[Javascript] Simplify Creating Immutable Data Trees With Immer

Immer is a tiny library that makes it possible to work with immutable data in JavaScript in a much more straight-forward way by operating on a temporarily draft state and using all the normal JavaScript API's and data structures. The first part of the lesson explains how to use Immer, and the second part of the lesson shows you how it can be used to simplify, for example, Redux reducers.

Immer has a few unique features:

• Supports arbitrary, deep modifications in immutable data trees
• Strongly typed
• Uses JavaScript native data structures, no new API to learn
• Automatically freezes any data that is produced

The whole point of Immer is that you can wrap you reducer function with the function immer provide, let's call 'produce'.  When you call it, if you don't do anything, it just return your previous state, if you do any partial mutation, if will keep original object untouched, instead creating immutable version return to you. therefore make code much simpler.

// Before

const byId = (state, action) => {
  switch (action.type) {
    case RECEIVE_PRODUCTS:
      return {
        ...state,
        ...action.products.reduce((obj, product) => {
          obj[product.id] = product
          return obj
        }, {})
      }
    default:
      return state
  }
}

// After

import produce from 'immer'

const byId = (state, action) =>
  produce(state, draft => {
    switch (action.type) {
      case RECEIVE_PRODUCTS:
        action.products.forEach(product => {
          draft[product.id] = product
        })
    }
  })

Or an improved version:

import produce from 'immer'

const byId = produce((draft, action) => {
  switch (action.type) {
    case RECEIVE_PRODUCTS:
    action.products.forEach(product => {
        draft[product.id] = product
    })
  }
})