At this moment, the current ECMA-Script 5.x implementations do not support this functionality. However, there is anticipation for the new ECMA-Script 6 (ES-Harmony) standard which will offer native module loading without the need for third-party libraries.
If you are interested, you can access working drafts of the upcoming standard here, or read informative blog posts like this one that discuss how this feature will operate in the near future.
Alternatively, you have the option to build from the ground up what RequireJS has already accomplished, avoiding the incorporation of third-party code into your project. However, reinventing the wheel may not be the most efficient use of your time when others have already paved the way!