moon indicating dark mode
sun indicating light mode

Understanding Reduce Through Types

September 25, 2019

I feel like I didn’t fully understand Array.prototype.reduce until I started getting into functional programming and learning about types and function type signatures. Once I understood the types, I feel like it unlocked a powerful tool that I naturally reach for any time I need to turn an array of things into something of a different type.

Starting with map’s type signature

Before we dig into reduce though, let’s take a look at the type signature for map. In Haskell, it looks like this:

map :: (a -> b) -> [a] -> [b]

This can be understood as map being a function that accepts two arguments:

  1. An (a -> b) function
  2. A list of as

And it returns a list of bs.

We have some transformation we want to apply to each element in the array and get back a new array with the transformations applied.

For the purpose of keeping as close to the simple type signature of Haskell and ignoring that map is actually on Array.prototype we’ll write our TypeScript implementation like this:

function map<A, B>(fn: (a: A) => B, as: A[]): B[] {

Similar to the Haskell type signature, we can see that we have a function that takes an A and returns a B, and an array of As. Then the return type is a an array of Bs.

Let’s see what the type signature becomes when we actually use it.

const length = (s: string) => s.length
// map<string, number>(
// fn: (s: string) => number,
// as: string[],
// ) => number[]
const result = map(length, ["hi", "there"])

We can see here that the types fill in the generics and we know that we’re going to get an array of numbers back.


Next let’s look at reduce in Haskell which is called foldl (short for fold left):

foldl :: Foldable t => (b -> a -> b) -> b -> t a -> b

This can be read as a function that accepts three arguments:

  1. A two argument function (b -> a -> b). The first argument in this function is a value of type b, the second argument to this function is a value of type a, and the return type is a value of type b
  2. An initial b value
  3. A Foldable type t of as. We will gloss over what this means, but suffice it to say that lists are “foldable” so they can be used with foldl, and this Foldable t means the t a is the “list of as”. If you want to learn more on this, look into Haskell’s typeclasses and higher kinded types.

A similar implementation in TypeScript (without the additional params in the reduce function such as index and array) would look like this:

function reduce<A, B>(
reducer: (accumulator: B, a: A) => B,
initialValue: B,
as: A[],
): B {
return as.reduce(reducer, initialValue)

So what is happening here?

The B type is the accumulator type.

Whatever type this is will be the type that gets returned by reduce.

In most languages you must supply an initial value for the accumulator. It iterates over each element in the list, passing the element into the reducer as the A value along with the accumulator result from the previous reducer invocation (or initial value for the first one), and the reducer must somehow combine it with the B value accumulator. Whatever combining means in the context of your reducer.

Because JavaScript has to be quirky…

I said in most languages you must supply the initial value, but in JavaScript if you don’t supply an initial value for the accumulator it will actually use the head of the list as the initial value for the accumulator and then reduce the tail. In this case, the type of the accumulator is by default the same as whatever the array is of. This is not normal, and I don’t recommend using this because it is implicit and others working in the same code may not be aware of this quirk.

The behavior is essentially this:

function reduceWithNoInitialValue<A>(
reducer: (accumulator: A, a: A) => A,
as: A[],
) {
const [head, ...tail] = as
return tail.reduce(reducer, head)

Basic Examples

Let’s see some basic examples and understand the types.


// 10
const result = [1, 2, 3, 4].reduce(
(accumulator, a) => accumulator + a,

In this case, both types A and B are numbers. Combining them in this context means adding them all together. We start with 0 as the initial value because when added to any other number, it doesn’t change it. It is the only sensible starting point for this.


What if we want to multiply the numbers to combine them though? We update our reducer to multiply instead of add, then we update the initial value to be 1 instead of 0 for the same reason as before. It’s the only viable starting value for multiplication.

// 24
const result = [1, 2, 3, 4].reduce(
(accumulator, a) => accumulator * a,

Building other array operations with reduce

Now let’s change the type of the accumulator and thus the result!


What if we wanted to implement our own version of the Array.prototype.some function?

Since we know Array.prototype.some returns a boolean, we know the accumulator (type B) must be a boolean!

What kind of operation should we perform to combine a value of type A with our boolean accumulator though?

Well, if any invocation of the predicate returns true, we want the entire result to be true, right?

This sounds a lot like or!

