flatten.js

function isArray(obj) {
  return toString.call(obj) === '[object Array]';
};

function flatten(branch, flat, max, depth) {
  flat = flat || [ ]; max = max || 1;
  depth = ( depth ? depth + 1 : 1);
  for(var i in branch) {
    if(isArray(branch[i])) {
      if((max < 0) || (depth <= max)) {
        flatten(branch[i], flat, max, depth);
      } else { flat.push(branch[i]); }
    } else { flat.push(branch[i]); }
  }
  return flat;
};

var test = [ 1, [ 2, [ 3, 4 ], [ 5 ], 6 ], 7 ];

console.log(flatten(test, [ ], -1, -1))