JavaScript Engines and Hermes

React Native runs on JavaScript and, as mentioned in Chapter 2, Understanding the Essentials of JavaScript and TypeScript, JavaScript needs a JavaScript engine to interpret and/or transform the code into executable machine code. There is no exception to this for React Native.

While there are quite a lot of different JS engines out there, only a few are used in React Native projects. This is due to the quite complex process to change the JS engine as well as the new Hermes engine, which is an engine developed for React Native and is going to be the default soon. Nevertheless, it is important and helpful to understand the different possible engines with their strengths and weaknesses.

In this theoretical chapter, we will cover the following topics:

• Understanding JavaScript engines
• Getting to know the Hermes engine
• Comparing key metrics