< return home


Transforming multiple inputs into a single output

This article was originally published on #dev-a-day.

Thanks to the recent popularity of the Flux state model and easy to use Flux implementations like Redux and MobX, reducers have become a hot topic. Don't worry if you're not familiar with complex state management models or how reducers apply in the context of Flux, that's a topic for a later article. Let's take a step back and look at reducers from the bottom up.

If you've worked much with JS, you may be familiar with Array.prototype.reduce. Let's forget that for a second (we'll come back to it) and look at a vanilla reducer function.

function reducer(val1, val2) {
  return val1 + val2;

reducer(1, 2); // 3

At their core, reducers take two inputs and return a single output. Reducers vary wildly in both purpose and implementation, and as such they can be very useful in a ton of different contexts. The example above is obviously a rather contrived example, but it's important to get a baseline understanding before we go any further.

In our reducer above, we take two numbers and add them together. The program flow looks like this:


Similarly, we could create a reducer that concatenates two strings:

function reducer(str1, str2) {
  return str1 + ' ' + str2;

reducer('Hello', 'world'); // 'Hello world'

Typical Usage

Again, pretty contrived. In the Real World™, reducers will generally combine two inputs of different types in some meaningful way. Generally, one input is piece of data and the second is a modifier for that data. Take, for example:

function reducer(user, timestamp) {
  return {
    lastUpdated: timestamp,

const user = {
  name: 'Jane Doe',
  lastUpdated: null,

reducer(user, new Date(2019, 2, 3).toLocaleDateString());
// {
//   name: 'Jane Doe',
//   lastUpdated: '2/3/2019',
// }

This reducer takes a piece of data (a user model) and returns a copy with an updated timestamp. Here's the data flow for this reducer (note that the inputs are of different types):


Still doesn't seem very useful, huh? Reducers may feel like overkill for small examples, but when working in a production environment, immutability is very important. Often, any number of application services may be operating on the same central data store, and we need ensure that each service can safely operate on that data and have its changes persisted, no matter when or how often it accesses the store. Let's look at an example:

const store = {
  count: 0,
  bar: 'bar',

function addOne() {
  store.count += 1;

function addTwo() {
  store.count += 2;

addOne(); // store.count === 1
addTwo(); // store.count === 3

This works fine for this example, but what happens if addOne and addTwo are called at the same time? This causes non-deterministic behavior, which means we can't say for sure what will happen and it may change from one run to the next. Why is this? Let's see what happens if addOne begins execution first:

// NOTE: these are called at the same time, not synchronously. This example
// won't work if you copy/paste it into your console, you would need to set
// up an asynchronous environment first.
addOne(); // store.count === 1
addTwo(); // store.count === 2

If both functions are called at the same time, they both start with the initial value of store, where count is 0. addOne will set count to 1, and then addTwo will set count to 2, effectively destroying the changes that addOne made. If they were called in the opposite order, addTwo's changes would be lost.

Deterministic Behavior

To prevent this and produce deterministic behavior, we should instead use reducers!

let store = {
  count: 0,
  bar: 'bar',

function addReducer(store, increment) {
  return {
    count: store.count + increment,

store = addReducer(store, 1); // store.count === 1
store = addReducer(store, 2); // store.count === 3

If you've used a Flux library before, this should look familiar. Flux reducers accept two parameters, the store and an action that describes how the store should change. Here's a barebones version of how Flux reducers manage state:

let store;

const initialState = { count: 0, bar: 'bar' };

function reducer(state = initialState, action) {
  switch (action.type) {
    case 'INCREMENT':
      return {
        count: state.count + 1,
    case 'DECREMENT':
      return {
        count: state.count - 1,
      return state;

// when the store is initialized
store = reducer(null, { type: '@@INIT' });

// when an action is dispatched
store = reducer(store, { type: 'INCREMENT' });

It's important that you spread state in your return value so no existing piece of state gets dropped. You also want to make sure to provide a default case to properly initialize state. If you want to learn more about Flux reducers, check out Redux's documentation here.

Array Reducers

Alright, now let's look at Array.prototype.reduce. I've put this off until the end because it's not actually a reducer! The function you pass to it is the reducer. If you're not familiar, here's the MDN:

The reduce() method executes a reducer function (that you provide) on each member of the array resulting in a single output value.

Your reducer function takes up to four arguments, but you'll generally just use the first two:

const data = [1, 2, 3, 4];

function reducer(accumulator, current) {
  return accumulator + current;

function sumArray(arr) {
  return arr.reduce(reducer, 0);

sumArray(data); // 10

Reducing an array passes over each element in the array and calls your reducer function with the accumulator and the current element in the array. When you call reduce, you provide a starting value, which is what accumulator is set to when running the reducer on the first element. The reducer function is usually written inline to save on space. The accumulator is often abbreviated acc, or renamed to better fit the task at hand. The current value paramter is also often renamed.

Let's walk through what's happening here:

// initialize
let acc = 0;

// first element (1)
let current = data[0];
acc = reducer(acc, current); // acc === 1

// second element (2)
current = data[1];
acc = reducer(acc, current); // acc === 3

// third element (3)
current = data[2];
acc = reducer(acc, current); // acc === 6

// fourth element (4)
current = data[3];
acc = reducer(acc, current); // acc === 10

// return final accumulator value
return acc;

To better visualize this, here's a flow diagram:


A: accumulator
B: current
C: return value

Using a reducer for basic addition is somewhat overkill (but still occasionally necessary!). Here's a more realistic example:

const users = [
  { id: 1, name: 'John Doe' },
  { id: 2, name: 'Jane Doe' },
  { id: 3, name: 'Foo Bar' },

function keyBy(arr, property) {
  return arr.reduce((acc, current) => {
    acc[current[property]] = current;
    return acc;
  }, {});

keyBy(users, 'id');
// {
//   1: { id: 1, name: 'John Doe' },
//   2: { id: 2, name: 'Jane Doe' },
//   3: { id: 3, name: 'Foo Bar' },
// }

Look at that, we've just implemented lodash's _.keyBy, albeit a bit stripped down. This array reducer will take an array of objects and return a single object with each array element set to the key indicated by property. A key takeaway here when working with more complicated array reducers is to make sure you return the accumulator from your reducer function, otherwise you'll do a whole lot of nothing! If you ever run into issues with array reducers, this is usually a good first thing to check.

If you're feeling especially cheeky, we can spice this up with a bit of ES6 magic:

function keyBy(arr, property) {
  return arr.reduce(
    (acc, current) => ({
      [current[property]]: current,

Everybody loves less typing, right?


Alright, if you've made it this far, give yourself a pat on the back - you now know as much as I do about reducers. Be sure to check back tomorrow, when we'll dive into everybody's favorite: transducers! See you then.

ramblings by Aaron Ross, otherwise known as superhawk610
> ...
© 2023 all rights reserved
built with Gatsby