In simple words, a Javascript engine is the one in charge of interpreting and executing javascript code.

There are several engines on the market today, one of the most popular is V8.

V8 is an Open Source engine used by Google Chrome and Chromium-based browsers and tools.

Today it is the most used Javascript engine.

Who uses?

It is mainly used by the Google Chrome browser but it is also used by large projects such as:

• NodeJS
• MongoDB
• Couchbase
• Electron
• and more

Refs:

https://en.wikipedia.org/wiki/V8_(JavaScript_engine)