We make use of state to keep track of application data. States change as users interact with an application. When this happens, we need to update the state that is displayed to the user, and we do this using React’s setState.

Since states are not meant to be updated directly (because React’s state has to be immutable), things can get really complicated as states become more complex. They become difficult to understand and follow.

This is where Immer comes in and that’s what we’re going to look at in this post. Using Immer, states can be simplified and much easier to follow. Immer makes use of something called “draft” which you can think of as the copy of your state, but not the state itself. It’s as though Immer hit CMD+C on the state and then cmd+V’d it somewhere else where it can be safely viewed without disturbing the original copy. Any updates you need to make happen on the draft, and the parts of the current state that change on the draft is updated.

Let’s say your application’s state looks like this;

this.state = { name: 'Kunle', age: 30, city: 'Lagos, country: 'Nigeria'

This user happens to be celebrating his 31st birthday and which means we need to update the age value. With Immer running behind the scenes, a replica of this state will be made.

Now imagine the replica is made and handed over to a messenger, who gives the newly copied version of the state to Kunle. It means there are now two copies available — the current state and the draft copy that was handed over. Kunle then changes the age on the draft to 31. The messenger then returns to the application with the draft, compares both versions, and only updates the age since that’s the only part of the draft that changed.

It does not break the idea of an immutable state, as the current state does not get updated directly. Immer basically makes it convenient to work with immutable state.

Let’s look at an example of this at work

Say you want to build a traffic light for your community, you can give it a shot using Immer for your state updates.

See the Pen
Traffic Light Example with Reactjs
by CarterTsai (@CarterTsai)
on CodePen.

Using Immer, the component will look like this:

const {produce} = immer class App extends React.Component { state = { red: 'red', yellow: 'black', green: 'black', next: "yellow" } componentDidMount() { this.interval = setInterval(() => this.changeHandle(), 3000); } componentWillUnmount() { clearInterval(this.interval); } handleRedLight = () => { this.setState( produce(draft => { draft.red = 'red'; draft.yellow = 'black'; draft.green = 'black'; draft.next = 'yellow' }) ) } handleYellowLight = () => { this.setState( produce(draft => { draft.red = 'black'; draft.yellow = 'yellow'; draft.green = 'black'; draft.next = 'green' }) ) } handleGreenLight = () => { this.setState( produce(draft => { draft.red = 'black'; draft.yellow = 'black'; draft.green = 'green'; draft.next = 'red' }) ) } changeHandle = () => { if (this.state.next === 'yellow') { this.handleYellowLight() } else if (this.state.next === 'green') { this.handleGreenLight() } else { this.handleRedLight() } } render() { return ( 
</div> ); } };

produce is the default function we get from Immer. Here, we pass it as a value to the setState() method. The produce function takes a function which accepts draft as an argument. It is inside this function that we can then set the draft copy with which we want to update our state.

If that looks complicated, there is another way to write this. First, we create a function.

const handleLight = (state) => { return produce(state, (draft) => { draft.red = 'black'; draft.yellow = 'black'; draft.green = 'green'; draft.next = 'red' });

We are passing the current state of the application, and the function which accepts draft as arguments to the produce function. To make use of this inside our component, we do this;

handleGreenLight = () => { const nextState = handleLight(this.state) this.setState(nextState)

Another example: A shopping list

If you have been working with React for a while now, then you’re not a stranger to the spread operator. With Immer, you need not make use of the spread operator, especially when working with an array in your state.

Let’s explore that a little further by creating a shopping list application.

See the Pen
immer 2 – shopping list
by Kingsley Silas Chijioke (@kinsomicrote)
on CodePen.

