array.flatten.js

'use strict';

/**
 * Flattens a deep-nested array of integers (numbers)
 * @param {Number[]} inpArray A possibly deep-nested array of numbers.
 *    The level of nesting is limited by the maximum stack size of the
 *    JavaScript engine (the lowest we have seen is IE6 at 1130)
 * @return {Number[]} Returns a flattened version of the input array
 */
function flatten (inpArray) {
  const outputArray = [];
  
  // We will do this optimistically, and catch any errors.
  try {
    inpArray.forEach(item => {
      if (typeof item === 'number') {
        // Push this item to the output array and move to the next item
        outputArray.push(item);
        return;
      }
      
      // This is not a number. We assume it's an array
      // We will attempt to flatten the array recursively, optimistically
      const flt = flatten(item);
      
      flt.forEach(innerItem => {
        outputArray.push(innerItem);
      });
    });
  } catch (e) {
    throw new Error('Supplied parameter is not a nested array of numbers');
  }
  
  return outputArray;
}

module.exports = flatten;

array.flatten.spec.js

'use strict';

// This test makes use of mocha+chai. It is assumed you have installed mocha and chai through npm. Use BDD.
const chai = require('chai');
const expect = chai.expect;
const flatten = require('./array.flatten.js');

describe('flatten', () => {
    const testFn = (...args) => () => { flatten.apply(null, args); };
    
    it('requires first parameter to be an array', () => {
        expect(testFn()).to.throw(Error);
        expect(testFn(1)).to.throw(Error);
        expect(testFn(29.34)).to.throw(Error);
        expect(testFn('a string')).to.throw(Error);
        expect(testFn(null)).to.throw(Error);
        expect(testFn(undefined)).to.throw(Error);
        expect(testFn({})).to.throw(Error);
        expect(testFn(NaN)).to.throw(Error);
        expect(testFn(true)).to.throw(Error);
        expect(testFn(new Date())).to.throw(Error);
        expect(testFn(4, [1, 2, 3])).to.throw(Error);
        expect(testFn([1, 2, 3])).to.not.throw(Error);
        expect(testFn([1, 2, 3], 'additional parameter')).to.not.throw(Error);
    });

    it('requires the provided array to contain only numbers', () => {
      expect(testFn([1, 2, 3, 'a'])).to.throw(Error);
      expect(testFn([1, 2, 3, 'b', 'a'])).to.throw(Error);
      expect(testFn(['string'])).to.throw(Error);
      expect(testFn([true])).to.throw(Error);
      expect(testFn([[1, 2, 3, 4], ['a', 'string']])).to.throw(Error);
      expect(testFn([1, 2, 3, [6, ['a']]])).to.throw(Error);
      expect(testFn([1, 2, 3, undefined])).to.throw(Error);
      expect(testFn([1, 2, 3, null])).to.throw(Error);
      expect(testFn([1, 2, 3, NaN])).to.not.throw(Error);
      expect(testFn([1, 2, 3, {}])).to.throw(Error);
      expect(testFn([1, 2, 3, {forEach: ''}])).to.throw(Error);
      expect(testFn([1, 2, 3, /a-z/])).to.throw(Error);
    });
    
    it('returns expected values', () => {
      expect(flatten([1, 2, 3])).to.eql([1, 2, 3]);
      expect(flatten([1, 2, 3, 1])).to.eql([1, 2, 3, 1]);
      expect(flatten([1, [2], 3])).to.eql([1, 2, 3]);
      expect(flatten([1, 2, 3.9])).to.eql([1, 2, 3.9]);
      expect(flatten([1, [2, 3]])).to.eql([1, 2, 3]);
      expect(flatten([[1,2,[3]],4])).to.eql([1, 2, 3, 4]);
      expect(flatten([[1,2,[3]],4, [5, [6], [7, [8], [9]]]])).to.eql([1, 2, 3, 4, 5, 6, 7, 8, 9]);
    });
});