Equivalent pure functions for Functional programming utility functions like .map(), .filter() in ES2015

findKey.js

/**
 * A pure function to find key from object, matching a predicate 
 * similar to https://lodash.com/docs/4.17.4#findKey or Array.findIndex()
 * @param {Object} obj: The object to find the specified key from
 * @param {Function} fn: The predicate function
 */
const findKey = (obj, fn) => Object.keys(obj).find(k => fn(obj[k]));


//Test code
const data = { a: 1, b: 2, c: 3 };
console.log(findKey(data, (x) => x > 2));

len.js

/**
 * A pure function to find length of a list 
 * @param {Array} [first, ...rest]: The list to find the length of
 */
const len = ([first, ...rest]) =>
  !first ? 0 : (1 + len(rest));

// Test code
console.log(len([1,2,3,4,5]));

map.js

/**
 * A pure function to map a list 
 * @param {Array} [first, ...rest]: The list to map
 * @param {Function} fn: The function to apply to the list
 */
const map = ([first, ...rest], fn) =>
  !first ? [] : [fn(first), ...map(rest, fn)];

// Test code
console.log(map([1,2,3,4,5], Math.sqrt));

omit.js

/**
 * A pure function to pick specific keys from object, similar to https://lodash.com/docs/4.17.4#omit
 * @param {Object}obj: The object to pick the specified keys from
 * @param {Array}keys: A list of all keys to omit from obj
 */
const omit = (obj, keys) => 
  Object.keys(obj)
    .filter(i => !keys.includes(i))
    .reduce((acc, key) => {
      acc[key] = obj[key];
      return acc;
    }, {})

console.log(omit(data, ['a', 'c']));

pick.js

/**
 * A pure function to pick specific keys from object, similar to https://lodash.com/docs/4.17.4#pick
 * @param {Object}obj: The object to pick the specified keys from
 * @param {Array}keys: A list of all keys to pick from obj
 */
const pick = (obj, keys) => 
  Object.keys(obj)
    .filter(i => keys.includes(i))
    .reduce((acc, key) => {
      acc[key] = obj[key];
      return acc;
    }, {})

// Test code
const data = { a: 1, b: 2, c: 3 };
console.log(pick(data, ['a', 'c']));

range.js

/**
 * A pure function to generate a range of numbers
 * @param {Number} start: The start of the range, including this value
 * @param {Number} end: The end of the range, excluding this value
 */
const range = (start, end) => Array.from({length: (end - start)}, (v, k) => k + start);

//Test code
console.log(range(1, 10));