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Picture this: you’ve just launched your latest React Native app, and it’s a hit! People love the design, the features, the experience—but there’s one nagging issue. Some users start to complain that your app takes too long to load, especially on older devices or in regions of the world where fast wireless connections are not common.
You check the code, and everything seems fine at first glance. But deep down, you know the truth: something is wrong with your app’s loading process, which might be slowing things down.
App bundle size often impacts application size and performance. Although its influence was greatly reduced with the introduction of Hermes and its ahead-of-time compilation, it wasn’t eliminated completely.
Hermes and ahead-of-time compilation don’t ring any bells? Watch this video to learn more about them.
While parsing is no longer done at runtime, the code still needs to be executed, which may increase memory footprint and startup time. This is especially critical for iOS App Clips and Android Instant Apps, which have strict size limits and require careful optimization to meet platform constraints. It also applies to over-the-air updates, as the larger the bundle, the longer it will take to download—especially in bandwidth-constrained scenarios and regions of the world.
What can go wrong? Many things. You might include a huge library only to use a small portion of it. Other times, your dependencies may require different versions of the same library, and both end up in your bundle, wasting space. It’s not easy to catch all of these issues with the naked eye. And that’s where Expo Atlas comes in.
Enter Expo Atlas
I would compare Expo Atlas to an X-ray machine. It allows you to peek behind the curtain and inspect your React Native app’s bundle, module by module. Think of it as a map that shows you how all the pieces of your app fit together, from external dependencies to internal modules. It’s incredibly useful for understanding exactly what contributes the most to your bundle size.
Here’s how it works: once you enable Expo Atlas, it hooks into the Metro bundler to serialize the dependency tree and later uses it to display detailed insights about your app’s structure. It can break down the bundle, highlight large dependencies, and show you which parts of your app take up the most space. With this information, you can make informed decisions about what to optimize or remove.
Enabling Expo Atlas in an Expo project
Sounds like rocket science? Don’t worry! It’s dead simple to use. If you’re working with an Expo project, all you need to do is set the environment variable EXPO_UNSTABLE_ATLAS and run expo start like this: EXPO_UNSTABLE_ATLAS=true expo start. The last step is to press Shift + M to access additional tools and select the ‘Open expo-atlas’ option. A new web browser window will pop up, giving you access to an interactive diagram and allowing you to explore it as deeply as you want.
Key insights from Expo Atlas analysis
This analysis provides you with valuable insights into key metrics and relationships.
Assess the size of each dependency
You can now analyze your bundle and determine the percentage of total size taken up by each dependency. The same goes for your code—you can check if the new feature you’re about to release is too large.
Understand the purpose of each module
When examining an individual module, you can easily identify why it’s included by seeing which other modules depend on it. Additionally, you can view which modules this particular one relies on.
Compare code before and after transpilation
You can also compare your source code before and after transpilation, which can be especially helpful when debugging tricky issues where something appears off in the output code, but nothing seems unusual in the source code.
This analysis can also be applied to all variants of your application bundle across different platforms.
How does Expo Atlas compare to similar tools?
You’ve probably heard of other tools that also give you insights about your bundle contents. What would make you switch, for example, from Source Map Explorer to Expo Atlas?
The key difference is that Expo Atlas is purpose-built for React Native and integrates seamlessly with the Expo ecosystem. Source Map Explorer doesn’t implement any React Native-specific heuristics and relies solely on source maps. They can be used for peeking into a React Native bundle, but unfortunately, they often lack information about the origins of certain parts of the bundle, such as code generated by Babel plugins. These sections show up as unmapped regions, which can account for as much as 30% of your bundle size. Expo Atlas, on the other hand, is able to resolve these unmapped regions because it uses the dependency graph of Metro—which gives you a far more complete and actionable view of your app’s structure.
That said, Expo Atlas focuses primarily on JavaScript-level insights and doesn’t directly analyze native dependencies or native modules in your app. It’s still a handy tool for anyone focused on building efficient React Native apps.
Using Expo Atlas with vanilla React Native
Now, here’s the catch: Expo Atlas is designed to work seamlessly with Expo projects. If you’re working on a vanilla React Native app, you’re probably thinking, “Great, but what about me?”
Normally, it’s tricky to get Expo Atlas running in a non-Expo environment. That’s because it relies on Expo’s Metro serializer to, well, serialize the dependency graph and expose it for further analysis. But don’t worry, there’s a solution. By recreating Metro’s built-in serializer and chaining it with the Expo Atlas serializer, you can achieve the same outcome in your vanilla React Native app.
Sounds complicated? It doesn’t have to be. If you don’t want to roll up your sleeves and write custom serializers, there’s an easier way: the expo-atlas-without-expo package. This package does it all for you, so the only thing left to do is install it and use the exposed serializer in place of the original one. That’s it! You’ll have access to all the features of Expo Atlas without needing to use the full Expo framework. There are no performance or feature penalties when using this approach.
Why Expo Atlas matters
It’s not easy to build a great application without a way to inspect it from the inside. There are many dangers along the way, just waiting for us to make a mistake and underestimate the power of duplicated or simply large dependencies.
Expo Atlas gives you the opportunity to precisely strike back, without guessing or following your gut. It’s especially important in regions where fast wireless connections are not yet common, as your app size may be a significant culprit for adoption. It also applies to you if you use over-the-air update solutions or work on iOS App Clips or Android Instant Apps.
Whether you’re working in the Expo ecosystem or using vanilla React Native, there’s no excuse for flying blind when it comes to your app’s bundle—and using tools like Expo Atlas for React Native ensures you’re not shipping bloated dependencies or slowing down app startup time.