What should the initial value be? true or false? If we’re using or to combine a bunch of booleans, we can’t start with true, or the result will always be true. false must be the initial value.

const or = (a: boolean, b: boolean) => a || b
const some = <A>(
predicate: (a: A) => boolean,
as: A[],
) =>
(accumulator, a) => or(accumulator, predicate(a)),
const isEven = n => n % 2 === 0
const resultOne = some(isEven, [1, 3, 5]) // false
const resultTwo = some(isEven, [1, 3, 4]) // true


Similarly, we can implement Array.prototype.every using reduce!

const and = (a: boolean, b: boolean) => a && b
const every = <A>(
predicate: (a: A) => boolean,
as: A[],
) =>
(accumulator, a) => and(accumulator, predicate(a)),
const isEven = n => n % 2 === 0
const resultOne = every(isEven, [1, 4, 6]) // false
const resultTwo = every(isEven, [2, 4, 6]) // true

This time, the we’ve flipped the logic! We initialize with true, and now we and the accumulator together with the result of the predicate. If a single false value is returned from the predicate, the entire expression will be false!


How about Array.prototype.filter?

Well, the accumulator needs to be the same type as the original array because all we’re doing is removing elements in the array that don’t return true from the predicate, right?

function filter<A>(
predicate: (a: A) => boolean,
as: A[],
): A[] {
return as.reduce(
(accumulator, a) =>
predicate(a) ? [...accumulator, a] : accumulator,
[] as A[],
// [2, 4]
const result = filter(n => n % 2 === 0, [1, 2, 3, 4])

If the predicate returns true, then we add the current element to the new accumulator, otherwise we just return the accumulator as is without adding the element.


Similarly, we can also implement our own map using reduce!

The only difference is the return type is not the same as the original. If we started with A[] and we have an (a: A) => B function, we need to return B[].

function map<A, B>(
fn: (a: A) => B,
as: A[],
): B[] {
return as.reduce(
(accumulator, a) => [...accumulator, fn(a)]),
[] as B[],
// [6, 2, 3]
const result = map(s => s.length, ["reduce", "is", "rad"])

Reducing to objects

There are many scenarios where you want the accumulator — let’s look at a few.

Creating a map of entities by id

Let’s say you’ve got a very long list of objects and you know that you’re going to need to look them up by a unique id field.

type Person = {
id: string
name: string
email: string
const people: Person[] = [
id: "1",
name: "Kevin Kipp",
email: "",
// 10,000 more...

Using [].find() will be expensive every time you perform the lookup because it has to iterate through the list one by one looking for the right person every time you want to access them by id.

We can create an object that looks like this using reduce:

const personById = {
"1": {
id: "1",
name: "Kevin Kipp",
email: "",
// 10,000 more by id
// can simply access now by key!
const kevin = personById["1"]

How do we create this? Well, since an object is what we want, an empty object {} should be used as the accumulator.

type MapById<T> = {
[key: string]: T | undefined
const peopleById = people.reduce(
(acc, person) => ({ ...acc, []: person }),
{} as MapById<Person>,

Transforming values on objects

What if we want to do something like iterate over each key of an object and perform a transformation on it? Object.entries gives us a list of [key, value] tuples which we can then reduce onto a new object!

const original = {
foo: 1,
bar: 2,
baz: 3,
// {foo: 2, bar: 4, baz: 6}
const result = Object.entries(original).reduce(
(accumulator, [key, value]) => ({
[key]: value * 2,
{} as typeof original,

Passing state through accumulators

You can even pass state along in the accumulator. Let’s say you wanted to create a function that counts the most frequently occuring string in a list of strings.

function findMostFrequentWord(words: string[]) {
const { mostFrequentWord } = words.reduce(
(accumulator, word) => {
const { mostFrequentWord, words } = accumulator
const count = (words[word] || 0) + 1
return {
words: {
[word]: count,
mostFrequentWord.count >= count
? mostFrequentWord
: {
words: {},
mostFrequentWord: {
word: "",
count: 0,
return mostFrequentWord
// {word: "foo", count: 3}
const result = findMostFrequentWord([

Here the actual result we’re really interested in is the mostFrequentWord which we destructure from the accumulator when we’re done, but we also pass the words object along while reducing so we can keep counts of occurances. This allows us to perform computations like this iterating only once over the list without actually mutating anything directly!

As an exercise, try solving this without using reduce and see if you can do it without mutating anything and tweet me your findings @kevin_kipp!

This was a pattern that I hadn’t come across anywhere until I started playing with pure functional languages that don’t allow any mutation at all.


The reduce function is incredibly powerful and hopefully a bit more clear now.

I think the two key takeaways here are:

  1. You can use reduce any time you need to turn an array into a value of some other type that may not even be an array. Use the regular other array helpers if they fit your need, but if not it’s very likely that reduce can do what you need.
  2. The accumulator’s type is the same as the return type. Start by asking what the type of the value you need is. Once you know that, you know what the types of arguments your reducer will have which makes implementing the reducer much easier!

Please let me know if you have any questions on Twitter!