js array flatten

flatten.js

const assert = require('assert');

function flatten(array) {
  // Return argument if it isn't an array
  if (!Array.isArray(array)) {
    return array;
  }

  const result = [];

  // I should deep copy array here since function parameters should be immutable,
  // but it's kind out of scope for this task
  let nodes = array;
  let node;

  if (!nodes.length) {
    return result;
  }

  node = nodes.pop();

  do {
    if (Array.isArray(node)) {
      nodes.push.apply(nodes, node);
    } else {
      result.push(node);
    }
  } while (nodes.length && (node = nodes.pop()) !== undefined);

  result.reverse();

  return result;
}

[
  {
    actual: flatten([[1, 2, [3]], 4]),
    expected: [1, 2, 3, 4]
  },
  {
    actual: flatten([1, [2]]),
    expected: [1, 2]
  },
  {
    actual: flatten([1, 2, 3, 4]),
    expected: [1, 2, 3, 4]
  },
  {
    actual: flatten([[[[]]], [[]], []]),
    expected: []
  },
  {
    actual: flatten(null),
    expected: null
  },
  {
    actual: flatten(5),
    expected: 5
  }
].forEach(testCase => assert.deepEqual(testCase.actual, testCase.expected));