Maksim IvanovProgramming tutorials

Reducers VS Transducers

November 28, 2017

Sweet chunk of functional paradigm for you today. I don’t know why did I write “versus” while they compliment each other. Anyway, let’s get to the good stuff…


Simply speaking a Reducer is a function that takes an accumulation and a value, and then returns a new accumulation.

You are already familiar with reducers if you’ve used the Array.prototype.reduce() method. The .reduce() function itself is not a reducer! It iterates over a collection and passes values to it’s “callback” that is a reducer here.

Let’s imagine that we have an array with five numbers: [1, 2, 3, 14, 21] and we want to find the biggest of them.

const numbers = [1, 2, 3, 14, 21];

const biggestNumber = numbers.reduce(
  (accumulator, value) => Math.max(accumulator, value)

// 21

The arrow function here is a reducer. The .reduce() method only takes the result of previous reduction and calls the reducer with it and next element of an array.

Reducers can work with any kinds of values. The only rule is that the accumulation you return should have the same type that the accumulation you pass in.

For example you can easily create a reducer that will work with strings:

const folders = ['usr', 'var', 'bin'];

const path = folders.reduce(
  (accumulator, value) => `${accumulator}/${value}`
, ''); // Here I passed empty string as an initial value

// /usr/var/bin

Actually it’s better to illustrate without Array.reduce() method. Look:

const stringReducer = (accumulator, value) => `${accumulator} ${value}`

const helloWorld = stringReducer("Hello", "world!")

// Hello world!

Map And Filter As Reducers

The other cool thing about reducers is that you can chain them to perform a series of operations on some data. This opens up huge possibilities for composition and reuse of small reducer functions.

Let’s say you have an ordered array of numbers. You want to get even numbers from it and then multiply by 2.

The ordinary way to do it would be to use .map and .filter functions:

[1, 2, 3, 4, 5, 6]
  .filter((x) => x % 2 === 0)
  .map((x) => x * 2)

But what if your array had 1000,000 elements? You have to loop through the whole array for every operation, that’s extremely ineffective.

We need some way to combine the functions we passed to map and filter. But we can’t do this as they have different interface. The function that we passed to filter is called predicate and it takes a value and returns True or False depending on inner logic. And the function we passed to map is transformer function. It takes a value and returns transformed value.

We can achieve this with reducers, let’s create our own reducer version of .map and .filter functions.

const filter = (predicate) => {
  return (accumulator, value) => {
    return accumulator;

const map = (transformer) => {
  return (accumulator, value) => {
    return accumulator;

Great, we used decorator functions to wrap our reducers. Now we have map and filter functions that return reducers that can be passed to Array.reduce() method!

[1, 2, 3, 4, 5, 6]
  .reduce(filter((x) => x % 2 === 0), [])
  .reduce(map((x) => x * 2), [])

Great, now we have a chain of .reduce function calls, but we still can’t compose our reducers! Good news is there is only one step left. To be able to compose reducers we need to be able to pass them to each other.

Transducers FTW

Let’s update our filter function so it would also accept reducer as an argument. We are going to decompose it and instead of pushing value to accumulator we’ll allow the passed in reducer to perform it’s logic.

const filter = (predicate) => (reducer) => {
  return (accumulator, value) => {
      return reducer(accumulator, value);
    return accumulator;

This pattern where we take a reducer as an argument and return another reducer is called transducer. As it’s a combination of transformer and reducer (we take a reducer and transform it).

const transducer => (reducer) => {
  return (accumulator, value) => {
    // Some logic involving passed in reducer

So basically transducer looks like this (oneReducer) => anotherReducer.

Let’s rewrite our mapping reducer as well:

const map = transformer => reducer => {
  return (accumulator, value) => {
    return reducer(accumulator, transformer(value));

Also we need to add a final reducer that will push values to array for us:

const finalReducer = (acc, val) => {
  return acc;

Now we can combine our mapping reducer and filtering transducer and do our calculations in one run.

const evenPredicate = (x) => x % 2 === 0;
const doubleTransformer = (x) => x * 2;

const filterEven = filter(evenPredicate);
const mapDouble = map(doubleTransformer);

[1, 2, 3, 4, 5, 6]
  .reduce(filterEven(mapDouble(finalReducer)), []);

Actually we could make our map method a transducer as well and continue this composition indefinitely.

But just imagine having to compose more than 2 transducers. We have to find more convenient way to compose them.

Better Composition

Basically we need something that would take a number of functions and compose them in that order.

compose(fn1, fn2, fn3)(x) => fn1(fn2(fn3(x)))

Luckily a lot of libraries provide this kind of function. For instance RamdaJS. But for educational purposes let’s create our own version.

const compose = (...functions) =>
  functions.reduce((accumulation, fn) =>
    (...args) => accumulation(fn(args)), x => x)

The function is very compact, let’s break it down.

Imagine that we called that function like this compose(fn1, fn2, fn3)(x).

First look at the x => x part. In lambda calculus it’s called identity function. It just returns whatever it takes as an argument without changing. We need it here to start our unfolding.

So after fist iteration we’ll have that identity function (for convenience let’s call it I) called with the fn1 function as an argument:

  (...args) => accumulation(fn(args))

  // STEP 1
  // We pass our fn1 to accumulation
  (...args) => accumulation(fn1(args))

  // STEP 2
  // Here we basically substitute accumulation with I
  // and fn and fn1
  (...args) => I(fn1(args))

Yay, we calculated the accumulation value after the first iteration. Let’s do the second one:

  (...args) => I(fn1(args)) // Our new accumulation

  // STEP 3
  // Now we pass fn2 to our accumulation
  (...args) => accumulation(fn2(args))

  // Step 4
  // Lets substitute "accumulation" with it's current value
  (...args) => I(fn1(fn2(args)))

I think you got the idea. Now just repeat steps 3 and 4 for fn3 and voila, you’ve converted your compose(fn1, fn2, fn3)(x) to fn1(fn2(fn3(x))).

Now we can compose our map and filter like this:

[1, 2, 3, 4, 5, 6]


I suppose you already knew about reducers, and if not – you’ve learned a nice abstraction to work with collections. Reducers are great to fold different data structures.

Also you’ve learned how to do your computations effectively using transducers.

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