A reducer is a function that is able to process our message, our Action. A reducer takes the existing state and applies the message on it. The end result is a new state. A reducer typically operates on a slice of state.

Defining a reducer

A reducer typically looks like this:

function reducer(state = [], action) {
  switch(action.type) {
    case 'CREATE_ITEM':
      return [ ...state, { ... action.payload}];
    case 'REMOVE_ITEM':
      return state.filter(item => !==;
      return state;

Using the reducer is done by invoking it:

let id = 1;
const actionCreator = (title) =>  ({ type: 'CREATE_ITEM', payload: { id: counter++, title } })

let state = reducer([], actionCreator('item1'));
// [{ title: 'item1' }]

state = reducer([], actionCreator('item2'));
// [{ title: 'item1' }, { title: 'item2' }]

What we can see above is how we can use the reducer and take initial state, and apply an action to it. We also see how we can take an existing state and another action and simply create a new state which consists of the old state + action.

Reducer Types

There are different types of reducers. We have shown a list reducer so far but it is possible to define reducers for:

  • lists
  • objects
  • primitives

Object reducer

We have already showcase the list reducer so let's have a look at an object reducer.

The aim of an object reducer is simply to either load the object or update parts of it, if we are editing it for example. Let's show some code:

const reducer = (state = {}, action) => {
  switch(action.type) {
    case 'LOAD_ITEM':
    case 'UPDATE_ITEM':
      return { ...state, ...action.payload }
    case 'REMOVE_ITEM':
      return null;

Primitive reducer

This is quite an easy one. It looks like this if we are dealing with an integer:

const reducer = (state = 0, action) => {
  switch(action.type) {
    case 'INCREMENT':
      return state + 1;
    case 'DECREMENT':
      return state -1;
      return state;

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