mutate-using-immer.js

import produce from 'immer'

const state = {
  activeUserId: 123,
  activeSessionIndex: 0,
  sessions: {
    123: [
      {
        products: [{ id: 456, name: 'iPhone' }]
      }
    ]
  }
}

function log(data) {
  console.log(JSON.stringify(data, null, 2))
}

function addItem(currentState, item) {
  return produce(currentState, s => {
    s.sessions[s.activeUserId][s.activeSessionIndex].products.push(item)
  })
}

log(state)
log(addItem(state, { id: 789, name: 'iPad' }))

/*
{
  "activeUserId": 123,
  "activeSessionIndex": 0,
  "sessions": {
    "123": [
      {
        "products": [
          {
            "id": 456,
            "name": "iPhone"
          }
        ]
      }
    ]
  }
}
index.js:28 {
  "activeUserId": 123,
  "activeSessionIndex": 0,
  "sessions": {
    "123": [
      {
        "products": [
          {
            "id": 456,
            "name": "iPhone"
          },
          {
            "id": 789,
            "name": "iPad"
          }
        ]
      }
    ]
  }
}
*/