Function to flatten any array, no matter how many dimensions it has. I came up with it messing around after I got stumped on a test question re: flattening multi-dimensional arrays.

flatten().js

function flatten (arr) {
  // create a property on the function object that acts as a 
  // 'static'-ish variable array to hold the output 
  // and another for the length of the original array (you'll see why later)
  // this is pretty clever and not my idea but
  flatten.output = flatten.output || [];
  flatten.origLength = flatten.origLength || arr.length;
  // loop over array
  for (var i = 0; i < arr.length; i++) {
    // if arr[i] is itself an array
    if (arr[i] instanceof Array) {
      // call flatten passing in arr[i] in order to go down into the nested array and flatten that
      // this will keep happening until the function gets to the 'bottom' of that 'nest'
      flatten(arr[i]);
    // otherwise
    } else {
      // push arr[i] onto flatten.output array
      flatten.output.push(arr[i]);
    }
  }
  // store the output property in a new variable since I'm going to be deleting it in a second
  var output = flatten.output
  // if i === length of the original array, 
  // that means I'm at the end of the WHOLE multi-dimensional array
  // and not just the end of one of the nested arrays
  if (i === flatten.origLength) {
    // delete the properties so the function is re-usable
    // if I don't do this, the next time the function is called these properties still exist
    // and that obviously screws everything up
    delete flatten.origLength;
    delete flatten.output;
  }
  // return output array
  return output;
}

var test = [1,2,3,[4,5,[6,7,[8,[9,10]]]],'one','two',['dog',5,['cat','bird']],[{key: 'value'},[{key: 'value', key2: 'value2', foo: function () {return 'bar';}, nest: {nest: 'nested'}, nestArr: ['whoa','man']}]]];

flatten(test);
/*
  [ 1,
    2,
    3,
    4,
    5,
    6,
    7,
    8,
    9,
    10,
    'one',
    'two',
    'dog',
    5,
    'cat',
    'bird',
    { key: 'value' },
    { key: 'value',
      key2: 'value2',
      foo: [Function],
      nest: { nest: 'nested' },
      nestArr: [ 'whoa', 'man' ] } ]
  */