' ], x.slice(1,3))

Now y will be equal to [ 'a', 'second', 'b', 'c' ]. So the above will get the first (0-th) element from the x array and concat it with another element (second) and the remaining elements of x. Remember that x is not modifyied since concat will create a new array.

In a similar fashion to objects, instead of using concat it is much easier to use the spread syntax. The spread syntax for an array will output its elements one after the other for them to be used by other arrays. Thus, continuing from the previous example, [...x] will return a new array with the elements of x (so it is similar to x.slice() or x.concat()), thus to re-generate the previous example you’ll do something like

let y = y=[...x.slice(0,1), 'second', ...x.slice(1,3)]

All three of concat, slice or the spread syntax will do a shallow copy (similar to how Object.assign works) so the same conclusions from the previous section are true here: If you have arrays inside other arrays (or objects) you’ll need to copy the inner arrays recursively.

More complex cases

We’ll now take a look at some more complex cases and see how quickly it gets difficult because of the shallow copying. Let’s suppose that our state is the following:

const state = {
  'user': {
    'first_name': 'John',
    'last_name': 'Doe',
    'address': {
      'city': 'Athens',
      'country': 'Greece',
      'zip': '12345'

and we want to assign a group attribute to the state. This can be easily done with assign:

let groups = [{
    'name': 'group1'

state = Object.assign({}, state, {
  'groups': groups

or spread:

state = {
  ...state, 'groups': groups

Notice that instead of 'groups': groups I could have used the shorthand syntax and written only groups and it would still work (i.e state = {...state, groups} is the same). In all cases, the resulting state will be:

  'user': {
    'first_name': 'John',
    'last_name': 'Doe',
    'address': {
      'city': 'Athens',
      'country': 'Greece',
      'zip': '12345'
  'groups': [{
    'name': 'group1'

From now on I’ll only use the spread syntax which is more compact.

Let’s try to change the user’s name. This is not as easy as the first example because we need to:

  • Create a new copy of the user object with the new first name
  • Create a new copy of the state object with the new user object created above

This can be done in two steps like this:

let user ={...state['user'], 'first_name': 'Jack'}
state = {...state, user}

or in one step like this:

state = {...state, 'user':{
  ...state['user'], 'first_name': 'Jack'}

The single step assignment is the combination of the two step described above. It is a little more complex but it saves typing and is prefered because it allows the reducer function to have a single expression. Now let’s try to modify the user’s zip code. We’ll do it in three steps first:

let address ={...state['user']['address'], 'zip': '54321'}
user ={...state['user'], address}
state = {...state, user}

And now in one:

state = {...state, 'user': {
  ...state['user'], 'address': {
    ...state['user']['address'], 'zip': 54321

Now, as can be seen in the above examples, modifying (without mutating) a compex state object is not very easy - it needs much thinking and is too error prone! This will be even more apparent when we also get the array modifications into the equation, for example by adding another two groups:

state = {
  ...state, groups: [
    {name: 'group2', id: 2},
    {name: 'group3', id: 3}

The above copies the existing state and assigns to it a new groups object by copying the existing groups and appending two more groups to that array! The state now will be:

  user: {
    first_name: 'Jack',
    last_name: 'Doe',
    address: { city: 'Athens', country: 'Greece', zip: 54321 }
  groups: [
    { name: 'group1' },
    { name: 'group2', id: 2 },
    { name: 'group3', id: 3 }

As a final examply, how can we add the missing id attribute to the first group? Following the above techniques:

state = {
  ...state, groups: [
    {...state['groups'][0], 'id': 1},

One more time what the above does?

  • Creates a new object and copies all existing properties of state to it
  • Creates a new array which assigns it to the new state’s groups attribute
  • For the first element of that array it copies all attributes of the first element of state[‘groups’] and assings it an id=1 attribute
  • For the remaining elements of that array it copies all elements of state[‘groups] after the first one

Now think what would happen if we had an even more complex state with 3 or 4 nested levels!

Immutability’s little helpers

As you’ve seen from the previous examples, using immutable objects is not as easy as seems from the toy examples. Actually, drilling down into complex immutable objects and returning new ones that have some values changed is a well-known problem in the functional world and has already a solution called “lenses”. This is a funny name but it more or less means that you use a magnifying lens to look at exactly the value you want and modify or retrieve it. The problem with lenses is that although they solve the problem I mention is that if you want to use them you’ll need to dive deep into functional programming and also you’ll need to include an extra library to your project (even if you only want this specific capability).