Here’s the component we’re working with:

class App extends React.Component { constructor(props) { super(props) this.state = { item: "", price: 0, list: [ { id: 1, name: "Cereals", price: 12 }, { id: 2, name: "Rice", price: 10 } ] } } handleInputChange = e => { this.setState( produce(draft => { draft[event.target.name] = event.target.value })) } handleSubmit = (e) => { e.preventDefault() const newItem = { id: uuid.v4(), name: this.state.name, price: this.state.price } this.setState( produce(draft => { draft.list = draft.list.concat(newItem) }) ) }; render() { return ( <React.Fragment> <section className="section"> 

Create your shopping list

<button className="button is-grey">Submit</button> </form> </div>
{ this.state.list.length ? ( this.state.list.map(item => (
  • {item.name}


)) ) :

Your list is empty

</section> </React.Fragment> ) } } ReactDOM.render( <App />, document.getElementById('root') );

As items are added to the list, we need to update the state of the list to reflect those new items. To update the state of list using setState(), we’ll have to do this:

handleSubmit = (e) => { e.preventDefault() const newItem = { id: uuid.v4(), name: this.state.name, price: this.state.price } this.setState({ list: [...this.state.list, newItem] })

If you have to update multiple states in the application, you’ll have to do a ton of spreading to create a new state using the old state and the additional value. Which can look more complex as the number of changes increases. With Immer, it becomes very easy to do that, as we did in the example above.

What if we want to add a function that gets called as a callback after the state update?In this case, let’s say we are keeping a tally of the number of items in the list and the total price of all the items.

See the Pen
immer 3 – shopping list
by Kingsley Silas Chijioke (@kinsomicrote)
on CodePen.


Say we want to calculate the amount that will be spent based on the price of items in the list, we can have the handleSubmit function look like this:

handleSubmit = (e) => { e.preventDefault() const newItem = { id: uuid.v4(), name: this.state.name, price: this.state.price } this.setState( produce(draft => { draft.list = draft.list.concat(newItem) }), () => { this.calculateAmount(this.state.list) } )

First, we create an object using the data entered by the user, which we then assign to newItem. To update our application’s state, we make use of .concat() which will return a new array that’s comprised of the previous items and the new item. This updated copy is now set as the value of draft.list, which can then be used by Immer to update the state of the application.

The callback function gets called after the state update. It’s important to note that it makes use of the updated state.

The function we want to call will look like this:

calculateAmount = (list) => { let total = 0; for (let i = 0; i < list.length; i++) { total += parseInt(list[i].price, 10) } this.setState( produce(draft => { draft.totalAmount = total }) )

Let’s look at Immer hooks

use-immer is a hook that allows you to manage state in your React application. Let’s see this in action using a classic counter example.

import React from "react";
import {useImmer} from "use-immer"; const Counter = () => { const [count, updateCounter] = useImmer({ value: 0 }); function increment() { updateCounter(draft => { draft.value = draft.value +1; }); } return ( 

Counter {count.value}

); } export default Counter;

useImmer is similar to useState. The function returns the state and an updater function. When the component loads at first, the value of the state (which is count in this example), is the same as the value passed to useImmer. Using the updater function which is returned, we can then create an increment function to increase the value of the count.

There is also a useReducer-like hook for Immer.

import React, { useRef } from "react";
import {useImmerReducer } from "use-immer";
import uuidv4 from "uuid/v4"
const initialState = [];
const reducer = (draft, action) => { switch (action.type) { case "ADD_ITEM": draft.push(action.item); return; case "CLEAR_LIST": return initialState; default: return draft; }
const Todo = () => { const inputEl = useRef(null); const [state, dispatch] = useImmerReducer(reducer, initialState); const handleSubmit = (e) => { e.preventDefault() const newItem = { id: uuidv4(), text: inputEl.current.value }; dispatch({ type: "ADD_ITEM", item: newItem }); inputEl.current.value = ""; inputEl.current.focus(); } const handleClear = () => { dispatch({ type: 'CLEAR_LIST' }) } return ( 
    {state.map(todo => { return
  • {todo.text}
  • ; })}
); } export default Todo;

useImmerReducer takes in a reducer function and the initial state, and it returns both state and the dispatch function. We can then loop through the state to display the items we have. We dispatch an action when submitting a todo item and clearing the list of them. The dispatched action has a type which we use in determining what to do in the reducer function.
In the reducer function, we make use of draft like we did before, instead of state. With that, we have a convenient way of manipulating the state of our application.

You can find the code used in the above example on GitHub.

That’s a look at Immer!

Going forward, you can begin to make use of Immer in your next project, or even slowly begin to use it in the current project you’re working on. It has proven to aid in making state management convenient.

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