Update patterns

Working with immutable data, before Immer, used to mean learning all the immutable update patterns.

To help 'unlearning' those patterns here is an overview how you can leverage the built-in JavaScript APIs to update objects and collections:

Object mutations

import {produce} from "immer"

const todosObj = {
    id1: {done: false, body: "Take out the trash"},
    id2: {done: false, body: "Check Email"}
}

// add
const addedTodosObj = produce(todosObj, draft => {
    draft["id3"] = {done: false, body: "Buy bananas"}
})

// delete
const deletedTodosObj = produce(todosObj, draft => {
    delete draft["id1"]
})

// update
const updatedTodosObj = produce(todosObj, draft => {
    draft["id1"].done = true
})

Array mutations

import {produce} from "immer"

const todosArray = [
    {id: "id1", done: false, body: "Take out the trash"},
    {id: "id2", done: false, body: "Check Email"}
]

// add
const addedTodosArray = produce(todosArray, draft => {
    draft.push({id: "id3", done: false, body: "Buy bananas"})
})

// delete by index
const deletedTodosArray = produce(todosArray, draft => {
    draft.splice(3 /*the index */, 1)
})

// update by index
const updatedTodosArray = produce(todosArray, draft => {
    draft[3].done = true
})

// insert at index
const updatedTodosArray = produce(todosArray, draft => {
    draft.splice(3, 0, {id: "id3", done: false, body: "Buy bananas"})
})

// remove last item
const updatedTodosArray = produce(todosArray, draft => {
    draft.pop()
})

// remove first item
const updatedTodosArray = produce(todosArray, draft => {
    draft.shift()
})

// add item at the beginning of the array
const addedTodosArray = produce(todosArray, draft => {
    draft.unshift({id: "id3", done: false, body: "Buy bananas"})
})

// delete by id
const deletedTodosArray = produce(todosArray, draft => {
    const index = draft.findIndex(todo => todo.id === "id1")
    if (index !== -1) draft.splice(index, 1)
})

// update by id
const updatedTodosArray = produce(todosArray, draft => {
    const index = draft.findIndex(todo => todo.id === "id1")
    if (index !== -1) draft[index].done = true
})

// filtering items
const updatedTodosArray = produce(todosArray, draft => {
    // creating a new state is simpler in this example
    // (note that we don't need produce in this case,
    // but as shown below, if the filter is not on the top
    // level produce is still pretty useful)
    return draft.filter(todo => todo.done)
})

Nested data structures

import {produce} from "immer"

// example complex data structure
const store = {
    users: new Map([
        [
            "17",
            {
                name: "Michel",
                todos: [
                    {
                        title: "Get coffee",
                        done: false
                    }
                ]
            }
        ]
    ])
}

// updating something deeply in-an-object-in-an-array-in-a-map-in-an-object:
const nextStore = produce(store, draft => {
    draft.users.get("17").todos[0].done = true
})

// filtering out all unfinished todo's
const nextStore = produce(store, draft => {
    const user = draft.users.get("17")
    // when filtering, creating a fresh collection is simpler than
    // removing irrelevant items
    user.todos = user.todos.filter(todo => todo.done)
})

Note that many array operations can be used to insert multiple items at once by passing multiple arguments or using the spread operation: todos.unshift(...items).

Note that when working with arrays that contain objects that are typically identified by some id, we recommend to use Map or index based objects (as shown above) instead of performing frequent find operations, lookup tables perform much better in general.