The Superpowers of Array.reduce()
Tags: javascript, ecmascript, webdev
Published: 2020-01-11 11:26:01

ECMAScript 5 introduced many [awesome features](https://www.w3schools.com/js/js_es5.asp) in 2009, the majority of them being array methods like `isArray`, `forEach`, `map`, `filter`, `every`, `some`. But let's talk about my favorite one: `reduce`.

* * * * *

The reduce method
=================

The `reduce` method executes a `reducer` callback function (provided by the user) on each element of the array, resulting in a single output value.

reducer
-------

The `reducer` function takes four arguments:

-   Accumulator (acc)
-   Current value (cur)
-   Current index (idx)
-   Source array (src)

Your `reducer` function's return value is assigned to the accumulator, whose value is remembered across each iteration throughout the array and ultimately becomes the final, single resulting value.

Important: On each iteration, you must return the accumulator value for the next iteration (which will eventually be the final return value), or else the accumulator's next (and ultimately final) value will be `undefined`.

initialValue
------------

The `reduce` method takes a second optional argument: `initialValue`.
If not provided, the initial value of the accumulator will be the first element of the array, and the first iteration will point to the second element. If `initialValue` is provided, it will be the initial value of the accumulator, and the first iteration will point to the first element of the array.

* * * * *

Examples
========

Summing numbers with/without an initial value
---------------------------------------------

```javascript
const numbers = [1, 2, 3];

// Without initialValue
const sum = numbers.reduce(
  (accumulator, currentValue) => accumulator + currentValue
);

// Prints 6
console.log(sum);

// With initialValue
const initialValue = 3;
const sumWithInitialValue = numbers.reduce(
  (accumulator, currentValue) => accumulator + currentValue
, initialValue);

// Prints 9
console.log(sumWithInitialValue);
```

Without the initial value, the first iteration will have `accumulator` pointing to the first element of the array (1), and `currentValue` pointing to the second element of the array (2).

Given the initial value, the first iteration will have an `accumulator` with the value of the given initial value (3), and `currentValue` will point to the first element of the array (1).

Counting the number of occurrences in an array
----------------------------------------------

Let's count the number of occurrences of words in the following and store the results in a map:

*How much wood would a woodchuck chuck
If a woodchuck could chuck wood?
He would chuck, he would, as much as he could,
And chuck as much as a woodchuck would
If a woodchuck could chuck wood.*

```javascript
const sentence = "how much wood would a woodchuck chuck" +
  "if a woodchuck could chuck wood " +
  "he would chuck he would as much as he could " +
  "and chuck as much as a woodchuck would " +
  "if a woodchuck could chuck wood";

const words = sentence.split(" ");

const occurencesMap = words.reduce(
  (occurences, word) => {
    const numOfOccurences = (occurences.get(word) || 0) + 1;
    occurences.set(word, numOfOccurences);
    return occurences;
  }
, new Map());

const numOfWoodchucks = occurencesMap.get("woodchuck");

// 4
console.log(numOfWoodchucks);
```

We initialize an empty map and use it as the initial value of the accumulator, initializing or updating the number of occurrences of each word as we iterate over the words in the sentence.

These are only two examples, but by now you must have realized how awesome `reduce` is, right?

It allows you to take an array and reduce its values to basically anything that can be derived from the data it holds. It also allows you to return any type of data, regardless of the type of the elements of the array.

* * * * *

One Method to Rule Them All?
============================

Revisiting other ES5 array methods, we can see that each method uses the given callback function on the array and returns some kind of result.

For example:

-   `map` transforms each element of the array, returning a new array.
-   `every` checks if the given condition applies to every element in the array, returning the corresponding Boolean value.

Looks familiar, right?

Using what we already know, let's try to use `reduce` to implement other ES5 array methods.

Note: we'll add the new methods to `Array`'s prototype in each example, where `this` will point to the array on which we're operating.

* * * * *

map
===

The [map()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map) method creates a new array where each original element is transformed by the given `transformer` callback.

Usage
-----
```javascript
const array = [1, 2, 3];
const doubled = array.map(num => num * 2);

// Prints [2, 4, 6]
console.log(doubled);
```

Using a transformer callback that doubles every number in the array, we get a new array where every element is twice its original value.

With `reduce`
-------------

`map` operates on an array and returns a new array, so the accumulator has to be an array.

```javascript
Array.prototype.mapWithReduce = function(transformer) {
  return this.reduce((newArray, currentElement) => {
    const newElement = transformer(currentElement);
    newArray.push(newElement);
    return newArray;
  }, []);
}

const array = [1, 2, 3];
const doubled = array.mapWithReduce(num => num * 2);

// Prints [2, 4, 6]
console.log(doubled);
```

Using `reduce`, we start with an empty array accumulator and iterate over the array. We then apply the transformer callback on each element and push it to the accumulating array.

* * * * *

filter
======

The [filter()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter) method creates a new array with all elements that pass the test implemented by the provided function.

Usage
-----
```javascript
const array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const evenOnly = array.filter(num => num % 2 === 0);

// Prints [2, 4, 6, 8, 10]
console.log(evenOnly);
```
Using a test callback that filters out all odd numbers, we get a new array with all the even elements of the original array.

With `reduce`
-------------

Just like the previous example, `filter` also operates on an array and returns a new array, so the accumulator has to be an array.

```javascript
Array.prototype.filterWithReduce = function(tester) {
  return this.reduce((newArray, currentElement) => {
    if (tester(currentElement)) {
      newArray.push(currentElement);
    };
    return newArray;
  }, []);
}

const array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const evenOnly = array.filterWithReduce(num => num % 2 === 0);

// Prints [2, 4, 6, 8, 10]
console.log(evenOnly);
```

Using `reduce`, we start with an empty array accumulator and iterate over the array. We then use the tester callback to check if each element should be pushed to the accumulating array.

