Flattening An Array of Arbitrarily Nested Arrays

flatten.js

/**
 * Flattens an array of arbitrarily nested arrays
 * @param  {Array} sourceArray [array of arbitrarily nested arrays]
 * @return {Array}             [flattened array]
 */
function flatten(sourceArray) {
  return sourceArray.reduce(
    (flattenedArray, currentArrayElement) => {
      // append current array element to end of array
      return flattenedArray.concat(
        Array.isArray(currentArrayElement) ?
          flatten(currentArrayElement) : // recursively flatten, if necessary
          currentArrayElement
      );
    },
    [] // initialize to empty array
  );
}

module.exports = flatten;

flatten.spec.js

/*
 * Simple test cases to provide a sanity check
 * using assert (rather than mocha+chai or jasmine), for simplicity
 */

const assert = require('assert');
const flatten = require('./flatten');

const testCases = [
  {
    test: [1],
    expected: [1]
  },
  {
    test: [[1]],
    expected: [1]
  },
  {
    test: [[[[[1]]]]],
    expected: [1]
  },
  {
    test: [1, 2, 3, 4],
    expected: [1, 2, 3, 4]
  },
  {
    test: [[1, 2, [3]], 4],
    expected: [1, 2, 3, 4]
  },
  {
    test: [[[1, 2], [3]], 4],
    expected: [1, 2, 3, 4]
  },
  {
    test: [[[[[[1]]]]], 2, 3, 4],
    expected: [1, 2, 3, 4]
  },
  {
    test: [],
    expected: []
  },
  {
    test: [[]],
    expected: []
  },
  {
    test: [[], []],
    expected: []
  }
];

testCases.forEach(testCase => {
  const actual = flatten(testCase.test);
  assert.equal(JSON.stringify(testCase.expected), JSON.stringify(actual));
});