Flatten array

flatten.js

function arrayEquals(actual, expected) {
  return actual.every(function(element, i) {
    if (Array.isArray(element)) {
      return arrayEquals(element, expected[i]);
    }
    return element === expected[i];
  })
}

function flatten (arr) {
  var result = [];
  for (var i=0; i < arr.length; i++) {
    if(Array.isArray(arr[i])) {
      var nested = flatten(arr[i]);
      result = result.concat(nested);
    } else {
      result.push(arr[i]);
    }
  }
  return result
}

function assert (actual, expected) {
  var result = arrayEquals(actual, expected);
  console.log(result ? 'SUCCESS' : ('FAILURE: ' + JSON.stringify(actual) + ' not equal to ' + JSON.stringify(expected)));
}

var testArr1 = [1, 2, [3, 4]];
var testArr2 = [1, [2, [10, 20], 3, 4], 5, [6, 7]];

assert(flatten(testArr1), [1, 2, 3, 4]); // SUCCESS 
assert(flatten(testArr2), [1, 2, 10, 20, 3, 4, 5, 6, 7]); // SUCCESS

// using language trick to flatten - 1 level deep arrays
assert([].concat.apply([], testArr1), [1, 2, 3, 4]); // SUCCESS 
assert([].concat.apply([], testArr2), [1, 2, [10, 20], 3, 4, 5, 6, 7]); // SUCCESS 
assert([].concat.apply([], testArr2), [1, 2, 10, 20, 3, 4, 5, 6, 7]); // FAILURE