Getting to know the Hermes engine

Hermes was brought to the React Native community at the React Native EU conference in 2019. Back then, it was already in production in Facebook’s apps for more than a year. It is completely built with mobile in mind, which changes the architectural approach completely. The following figure shows how a modern JS engine works.

Figure 8.1 – Modern JS engine pipeline (inspired by Tsvetan Mikov)

When creating and building JavaScript code, usually there is some transcompiling done to backward-compatible JS code and some JS code minification. This minified JS bundle is then sent to a device and gets executed. JS engines such as JavaScriptCore or V8 try to optimize the execution using just-in-time compilation, which, as described before, is a quite complex process and may store and optimize the wrong code statements. Hermes changes the way this is done completely.

The following figure shows how optimization and compilation...