toJson

/*
* @param {Object|Array|Date|string|number} obj Input to be serialized into JSON.
* @param {boolean=} pretty If set to true, the JSON output will contain newlines and whitespace.
* @returns {string|undefined} JSON-ified string representing `obj`.
*/

function toJson(obj, pretty) {
  if (typeof obj === 'undefined') return undefined;
  return JSON.stringify(obj, toJsonReplacer, pretty ? '  ' : null);
}

toJsonReplacer

function toJsonReplacer(key, value) {
  var val = value;

  if (typeof key === 'string' && key.charAt(0) === '$') {
    val = undefined;
  } else if (isWindow(value)) {
    val = '$WINDOW';
  } else if (value &&  document === value) {
    val = '$DOCUMENT';
  } else if (isScope(value)) {
    val = '$SCOPE';
  }

  return val;
}