For completeness, here’s the the docs on lens from Ramda which is a well known Javascript functional library. This needs you to understand what is prop, what is assoc and then how to use the lens with view, set and over. For me, these are way too much things to remember for such a specific thing. Also, notice that the minified version of Ramda is around 45 kb which is not small. Yes, if I wanted to fully use Ramda or a similar library I’d be delighted to use all these techniques and include it as a dependency - however most people prefer to stick with more familiar (and more procedural) concepts.

The helpers I’m going to present here are more or less a poor man’s lens, i.e you will be able to use the basic functionality of a lens but…

  • without the peculiar syntax and
  • without the need to learn more functional concepts than what you’ll want and
  • without the need to include any more external dependencies

Pretty good deal, no?

In any case, a lens has two parts, a get and a set. The get will be used to drill down and retrieve a value from a complex object while the set will be used to drill down and assign a value to a complex object. The set does not modify the object but returns a new one. The get lens is not really needed since you can easily drill down to an object using the good old index syntax but I’ll include it here for completenes.

We’ll start with the get which seems easier. For this, I’ll just create a function that will take an object and a path inside that object as parameter and retrieve the value at that path. The path could be either a string of the form ‘a.0.c.d’ or an array [‘a’, ‘0’, ‘c’, ‘d’] - for numerical indeces we’ll consider an array at that point.

Thus, for the object {'a': [{'b': {'c': {'d': 32} }}]} when the lens getter is called with either 'a.0.b.c' or [‘a’, 0, ‘b’, ‘c’] as the path, it should return {'d': 32}.

To implement the get helper I will use a functional concept, reduce. I’ve already explained this concept in my previous react-redux tutorial so I urge you to read that article for more info. Using reduce we can apply one by one accumulatively the members of the path to the initial object and the result will be the value of that path. Here’s the implementation of pget (from property get):

const objgetter = (accumulator, currentValue) => accumulator[currentValue];
const pget = (obj, path) =>  (
    (typeof path === 'string' || path instanceof String)?path.split('.'):path
).reduce(objgetter, obj)

I have defined an objgetter reducer function that gets an accumulated object and the current value of the path and just returns the currentValue index of that accumulated object. Finally, for the get lens (named pget) I just check to see if the path is a string or an array (if it’s a string I split it on dots) and then I “reduce” the path using the objgetter defined above and starting by the original object as the initial value. To understand how it is working, let’s try calling it for an object:

const s1 = {'a': [{'b': {'c': {'d': 32} }}]}
console.log(pget(s1, ['a', 0, 'b', 'c']))

The above pget will call reduce on the passed array using the defined objgetter above as the reducer function and s1 as the original object. So, the reducer function will be called with the following values each time:

accumulator currentvalue
s1 'a'
s1['a'] 0
s1['a'][0] 'b'
s1['a'][0]['b'] 'c'

Thus the result will be exactly what we wanted {'d' :32}. An interesting thing is that it’s working fine without the need to differentiate between arrays and objects because of how index access [] works.

Continuing for the set lens (which will be more difficult), I’ll first represent a simple version that works only with objects and an array path but displays the main idea of how this will work: It uses recursion i.e it will call itself to gradually build the new object. Here’s how it is implemented:

const pset0 = (obj, path, val) => {
  let idx = path[0]

  if(path.length==1) {
    return {
      ...obj, [idx]: val
  } else {
    let remaining = path.slice(1)
    return {
      [idx]: pset0(...[obj[idx]], remaining, val)

As already explained, I have assumed that the path is an array of indeces and that the obj is a complex object (no arrays in it please); the function returns a new object with the old object’s value at the path be replaced with val. This function checks to see if the path has only one element, if yes it will assign the value to that attribute of the object it retrieve. If not, it will call itself recursively by skipping the current index and assign the return value to the current index of the curent object. Let’s see how it works for the following call:

const s2 = {a0: 0, a: {b0: 0, b: {c0: 0, c: 3}}}
console.log(pset0(s2, ['a', 'b', 'c'], 4))
# Call Call parameters Return
1 pset0(s2, [‘a’, ‘b’, ‘c’], 4) {…s2, [‘b’]: pset0(s2[‘a’], [‘b’, ‘c’], 4) }
2 pset0(s2[‘a’], [‘b’, ‘c’], 4) {…s2[‘a’], [‘c’]: pset0(s2[‘a’][‘b’], [‘c’], 4) }
3 pset0(s2[‘a’][‘b’], [‘c’], 4) {…s2[‘a’][‘b’], [‘c’]: 4}

