immer
Last updated
Last updated
Create the next immutable state tree by simply modifying the current tree
Did Immer make a difference to your project? Consider buying me a coffee!
NPM: npm install immer
Yarn: yarn add immer
CDN: Exposed global is immer
Unpkg: <script src="https://unpkg.com/immer/dist/immer.umd.js"></script>
JSDelivr: <script src="https://cdn.jsdelivr.net/npm/immer/dist/immer.umd.js"></script>
Egghead lesson covering all of immer (7m): Simplify creating immutable data trees with Immer
Introduction blogpost: Immer: Immutability the easy way
Immer (German for: always) is a tiny package that allows you to work with immutable state in a more convenient way. It is based on the copy-on-write mechanism.
The basic idea is that you will apply all your changes to a temporarily draftState, which is a proxy of the currentState. Once all your mutations are completed, Immer will produce the nextState based on the mutations to the draft state. This means that you can interact with your data by simply modifying it while keeping all the benefits of immutable data.
Using Immer is like having a personal assistant; he takes a letter (the current state) and gives you a copy (draft) to jot changes onto. Once you are done, the assistant will take your draft and produce the real immutable, final letter for you (the next state).
A mindful reader might notice that this is quite similar to withMutations
of ImmutableJS. It is indeed, but generalized and applied to plain, native JavaScript data structures (arrays and objects) without further needing any library.
Blog: by Workday Prism on why they picked Immer to manage immutable state The Search for a Strongly-Typed, Immutable State
Video tutorial: Using Immer with React.setState
The Immer package exposes a default function that does all the work.
produce(currentState, producer: (draftState) => void): nextState
There is also a curried overload that is explained below.
The interesting thing about Immer is that the baseState
will be untouched, but the nextState
will reflect all changes made to draftState
.
Immutability with normal JavaScript objects and arrays. No new APIs to learn!
Strongly typed, no string based paths selectors etc.
Structural sharing out of the box
Object freezing out of the box
Deep updates are a breeze
Boilerplate reduction. Less noise, more concise code.
Small: bundled and minified: 2KB.
Read further to see all these benefits explained.
Here is a simple example of the difference that Immer could make in practice.
After using Immer, that simply becomes:
Notice that it is not needed to handle the default case, a producer that doesn't do anything will simply return the original state.
Creating Redux reducer is just a sample application of the Immer package. Immer is not just designed to simplify Redux reducers. It can be used in any context where you have an immutable data tree that you want to clone and modify (with structural sharing).
Note: it might be tempting after using producers for a while, to just place produce
in your root reducer and then pass the draft to each reducer and work directly over such draft. Don't do that. It kills the point of Redux where each reducer is testable as pure reducer. Immer is best used when applying it to small individual pieces of logic.
Deep updates in the state of React components can be greatly simplified as well by using immer. Take for example the following onClick handlers (Try in codesandbox):
Passing a function as the first argument to produce
is intended to be used for currying. This means that you get a pre-bound producer that only needs a state to produce the value from. The producer function gets passed in the draft and any further arguments that were passed to the curried function.
For example:
This mechanism can also nicely be leveraged to further simplify our example reducer:
Note that state
is now factored out (the created reducer will accept a state, and invoke the bound producer with it).
If you want to initialize an uninitialized state using this construction, you can do so by passing the initial state as second argument to produce
:
Fun with currying
A random fun example just for inspiration: a neat trick is to turn Object.assign
into a producer to create a "spread" function that is smarter than the normal spread operator, as it doesn't produce a new state if the result doesn't actually change (details & explanation). Quick example:
During the run of a producer, Immer can record all the patches that would replay the changes made by the reducer. This is a very powerful tool if you want to fork your state temporarily and replay the changes to the original.
