function serialize(val, isArrayProp) {
  var i, max, str, keys, key, propVal, toStr;
  if (val === true) {
    return "!0";
  }
  if (val === false) {
    return "!1";
  }
  switch (typeof val) {
    case "object":
      if (val === null) {
        return null;
      } else if (val.toJSON && typeof val.toJSON === "function") {
        return serialize(val.toJSON(), isArrayProp);
      } else {
        toStr = objToString.call(val);
        if (toStr === "[object Array]") {
          str = "[";
          max = val.length - 1;
          for (i = 0; i < max; i++) {
            str += serialize(val[i], true) + ",";
          }
          if (max > -1) {
            str += serialize(val[i], true);
          }
          return str + "]";
        } else if (toStr === "[object Object]") {
          // only object is left
          keys = objKeys(val).sort();
          max = keys.length;
          str = "{";
          i = 0;
          while (i < max) {
            key = keys[i];
            propVal = serialize(val[key], false);
            if (propVal !== undefined) {
              if (str) {
                str += ",";
              }
              str += '"' + key + '":' + propVal;
            }
            i++;
          }
          return str + "}";
        } else {
          return JSON.stringify(val);
        }
      }
    case "function":
    case "undefined":
      return isArrayProp ? null : undefined;
    case "string":
      return val;
    default:
      return isFinite(val) ? val : null;
  }
}
A custom Babel options serializer

Intentional deviation from JSON.stringify:
1. Object properties are sorted before seralizing
2. The output is NOT a valid JSON: e.g.
   The output does not enquote strings, which means a JSON-like string '{"a":1}'
   will share the same result with an JS object { a: 1 }. This is not an issue
   for Babel options, but it can not be used for general serialization purpose
3. Only 20% slower than the native JSON.stringify on V8

This function is a fork from https://github.com/nickyout/fast-stable-stringify
@param {*} val Babel options
@param {*} isArrayProp
@returns serialized Babel options