flatten.js

function flatten(array) {
  var output = [];
  // iterate through all items in argument array 
  for (var j = 0; j < array.length; j++) {
    var value = array[j];
    // base case: value is not an array, so just push to output
    if (!Array.isArray(value)) {
      output.push(value);
    } else {
      // recursive case: if value is an array, flatten it to a 1D array.
      //  i.e. [[1]] will become [1] and store the flattened array in original
      //  'value' variable
      value = flatten(value);
      // because value is now a 1D array, you can push each item in it to
      // output
      for (var i = 0; i < value.length; i++) {
        output.push(value[i]);
      }
    }
  }
  // flatten will always return a 1D array i.e. [1,2,3], which allows us
  // to recursively call it 
  return output;
}