recursive function to flatten a js Array

steamroller.js

function steamroller(arr) {  

  for (var i = 0; i < arr.length; i++) {    
    if(Array.isArray(arr[i])) {
      // recurse: replace this array with all its elements
      // for instance, if arr is [1, [2, 3], 4]
      // then we want to return  [1,  2, 3,  4]
      var left  = arr.slice(0, i);
      var right = arr.slice(i + 1);
      return steamroller(
        left.concat( arr[i] ).concat(right)
      );
    }
  }
  
  // base case: we haven’t returned until this point, which
  // means that none of the elements of arr are arrays, so
  // arr is flat.
  return arr;
}

passes all tests on http://www.freecodecamp.com/challenges/bonfire-steamroller