Patches are useful in few scenarios:
To exchange incremental updates with other parties, for example over websockets
For debugging / traces, to see precisely how state is changed over time
As basis for undo/redo or as an approach to replay changes on a slightly different state tree
To help with replaying patches, applyPatches
comes in handy. Here is an example how patches could be used to record the incremental updates and (inverse) apply them:
The generated patches are similar (but not the same) to the RFC-6902 JSON patch standard, except that the path
property is an array, rather than a string. This makes processing patches easier. If you want to normalize to the official specification, patch.path = patch.path.join("/")
should do the trick. Anyway, this is what a bunch of patches and their inverse could look like:
For a more in-depth study, see Distributing patches and rebasing actions using Immer
Tip: Check this trick to compress patches produced over time.
Immer automatically freezes any state trees that are modified using produce
. This protects against accidental modifications of the state tree outside of a producer. This comes with a performance impact, so it is recommended to disable this option in production. It is by default enabled. By default, it is turned on during local development and turned off in production. Use setAutoFreeze(true / false)
to explicitly turn this feature on or off.
It is not needed to return anything from a producer, as Immer will return the (finalized) version of the draft
anyway. However, it is allowed to just return draft
.
It is also allowed to return arbitrarily other data from the producer function. But only if you didn't modify the draft. This can be useful to produce an entirely new state. Some examples:
Note: It is not possible to return undefined
this way, as it is indistinguishable from not updating the draft! Read on...
undefined
using nothing
So, in general, one can replace the current state by just return
ing a new value from the producer, rather than modifying the draft. There is a subtle edge case however: if you try to write a producer that wants to replace the current state with undefined
:
Versus:
The problem is that in JavaScript a function that doesn't return anything also returns undefined
! So immer cannot differentiate between those different cases. So, by default, Immer will assume that any producer that returns undefined
just tried to modify the draft.
However, to make it clear to Immer that you intentionally want to produce the value undefined
, you can return the built-in token nothing
:
N.B. Note that this problem is specific for the undefined
value, any other value, including null
, doesn't suffer from this issue.
Immer exposes a named export original
that will get the original object from the proxied instance inside produce
(or return undefined
for unproxied values). A good example of when this can be useful is when searching for nodes in a tree-like state using strict equality.
Just want to know if a value is a proxied instance? Use the isDraft
function!
this
The recipe will be always invoked with the draft
as this
context.
This means that the following constructions are also valid:
void
Draft mutations in Immer usually warrant a code block, since a return denotes an overwrite. Sometimes that can stretch code a little more than you might be comfortable with.
In such cases, you can use javascripts void
operator, which evaluates expressions and returns undefined
.
Code style is highly personal, but for code bases that are to be understood by many, we recommend to stick to the classic draft => { draft.user.age += 1}
to avoid cognitive overhead.
The Immer package ships with type definitions inside the package, which should be picked up by TypeScript and Flow out of the box and without further configuration.
The TypeScript typings automatically remove readonly
modifiers from your draft types and return a value that matches your original type. See this practical example:
This ensures that the only place you can modify your state is in your produce callbacks. It even works recursively and with ReadonlyArray
s!
Note: Immer v1.9+ supports Typescript v3.1+ only.
By default produce
tries to use proxies for optimal performance. However, on older JavaScript engines Proxy
is not available. For example, when running Microsoft Internet Explorer or React Native on Android. In such cases, Immer will fallback to an ES5 compatible implementation which works identical, but is a bit slower.
produce
is exposed as the default export, but optionally it can be used as name import as well, as this benefits some older project setups. So the following imports are all correct, where the first is recommended:
Immer supports the following types of data:
All kinds of primitive values
Date
instances, but: only if not mutated, see below
Arrays, but: non-standard attributes are not supported (like: array.test = "Test"
)
Plain objects (objects that have as prototype either null
or Object
)
Functions, assuming they aren't mutated
Other value types (like class instances) can be stored in the tree, but note that Immer won't work inside those objects. In other words, if you modify a class instance, this will not result in clone and unmodified original, but just in a modified original.
