Flatten multidimensional arrays

flatten.js

function flatten(arr) {
  return arr.reduce(function (flat, toFlatten) {
    return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
  }, []);
}

function assertFlattenEquals(actual, expected, testName) {
  if (JSON.stringify(actual) === JSON.stringify(expected)) {
    console.log('\'' + testName + '\' test passed!');
  } else {
    console.log('FAILED: ' + testName + ', ' + actual + ' expected to be ' + expected);
  }
}

var actual = flatten([[1,2,[3]],4]);
var expected = [1, 2, 3, 4];

assertFlattenEquals(actual, expected, 'The array flattens correctly');