10 More Utility Functions Made with Reduce
By: Yazeed Bzadough
Tags: Reduce, JavaScript, Programming, Functional Programming, Technology

10 More Utility Functions Made with Reduce

This time, with a test suite!

Previously I wrote about 10 utility functions implemented with JavaScript’s reduce function. It was well-received, and I walked away with an even deeper appreciation for this magnificent multi-tool. Why not try 10 more?

Many of these were inspired by the awesome libraries Lodash and Ramda! I also wrote unit tests to ensure correct behavior. You can see everything on the Github repo here.

1. pipe

Parameters

...functions - Any number of functions.

Description

Performs left-to-right function composition. The first argument to a pipeline acts as the initial value, and is transformed as it passes through each function.

Implementation

const pipe = (...functions) => (initialValue) =>

functions.reduce((value, fn) => fn(value), initialValue);

Usage

const mathSequence = pipe(

(x) => x * 2,

(x) => x - 1,

(x) => x * 3

);



mathSequence(1); // 3

mathSequence(2); // 9

mathSequence(3); // 15

For more detail, I wrote an article on pipe here.

2. compose

Parameters

...functions - Any number of functions.

Description

Performs right-to-left function composition. The first argument to a pipeline acts as the initial value, and is transformed as it passes through each function.

Works like pipe , but in the opposite direction.

Implementation

const compose = (...functions) => (initialValue) =>

functions.reduceRight((value, fn) => fn(value), initialValue);

Usage

const mathSequence = compose(

(x) => x * 2,

(x) => x - 1,

(x) => x * 3

);



mathSequence(1); // 4

mathSequence(2); // 10

mathSequence(3); // 16

For more detail, I wrote an article on compose here.

3. zip

Parameters

list1 - List of items. list2 - List of items.

Description

Pairs items from two lists via index. If list lengths are not equal, the shorter list’s length is used.

Implementation

const zip = (list1, list2) => {

const sourceList = list1.length > list2.length ? list2 : list1; return sourceList.reduce((acc, _, index) => {

const value1 = list1[index];

const value2 = list2[index]; acc.push([value1, value2]); return acc;

}, []);

};

Usage

zip([1, 3], [2, 4]); // [[1, 2], [3, 4]] zip([1, 3, 5], [2, 4]); // [[1, 2], [3, 4]] zip([1, 3], [2, 4, 5]); // [[1, 2], [3, 4]] zip(['Decode', 'secret'], ['this', 'message!']);

// [['Decode', 'this'], ['secret', 'message!']]

4. intersperse

Parameters

separator - Item to insert. list - List of items.

Description

Inserts a separator between each element of a list.

Implementation

const intersperse = (separator, list) =>

list.reduce((acc, value, index) => {

if (index === list.length - 1) {

acc.push(value);

} else {

acc.push(value, separator);

} return acc;

}, []);

Usage

intersperse('Batman', [1, 2, 3, 4, 5, 6]);

// [1, 'Batman', 2, 'Batman', 3, 'Batman', 4, 'Batman', 5, 'Batman', 6] intersperse('Batman', []);

// []

5. insert

Parameters

index - Index to insert element at. newItem - Element to insert. list - List of items.

Description

Inserts an element at the given index. If the index is too large, element is inserted at the end of the list.

Implementation

const insert = (index, newItem, list) => {

if (index > list.length - 1) {

return [...list, newItem];

} return list.reduce((acc, value, sourceArrayIndex) => {

if (index === sourceArrayIndex) {

acc.push(newItem, value);

} else {

acc.push(value);

} return acc;

}, []);

};

Usage

insert(0, 'Batman', [1, 2, 3]);

// ['Batman', 1, 2, 3] insert(1, 'Batman', [1, 2, 3]);

// [1, 'Batman', 2, 3] insert(2, ['Batman'], [1, 2, 3]);

// [1, 2, ['Batman'], 3] insert(10, ['Batman'], [1, 2, 3]);

// [1, 2, 3, ['Batman']]

6. flatten

Parameters

list - List of items.

Description

Flattens a list of items by one level.

Implementation

const flatten = (list) => list.reduce((acc, value) => acc.concat(value), []);

Usage

flatten([[1, 2], [3, 4]]);

// [1, 2, 3, 4] flatten([[1, 2], [[3, 4]]]);

// [1, 2, [3, 4]] flatten([[[1, 2]], [3, 4]]);

// [[1, 2], 3, 4] flatten([[[1, 2], [3, 4]]]);

// [[1, 2], [3, 4]]

7. flatMap

Parameters

mappingFunction - Function to run on each list item. list - List of items.

Description

Maps each list item according to the given function, then flattens the result.

Implementation

// Kind of cheating, because we already implemented flatten 😉

const flatMap = (mappingFunction, list) => flatten(list.map(mappingFunction));

Usage

flatMap((n) => [n * 2], [1, 2, 3, 4]);

// [2, 4, 6, 8] flatMap((n) => [n, n], [1, 2, 3, 4]);

// [1, 1, 2, 2, 3, 3, 4, 4] flatMap((s) => s.split(' '), ['flatMap', 'should be', 'mapFlat']);

// ['flatMap', 'should', 'be', 'mapFlat']

8. includes

Parameters

item - Item to check the list for. list - List of items.

Description

Checks a list for a given element. If the element is found, returns true . Otherwise returns false .

Implementation

const includes = (item, list) =>

list.reduce((isIncluded, value) => isIncluded || item === value, false);

Usage

includes(3, [1, 2, 3]); // true

includes(3, [1, 2]); // false

includes(0, []); // false

9. compact

Parameters

list - List of items.

Description

Removes “falsey” values from a list.

Implementation

const compact = (list) =>

list.reduce((acc, value) => {

if (value) {

acc.push(value);

} return acc;

}, []);

Usage

compact([0, null, 1, undefined, 2, '', 3, false, 4, NaN]);

// [1, 2, 3, 4]

10. arrayIntoObject

Parameters

key - String to use as the new object key. list - List of items.

Description

Converts an array into an object, using the given key as the new object’s key.

Implementation

const arrayIntoObject = (key, list) =>

list.reduce((acc, obj) => {

const value = obj[key]; acc[value] = obj; return acc;

}, {});

Usage

const users = [

{ username: 'JX01', status: 'offline' },

{ username: 'yazeedBee', status: 'online' }

]; arrayIntoObject('username', users);

/*

{

JX01: {

username: 'JX01',

status: 'offline'

},

yazeedBee: { username: 'yazeedBee', status: 'online' }

}

*/ arrayIntoObject('status', users);

/*

{

offline: {

username: 'JX01',

status: 'offline'

},

online: { username: 'yazeedBee', status: 'online' }

}

*/

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Until next time!