* * * * *

every
=====

The [every()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/every) method tests whether all elements in the array pass the test implemented by the provided function. It returns a Boolean value.

Usage
-----
```javascript
const array = [1, 2, 3, 4, 5];
const result = array.every(num => num < 10);

// Prints true
console.log(result);
```
Using a callback function that tests every element in the array, we get a boolean that indicates whether all elements pass the test. In this case, all elements are smaller than 10, and thus `every` returns `true`.

With `reduce`
-------------

`every` operates on an array and returns a Boolean value, so the accumulator has to be a boolean.
```javascript
Array.prototype.everyWithReduce = function(tester) {
  return this.reduce((acc, currentElement) =>
    acc && tester(currentElement)
  , true);
}

const array = [1, 2, 3, 4, 5];
const result = array.everyWithReduce(num => num < 10);

// Prints true
console.log(result);
```

Using `reduce`, we start with a boolean accumulator value of `true` (we'll discuss the reason later on) and iterate over the array. We then chain the result of the tester callback to the accumulator using the logical AND (`&&`), to eventually return `true` if all elements pass the test, and `false` otherwise.

Why start with `true`?

If the array is empty, `every` returns `true` regardless of the test callback (even if the callback returns `false`).

Else, if all elements fulfill the condition, the chaining of the initial `true` value using the logical AND will eventually resolve to `true`. If not, the chaining will eventually resolve to `false`.

* * * * *

some
====

The [some()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/some) method tests whether at least one element in the array passes the test implemented by the provided function. It returns a Boolean value.

Usage
-----
```javascript
const array = [1, 2, 3, 4, 5];
const result = array.some(num => num > 3);

// Prints true
console.log(result);
```
Using a callback function that tests every element in the array, we get a boolean that indicates whether any element passes the test. In this case, the fourth element is larger than 3, and thus `some` returns `true`.

With `reduce`
-------------

`some` operates on an array and returns a boolean value, so the accumulator has to be a boolean.
```javascript
Array.prototype.someWithReduce = function(tester) {
  return this.reduce((acc, currentElement) =>
    acc || tester(currentElement)
  , false);
}

const array = [1, 2, 3, 4, 5];
const result = array.someWithReduce(num => num > 3);

// Prints true
console.log(result);
```

Using `reduce`, we start with a boolean accumulator value of `false` (we'll discuss the reason later on) and iterate over the array. We then chain the result of the tester callback to the accumulator using the logical OR (`||`), to eventually return `true` if any element passes the test, and `false` otherwise.

Why start with `false`?

If the array is empty, `some` returns `false` regardless of the test callback (even if the callback returns `true`).

Else, if any element fulfills the condition, the chaining of the initial `false` value using the logical OR will eventually resolve to `true`. If not, the chaining will eventually resolve to `false`.

* * * * *

Disclaimer (every and some)
===========================

The `every` method executes the provided callback function once for each element present in the array until it finds the one where callback returns a falsy value (a value that becomes `false` when converted to a boolean). If such an element is found, `every` immediately returns `false`.

Similarly, the `some` method executes the callback function once for each element present in the array until it finds the one where callback returns a *truthy* value (a value that becomes `true` when converted to a boolean). If such an element is found, `some` immediately returns `true`.

However, there's no pretty way to [terminate ](https://stackoverflow.com/questions/36144406/how-to-break-on-reduce-method)[reduce](https://stackoverflow.com/questions/36144406/how-to-break-on-reduce-method)[ mid-loop](https://stackoverflow.com/questions/36144406/how-to-break-on-reduce-method).
This means that while both implementations (both the original `every/some` method and the corresponding implementations using `reduce`) have a runtime of `O(n)`, the original implementations are likely to terminate without having to iterate over the entire array, making them more efficient.

* * * * *

filter + map
============

Given an array of numbers, what is the most efficient way to filter out all even elements and square the remaining ones (using ES5 methods)?

Let's try `filter` followed by `map`:
```javascript
const array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

const tester = num => num % 2 === 1;
const transformer = num => num * num;

const result = array.filter(tester).map(transformer);

// Prints [1, 9, 25, 49, 81]
console.log(result);
```

We create a tester function that keeps only odd elements and a transformer function that squares the given elements. We then use these two callback functions when chaining the `filter` and `map` methods, and return the desired array.

Let's use what we know about implementing `filter` and `map` with `reduce`, only this time let's combine them in one go.
```javascript
const array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

const tester = num => num % 2 === 1;
const transformer = num => num * num;

const result = array.reduce((newArray, currentElement) => {
  if (tester(currentElement)) {
    const newElement = transformer(currentElement);
    newArray.push(newElement); 
  }
  return newArray;
}, []);

// Prints [1, 9, 25, 49, 81]
console.log(result);
```

We use the same tester and transformer functions to test if each element should be kept in the array and transform it if it should.

This approach saves us the need to create an intermediate array of filtered values, and we get a slightly more efficient algorithm as we don't have to iterate over two different arrays (the original and the intermediate).

* * * * *

Conclusion
==========

These were a few examples of how to use the powerful `reduce` method to implement other ES5 methods.

How do you use it in your day-to-day coding? Share in the comments!

* * * * *

Sources
=======

-   [ES5 features](https://www.w3schools.com/js/js_es5.asp) by w3schools
-   [Javascript array methods](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array) by MDN web docs.
    Every method documentation in this article is taken from here.
-   Cover Photo by [Esteban Lopez](https://unsplash.com/@exxteban) on [Unsplash](https://unsplash.com/)