Recursive Reimplementation of JSON.stringify

stringify.js

// recursive reimplementation of JSON.stringify
var stringifyJSON = function(obj) {

  // null
  if (obj === null) {
    return "null";
  }

  // unstringifiable - functions and undefined
  if (obj === undefined || obj.constructor === Function) { return; }

  // strings
  if (obj.constructor === String) {
    return '"' + obj + '"';
  }

  // arrays
  if (obj.constructor === Array) {
    if (obj.length) {
      var partialJSON = [];

      for (var i = 0; i < obj.length; i++) {
        partialJSON.push(stringifyJSON(obj[i])); // recursion
      }

      return '[' + partialJSON.join(",") + ']';
    } else {
      return '[]';
    }
  }

  // objects
  if (obj.constructor === Object) {
    var keys = Object.keys(obj);
    if (keys.length) {
      var partialJSON = '';

      for (var i = 0; i < keys.length; i++) {
        var key = keys[i];

        if (!key || obj[key] === undefined || typeof key === 'function' || typeof obj[key] === 'function') {

        } else {
          if (i === keys.length - 1) {
            partialJSON += stringifyJSON(key) + ':' + stringifyJSON(obj[key]); // recursion
          } else {
            partialJSON += stringifyJSON(key) + ':' + stringifyJSON(obj[key]) + ','; // recursion
          }
        }
      }
      return '{' + partialJSON + '}';
    } else {
      return '{}';
    }
  }

  // everything else (numbers, booleans, etc.)
  return obj.toString();

};

This implementation cannot stringify nested object.