Array#flatten()

readme.md

Hello! :)

» npm t

 PASS  ./flatten.test.js
  flatten
    when input value is not Array
      when input value is undefined
        ✓ returns "undefined" (3ms)
      when input value is null
        ✓ returns "undefined"
      when input value is number
        ✓ returns "undefined" (1ms)
      when input value is string
        ✓ returns "undefined"
      when input value is object
        ✓ returns "undefined"
    when input value is Array
      when array is empty
        ✓ returns flattened array (1ms)
      when array is flat
        ✓ returns flattened array
      when array is not a flat
        ✓ returns flattened array
        ✓ returns flattened array

Test Suites: 1 passed, 1 total
Tests:       9 passed, 9 total
Snapshots:   0 total
Time:        0.775s, estimated 1s
Ran all test suites.

flatten.js

export const flatten = (arr) => {
  if (!(arr && Array.isArray(arr))) {
    return undefined;
  }

  return arr.reduce(
    (acc, currentValue) => (
      acc.concat(Array.isArray(currentValue)
        ? flatten(currentValue)
        : currentValue)
    ),
    []
  );
};

export default flatten;

flatten.test.js

import { flatten } from './flatten';

describe('flatten', () => {
  let result;
  let inputValue;

  beforeEach(() => result = flatten(inputValue));

  describe('when input value is not Array', () => {
    describe.each`
      input | type
      ${undefined} | ${undefined}
      ${null} | ${null}
      ${123} | ${'number'}
      ${'iHopeYouWillSeeIt'} | ${'string'}
      ${{ a: 1 }} | ${'object'}
    `('when input value is $type', ({ input, type }) => {
      beforeAll(() => inputValue = input);

      it('returns "undefined"', () => {
        expect(result).toBeUndefined();
      });
    });
  });

  describe('when input value is Array', () => {
    describe.each`
      input | comment | expectedValue
      ${[]} | ${'empty'} | ${[]}
      ${[1, 2, 3]} | ${'flat'} | ${[1, 2, 3]}
      ${[1, [2]]} | ${'not a flat'} | ${[1, 2]}
      ${[1, [[2]]]} | ${'not a flat'} | ${[1, 2]}
    `('when array is $comment', ({ input, comment, expectedValue }) => {
      beforeAll(() => inputValue = input);

      it(`returns flattened array`, () => {
        expect(result).toEqual(expectedValue);
      });
    });
  });
});

package.json

{
  "name": "flatten",
  "version": "1.0.0",
  "description": "flatten for Array",
  "main": "index.js",
  "scripts": {
    "test": "jest"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "@babel/core": "^7.4.4",
    "@babel/preset-env": "^7.4.4",
    "@types/jest": "^24.0.12",
    "babel-jest": "^24.7.1",
    "jest": "^24.7.1"
  },
  "jest": {
    "clearMocks": true,
    "testEnvironment": "node"
  },
  "babel": {
    "presets": [
      [
        "@babel/preset-env",
        {
          "targets": {
            "node": "current"
          }
        }
      ]
    ]
  }
}