If your project has a different folder structure than the React Native default structure, you need to override hermesCommand in your config as it is done here Make sure to test a deployment with Codepush. CodePush users: Hermes add a step when deploying with codepush: 1st Step: Bundle JS > 2nd Step (new): Convert to Hermes Bytcode, only the new version from appcenter-cli supports this. If you're using crash reporting (Sentry for instance) with custom build types (for AppCenter deployment for instance), there's an issue with crash reporting source maps on app center builds that can be fixed with this PRĥ. If you use TypeScript, make sure you target "lib": in the tsconfig.json. Hermes implements the JavaScript norm ES6. It is used in dependencies such as lingui (for pluralization), date-fns-tzģ. It is used in dependencies such as immer, or Intl support will be included in React Native 0.65 and above. Proxy support is included in React Native 0.64 and above. However it can come with a bit of gotchas to double-check:ġ. New! Read this article to know more about Hermes!Įnabling Hermes is simple and can be done following those two steps Hermes already does the parsing and compiling into bytecode at build time, which means your file will contain bytecode and not JavaScript (try to open it after switching to Hermes to see the difference), which will be much faster to start. If you use Hermes however, JavaScript is no longer packaged with your app. The JavaScript file is located at assets/. If you want to take a look at how much JavaScript you have in your app, you can download the APK file and unzip it (use unzip myapp.apk on MacOS). In our app for instance, we noticed that our APK included 3MB of JavaScript which needed to be parsed, compiled and executed at the start of the app. One way to measure time lapses locally and in production is with Firebase Performance.īasically, React Native used to come with a disadvantage for large apps in terms of startup performance. Indeed, the play store reports only the native startup time, but the app then needs to initialize the JavaScript and run it before being actually responsive. Measuring the startup time of a React Native app is not as straightforward as reading the vitals on the Play Store. For instance, Amazon found that every 100ms of latency cost them 1% in sales. We could hypothesize that the situation is similar to the web. In this study, the author shows that improving startup time by 53% improved the ratings of the app, and improved conversion rate by 2.2%. There's not a lot of studies done to measure the impact of improving startup time in the context of a mobile app. Google's recommendation is to have your app startup in less than 5 seconds. This sort of poor experience may cause a user to rate your app poorly or even abandon your app altogether. An app with a slow start time doesn't meet this expectation, and can be disappointing to users. Users expect apps to be responsive and fast to load.
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