flatten.js

function flatten(arr) {
  let flattened = [];

  for(let i in arr) {
    if(Array.isArray(arr[i])) {
      flattened = flattened.concat(flatten(arr[i]));
    } else {
      flattened.push(arr[i]);
    }
  }

  return flattened;
}

module.exports = flatten;

flatten.spec.js

const expect = require('chai').expect;

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

const tests = [
  [1, 2, 3, 4],
  [1, [2, 3], 4],
  [1, [2], [3, 5, 6, [11, 22, 44]], 4],
];

const expected = [
  [1, 2, 3, 4],
  [1, 2, 3, 4],
  [1, 2, 3, 5, 6, 11, 22, 44, 4],
];

describe('flatten', () => {
  it('should flatten', () => {
    for(let i in tests) {
      expect(flatten(tests[i])).to.deep.eq(expected[i]);
    }
  })
});

package.json

{
  "name": "flatten",
  "version": "1.0.0",
  "main": "flatten.js",
  "license": "MIT",
  "dependencies": {
    "chai": "^4.2.0",
    "mocha": "^5.2.0"
  },
  "scripts": {
    "test": "mocha -R spec ./flatten.spec.js"
  }
}