NerdWallet released our first cross platform mobile app at the start of 2017, about six months ago. We choose to use React Native, and we are still using 99% of the same code base for the iOS and Android apps. What have we learned so far?

Initial reasoning

We initially chose React Native because it was the only way to hit our goals. We didn’t have any native mobile engineers, and we had been asked to deliver an iOS and Android app from scratch in just five weeks. We also happened to have very solid React experience on the team. Additionally, the app overlapped very nicely with some features we already had on the web, and we thought we could get some code re-use there.

Code Reuse Deep Dive

In our case, we started with an intuition that 50% of the mobile app codebase could actually be NPM modules re-used from our web stack. Plus, we got the obvious savings of 100% code re-use between the iOS and Android app code.

After 6 months, we can look at our actual code re-use to get a better sense of where we actually ended up. Our current app has 26,000 lines of JavaScript. The NPM modules that we include from our web app stack total 40,000 lines of JavaScript. In practice, the maintenance of that shared code is virtually 100% offloaded to other teams.


Here is what it might look like if we had two separate native app code bases.


If you think of an app codebase as 2 unit of library code and 1 unit UX code, our app is about 1.2 units of code that we actually maintain. Writing two native apps would be about 6 units. The most interesting learning for me was just how much logic in an app is not related directly to the UI, as well as how easy it is to re-purpose NPM modules targeted at web.

Note: if you’re interested in reading more about leveraging NPM modules in React Native, see @parshap’s slides.


We have run into a number of areas where we are making compromises on the user experience. It’s not totally clear yet which of these are legitimate things that React Native is not good at, and which are due to us as a team not having focused on them.


We have seen persistent issues with app performance, particularly around the time to switch between tabs in the navigation. That issue seems to be due to a bug in our navigation library, react-native-router-flux. But it’s important to call out that performance is largely dependent on getting React basics right. Performance has been good enough that we have not dedicated time to look into it further. In general, we still don’t think there is anything that most apps need to do for which React Native will not perform well, except maybe custom animations.


Basic stock animations like screen transitions are available and perform well out of the box. For custom animations such as fading out a header as the user scrolls down on a list view, you need to write the animation yourself. We initially tried the naive solution where we control style properties from JavaScript. That did not perform well. But it was relatively easy to use the actual bridged animation primitives, at least for simple animations. We still don’t have a good sense of how difficult complex custom animations will be.


Initially we did not use the correct default fonts for each platform. It turned out to be relatively simple to use either San Francisco or Roboto based on the platform. Some font variants that are normally available are not included. This ended up being something that probably took more time to get right than a native app would, but if you pay attention to it the first time it should be smooth.

Biggest risks

The largest risk with React Native is still the immaturity of the platform. Releases come every two weeks with potentially breaking changes. But the larger risk is more existential; even though it’s still building in popularity, there is a chance that React Native fades away in the coming years as the framework ecosystem continues to evolve.


The second largest risk I see is a little more insidious. As we have ramped up the team with engineers who have prior mobile experience, they have begun to point out areas where we are not following best UX practices. Some of these are intentional. It’s very tempting once you have a cross platform app to make UX decisions to optimize for code re-use, not the best user experience. Even more troubling is the times when the engineers themselves don’t realize that a particular design is not following best practice UX; typically due to many of them not having mobile experience.

Question marks

In my mind, the biggest open question is around how these mobile native platforms continue to evolve. Both Android and iOS have their developer conferences this time of year. Neither are expected to announce anything ground breaking in terms of how we build apps. This is a natural trend as the platforms mature; there is less interesting stuff to do. This happened to the PC operating system platforms over the last 30 years. 10 years into mobile, are we starting to see the same? This could be the time when putting an abstraction layer on top starts to make more sense, and actually becomes a winning strategy.