JSON generator with circular reference censoring

flatten.js

function flatten(s, stack) {
  if(s === undefined || s === null || (typeof s === 'number' && isNaN(s))) {
    return 'null';
  } else if(s instanceof Date) {
    return flatten(s.toJSON(), stack);
  } else if(typeof s === 'object') {
    stack = stack ? {obj: s, parent: stack} : {obj: s};
    
    if((function cyclic(obj, stack, found) {
          return found || 
            (stack.parent ? cyclic(obj, stack.parent, stack.parent.obj === obj) : false);
        }(s, stack))) {
      return flatten('[circular reference]');
    } else if(s instanceof Array) {
      return ['[', s.map(function(v) {
        return flatten(v, stack);
      }).join(','), ']'].join('');
    } else {
      return ['{', Object.keys(s).filter(function(key) {
        return s[key] !== undefined;
      }).map(function(key) {
        return ['"', key, '"', ':', flatten(s[key], stack)].join('');
      }).join(','), '}'].join('');
    }
  } else if (typeof s === 'string') {
    return ['"', s, '"'].join('');
  } else {
    return s;
  }
}

After I didn't find a satisfying solution to compare simple data objects, and I've found that comparing JSON strings is a very simple and nice way to compare simple data structures, I came up with this solution to generate JSON while sanitizing circular references. I found existing solutions with object re-occurrence checks not ideal as they don't take into account the object hierarchy. This solution is keeping track of the "call stack" with a stack object during the recursive calls and tracing back to parent stacks for circular reference detection.

You can play around with this function here: http://jsbin.com/hefis/3/edit

I did not check the JSON.stringify implementations and therefore don't know if I missed something in the conversion process. Please comment if you have any suggestions.