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to Accumulate in the TypeScript Track

Published at Oct 18 2019 · 0 comments
Test suite

Implement the accumulate operation, which, given a collection and an operation to perform on each element of the collection, returns a new collection containing the result of applying that operation to each element of the input collection.

Given the collection of numbers:

  • 1, 2, 3, 4, 5

And the operation:

  • square a number (x => x * x)

Your code should be able to produce the collection of squares:

  • 1, 4, 9, 16, 25

Check out the test suite to see the expected function signature.


Keep your hands off that collect/map/fmap/whatchamacallit functionality provided by your standard library! Solve this one yourself using other basic tools instead.


Go through the setup instructions for TypeScript to install the necessary dependencies:



Install assignment dependencies:

$ yarn install

Making the test suite pass

Execute the tests with:

$ yarn test

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by changing xit to it.


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Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


import accumulate from './accumulate'

describe('accumulate()', () => {
  it('accumulation empty', () => {
    const accumulator = (e: number): number => e * e
    expect(accumulate([], accumulator)).toEqual([])

  it('accumulate squares', () => {
    const accumulator = (n: number): number => n * n
    const result = accumulate([1, 2, 3], accumulator)
    expect(result).toEqual([1, 4, 9])

  it('accumulate upcases', () => {
    const accumulator = (word: string): string => word.toUpperCase()
    const result = accumulate('hello world'.split(/\s/), accumulator)
    expect(result).toEqual(['HELLO', 'WORLD'])

  it('accumulate reversed strings', () => {
    const accumulator = (word: string): string => word.split('').reverse().join('')
    const result = accumulate('the quick brown fox etc'.split(/\s/), accumulator)
    expect(result).toEqual(['eht', 'kciuq', 'nworb', 'xof', 'cte'])

  it('accumulate recursively', () => {
    const result = accumulate('a b c'.split(/\s/), (char: string) => accumulate('1 2 3'.split(/\s/), (digit: string) => char + digit))

    expect(result).toEqual([['a1', 'a2', 'a3'], ['b1', 'b2', 'b3'], ['c1', 'c2', 'c3']])
type ACC = string|number;
// Ideally this would use generics, but I'm working on the assumption I shouldn't
// be changing the tests to add type assertions
type ACCUMULATOR = (acc: any) => any;

export function accumulate(acc: ACC[], accumulator: ACCUMULATOR): ACC[] {
    return acc.map((val) => accumulator(val));

export default accumulate;

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