Why Referential Transparency Makes TypeScript Code Easier for Humans and AI to Read

A software engineer auditing a mature TypeScript backend of roughly 1,500 source files argues that referential transparency — the ability to replace an expression with its value without changing program behavior — is the most practical principle in functional programming. The audit covered a NestJS application using pipe, ReaderTaskEither, Zod, and Prisma, where business logic had deliberately shifted toward typed functional patterns. The author contends that referentially transparent code reduces what a reviewer must know by making behavior local and dependencies explicit, rather than hidden in runtime state or object history. This benefit extends beyond human developers: AI coding agents operating within finite context windows also struggle when the meaning of a line depends on invisible history. The article uses concrete TypeScript examples — including pure functions versus UUID generators and class-based dependency injection — to illustrate where transparency breaks down and why that matters in production systems.