Recursively flattens arbitrarily nested arrays.

flatten-array.js

/**
 * Recursively flattens arbitrarily nested arrays.
 *
 * @param {Array} array The array to flatten.
 * @param {Array} [result=[]] The initial result value.
 * @returns {Array} Returns the new flattened array.
 */
function flattenArray(array, result = []) {
  if (!Array.isArray(array)) throw new TypeError('Called on non-array.');
  array.forEach(item => {
    if (Array.isArray(item)) {
      // Recursively flatten array.
      flattenArray(item, result);
      return;
    }
    result.push(item);
  });
  return result;
}

export default flattenArray;

flatten-array.spec.js

import flattenArray from '../../src/flatten-array';

describe('flattenArray', () => {
  it('it should flatten simple nesting', () => {
    const nestedArray = [1, 2, 3, [4]];
    const flatArray = flattenArray(nestedArray);
    expect(flatArray).to.be.an('array');
    expect(flatArray).to.deep.equal([1, 2, 3, 4]);
  });

  it('it should flatten complex nesting', () => {
    const nestedArray = [[1,2,[3]],4];
    const flatArray = flattenArray(nestedArray);
    expect(flatArray).to.be.an('array');
    expect(flatArray).to.deep.equal([1, 2, 3, 4]);
  });

  it('it should throw if passing a non array', () => {
    expect(flattenArray).to.throw(TypeError);
    expect(flattenArray.bind(null, 123)).to.throw(TypeError);
    expect(flattenArray.bind(null, [1, 2, 3])).not.to.throw(TypeError);
  });
});