Don't redefine draft like, draft = myCoolNewState
. Instead, either modify the draft
or return a new state. See Returning data from producers.
Immer assumes your state to be a unidirectional tree. That is, no object should appear twice in the tree, and there should be no circular references.
Class instances are not, and will not have first-class support. Read here why classes are a conceptual mismatch (and technically extremely challenging)
For example, working with Date
objects is no problem, just make sure you never modify them (by using methods like setYear
on an existing instance). Instead, always create fresh Date
instances. Which is probably what you were unconsciously doing already.
Since Immer uses proxies, reading huge amounts of data from state comes with an overhead (especially in the ES5 implementation). If this ever becomes an issue (measure before you optimize!), do the current state analysis before entering the producer function or read from the currentState
rather than the draftState
. Also, realize that immer is opt-in everywhere, so it is perfectly fine to manually write super performance critical reducers, and use immer for all the normal ones. Also note that original
can be used to get the original state of an object, which is cheaper to read.
Some debuggers (at least Node 6 is known) have trouble debugging when Proxies are in play. Node 8 is known to work correctly.
Always try to pull produce
'up', for example for (let x of y) produce(base, d => d.push(x))
is exponentially slower than produce(base, d => { for (let x of y) d.push(x)})
It is possible to return values from producers, except, it is not possible to return undefined
that way, as it is indistinguishable from not updating the draft at all! If you want to replace the draft with undefined
, just return nothing
from the producer.
Immer does not support built-in data-structures like Map
and Set
. However, it is fine to just immutably "update" them yourself but still leverage immer wherever possible:
Or a deep update in maps (well, don't use maps for this use case, but as an example):
react-copy-write Immutable state with a mutable API
redux-starter-kit A simple set of tools to make using Redux easier
immer based handleActions Boilerplate free actions for Redux
redux-box Modular and easy-to-grasp redux based state management, with least boilerplate
quick-redux tools to make redux development quicker and easier
bey Simple immutable state for React using Immer
immer-wieder State management lib that combines React 16 Context and immer for Redux semantics
robodux flexible way to reduce redux boilerplate
immer-reducer Type-safe and terse Redux reducers with Typescript
... and many more
Read the (second part of the) introduction blog.
For those who have to go back to thinking in object updates :-)
Here is a simple benchmark on the performance of Immer. This test takes 50,000 todo items and updates 5,000 of them. Freeze indicates that the state tree has been frozen after producing it. This is a development best practice, as it prevents developers from accidentally modifying the state tree.
These tests were executed on Node 9.3.0. Use yarn test:perf
to reproduce them locally.
Most important observation:
Immer with proxies is roughly speaking twice to three times slower as a handwritten reducer (the above test case is worst case, see yarn test:perf
for more tests). This is in practice negligible.
Immer is roughly as fast as ImmutableJS. However, the immutableJS + toJS makes clear the cost that often needs to be paid later; converting the immutableJS objects back to plain objects, to be able to pass them to components, over the network etc... (And there is also the upfront cost of converting data received from e.g. the server to immutable JS)
Generating patches doesn't significantly slow immer down
The ES5 fallback implementation is roughly twice as slow as the proxy implementation, in some cases worse.
(for those who skimmed the above instead of actually reading)
Q: Does Immer use structural sharing? So that my selectors can be memoized and such?
A: Yes
Q: Does Immer support deep updates?
A: Yes
Q: I can't rely on Proxies being present on my target environments. Can I use Immer?
A: Yes
Q: Can I typecheck my data structures when using Immer?
A: Yes
Q: Can I store Date
objects, functions etc in my state tree when using Immer?
A: Yes
Q: Is it fast?
A: Yes
Q: Idea! Can Immer freeze the state for me?
A: Yes
Special thanks to @Mendix, which supports its employees to experiment completely freely two full days a month, which formed the kick-start for this project.
A significant part of my OSS work is unpaid. So donations are greatly appreciated :)