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

  1. ...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

  1. ...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

  1. list1 - List of items.
  2. 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

  1. separator - Item to insert.
  2. 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

  1. index - Index to insert element at.
  2. newItem - Element to insert.
  3. 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

  1. 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

  1. mappingFunction - Function to run on each list item.
  2. 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

  1. item - Item to check the list for.
  2. 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

  1. 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

  1. key - String to use as the new object key.
  2. 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' }
}
*/

Want Free Coaching?

If you'd like to schedule a free call to discuss Front-End development questions regarding code, interviews, career, or anything else follow me on Twitter and DM me.

After that if you enjoy our first meeting, we can discuss an ongoing coaching to help you reach your Front-End development goals!

Thanks for reading

For more content like this, check out https://yazeedb.com!

Until next time!