Hermes, a JavaScript engine optimized for running React Native on Android

Interesting announcement from Facebook:

Hermes is an open-source JavaScript engine optimized for running React Native apps on Android. For many apps, simply enabling Hermes will result in improved start-up time, decreased memory usage and smaller app size. At this time Hermes is an opt-in React Native feature, and this guide explains how to enable it.

Here’s a side-by-side comparison recording by @nparashuram of one and the same app . The one of the left is with Hermes, the one on the right without:

To enable Hermes, simply edit your android/app/build.gradle file and set enableHermes to true (requires React Native 0.60.2 or newer):

project.ext.react = [
    entryFile: "index.js",
    enableHermes: true  // clean and rebuild if changing
]

One cd android && ./gradlew clean later and you’re good to go 🙂

Hermes Source (GitHub) →
React Native: Enabling Hermes →