Migrating a Legacy REST API to a Type-Safe NestJS Monorepo
A B2B payments platform needed to replace an aging Node.js API with production-grade TypeScript. We rebuilt from the ground up with NestJS, full TDD coverage, and a shared types package consumed by three frontend applications.
Zero regressions after migration
Test coverage from 12% to 94%
TypeScript-only monorepo with full CI/CD ownership transferred to client team
01The Challenge
The client's existing Node.js API had accumulated years of implicit type assumptions across three consuming frontends. Any change carried regression risk that the team mitigated only with manual QA cycles — slowing releases to once per month. There were no automated tests and no shared type contract between backend and frontend.
02Our Approach
We began with a complete type audit: every API response was modelled as a TypeScript interface in a shared `@company/types` package. We introduced NestJS incrementally alongside the existing Express routes, running both in parallel until full parity was verified by contract tests. TDD drove every new endpoint — no implementation was written before a failing test described its behaviour. The migration ran over three team sprints, with zero downtime.
03Technologies Used
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