Thus, the first time it will be called it will return a new object with the attributes of s2 but overriding its 'b' index with the return of the second call. The second call will return a new object with the attributes of s2['a'] but override it’s 'c' index with the return of the third call. Finally, the 3rd call will return an object with the attributes of s2['a']['b'] and setting the 'c' index to 4. The result will be as expected equal to:

{a0: 0, a: {b0: 0, b: {c0: 0, c: 4 }}}

Now that we’ve understood the logic we can extend the above function with the following extras:

  • support for arrays in the object using numerical indeces
  • support for array (['a', 'b']) or string path ('a.b') parameter
  • support for a direct value to set on the path or a function that will be applied on that value

Here’s the resulting set lens:

const pset = (obj, path, val) => {
  let parts = (typeof path === 'string' || path instanceof String)?path.split('.'):path
  const cset = (obj, cidx, val) => {
    let newval = val
    if (typeof val === "function") {
      newval = val(obj[cidx])
    if(Array.isArray(obj)) {
      return [
        ...obj.slice(0, cidx*1),
    } else {
      return {
        ...obj, [cidx]: newval

  let pidx = parts[0]
  if(parts.length==1) {
    return cset(obj, pidx, val)
  } else {
    let remaining = parts.slice(1)
    return cset(obj, pidx, pset(obj[pidx], remaining, val))

It may seem a little complex but I think it’s easy to be understood: The parts in the beginning will just check to see if the path is an array or a string and split the string to its parts. The cset function that follows is a local function that is used to make the copy of the object or array and set the new value. Here’s how it is working: It will first check to see if the val parameter is a function or a not. If it is a function it will apply this function to the object’s index to get the newvalue else it will just use val as the newvalue. After that it checks if the object it got is an array or not. If it is an array it will do the slice trick we saw before to copy the elements of the array except the newval which will put it at the index (notice that the index at that point must be numerical but that’s up to you to assert). If the current obj is not an array then it must be an object thus it uses the spread syntax to copy the object’s attributes and reassign the current index to newval.

The last part of pset is similar to the pset0 it just uses cset to do the new object/array generation instead of doing it in place like pset0 - as already explained, pset is called recursively until only one element remains on the path in which case the newval will be assigned to the current index of the current obj.

Let’s try to use pset for the following rather complex state:

let state2 = {
  'users': {
    'results': [
      {'name': 'Sera', 'groups': ['g1', 'g2', 'g3']},
      {'name': 'John', 'groups': ['g1', 'g2', 'g3']},
      {'name': 'Joe', 'groups': []}
    'pagination': {
      'total': 100,
      'perpage': 5,
      'number': 0
  'groups': {
    'results': [
    'total': 0

Let’s call it three times one after the other to change various attributes:

let new_state2 = pset(
            pset(state2, "users.results.2.groups.0", 'aa'),
        "users.results.0.name", x=>x.toUpperCase()),
    "users.total", x=>x+1),
'users.results.1.name', 'Jack')

And here’s the result:

    "users": {
        "results": [{
            "name": "SERA",
            "groups": ["g1", "g2", "g3"]
        }, {
            "name": "Jack",
            "groups": ["g1", "g2", "g3"]
        }, {
            "name": "Joe",
            "groups": ["aa"]
        "pagination": {
            "total": 101,
            "perpage": 5,
            "number": 0
    "groups": {
        "results": [],
        "total": 0

This should be self explanatory.

I’ve published the above immutable little helpers as an npm package: https://www.npmjs.com/package/poor-man-lens (yes I decided to use the poor man lens name instead of the immutable little helpers) - they are too simple and could be easily copied and pasted to your project but I’ve seen even smaller npm packages and I wanted to try to see if it is easy to publish a package to npm (answer: it is very easy - easier than python’s pip). Also there’s a github repository for these utils in case somebody wants to contribute anything or look at the source: https://github.com/spapas/poor-man-lens.

Notice that this package has been written in ES5 (and actually has a polyfil for Object.assign) thus you should probably be able to use it anywhere you want, even directly from the browser by directly including the pml.js file.


Using the above techniques you should be able to easily keep your state objects immutable. For simple cases you can stick to the spread syntax or Object.assign / Array.slice but for more complex cases you may want to consider either copying directly the pset and pget utils I explained above or just using the poor-man-lens npm package.