The Superpowers of Array.reduce() Method

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ECMAScript 5 introduced many awesome features 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

.

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.

Summing numbers with/without an initial value

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.

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.

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.

The map() method creates a new array where each original element is transformed by the given 

transformer

 callback.

Usage

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.

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.

The filter() method creates a new array with all elements that pass the test implemented by the provided function.

Usage

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.

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.

The every() method tests whether all elements in the array pass the test implemented by the provided function. It returns a Boolean value.

Usage

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.

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

.

The 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

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.

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

.

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 

reduce

 mid-loop.
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.

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

:

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.

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).

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!

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