What is javascript runtime..?

Maybe it’s referring to Google’s V8 engine.

• It is an engine for processing JavaScript in the browser and is used by Google Chrome.
• It’s open source.
• And it’s written in C++.
• It works on several platforms including mobile and embedded devices.

For more information see: https://code.google.com/p/v8/

If you google for “chrome javascript runtime“, you will get all these links to V8.