Most digital products eventually need three things: a mobile app, a web app, and a backend feeding them both. The traditional way to build this is three technologies, three codebases, and effectively three teams, with every feature implemented twice and every change coordinated across all of them. We build all three in one language. This post explains what that means concretely and why it's not just a developer preference, it directly affects your budget, your timeline, and how safely your product can evolve.
The stack, in one paragraph
Web app in Next.js. Mobile app in React Native. Backend in Nest.js. All three are TypeScript, one language, end to end. They live in a single repository (a 'monorepo'), with shared packages sitting between them: one for types (the definitions of your data), one for business logic (the rules of your product), one for utilities. The web app, the mobile app, and the server all import from the same shared source instead of maintaining their own copies.
What 'shared types' actually prevents
Here's the everyday disaster in three-technology projects: the backend team renames a field, say, 'price' becomes 'priceInCents'. The web team updates their copy. The mobile team doesn't get the memo. The mobile app now shows every product as 100× cheaper, and nobody notices until a customer does. This class of bug exists because each codebase keeps its own private description of the same data, and those descriptions drift apart silently.
With shared types, there is exactly one description of what a 'Product', an 'Order', or a 'User' looks like, and all three apps import it. If the backend changes a field, the web and mobile code that depends on it fails to compile immediately, on the developer's machine, with a precise list of every affected spot. The mismatch is caught in minutes at the desk instead of in production by your customers.
“Shared types turn 'the app broke because the server changed' from a production incident into a red underline in the editor.”
Shared logic: your business rules, written once
Types are the foundation; shared logic is where it gets valuable. Take something ordinary: how an order total is calculated, items, discount rules, delivery fee, VAT. In a traditional setup this logic is written three times, and sooner or later the web says €47.90 while the app says €48.20, and someone spends a day finding out why. In our setup it's one function in the shared package, used by the web, the mobile app, and the server. It cannot disagree with itself. The same applies to validation rules ('what counts as a valid booking?'), formatting, and permissions.
What this means for your budget and timeline
- One team, not three: the same developers work across web, mobile, and backend, no coordination overhead, no 'waiting for the backend team'
- Features cost less: the logic layer is written once; only the screens are built per platform
- Changes are safer: the compiler finds every place a change touches, across all three apps, automatically
- Onboarding is faster: a new developer learns one language and one repository, not three ecosystems
- Starting with one platform stays cheap: launch mobile-first, and when the web app comes later, it inherits the types and logic on day one
The honest limits
So everything is shared? No, and you wouldn't want it to be. The screens themselves are built separately, because a good mobile interface and a good web interface are genuinely different: different navigation, different gestures, different layouts. What's shared is everything underneath the surface, the data definitions, the rules, the calculations. In practice that's typically 30–50% of the total code, and it's precisely the part where bugs are most expensive. We share the foundations, not the wallpaper.
Why this should matter to a non-technical founder
You may never look at the code, but you'll feel this architecture in three ways: your quotes are lower because features aren't built twice; your product ships faster because one team moves without hand-offs; and two years in, your codebase is still one coherent system instead of three drifting ones, which is exactly what a technical co-founder, an investor's due diligence, or an acquiring company will look at. If you're planning a product that will someday live on both web and mobile, this is one of the highest-leverage decisions you'll make before a single screen is designed. Happy to walk you through how it would apply to your specific product, no jargon, promise.
