TypeScript

Describe data with types precise enough to rule out wrong values, then turn a value of unknown shape, such as a request body, into one TypeScript trusts, one check at a time.

Describe the shape of an object: required, optional and readonly properties, one interface extending another, methods an object may have, and records with fixed keys. Then see that TypeScript compares shapes, not names.

Write code once for every type it can hold, without giving up the checks: generic functions whose output type follows their input, keyof and indexed access types, and a generic repository constrained to things with an id, the shape of every repository in NestJS.

Derive types from types instead of writing them twice: Omit for what a client sends to create, Partial for what it sends to update, Pick for a summary, and a mapped type of your own. The same idea as NestJS's PartialType and OmitType, which apply these to DTO classes.

A service that receives its store through its constructor and hides what it keeps, the shape of every provider in NestJS: parameter properties, private and readonly, static members, getters, and a class that implements an interface.

A server waits on everything: databases, other services, timers. Run independent work at the same time, collect results whether each succeeded or not, give up after a deadline, and try again after a failure, with types that say what each promise carries.

@Controller('cats') and @Get(':id') are ordinary functions that run when a class is defined. Write your own: a class decorator and method decorators that record routes, and one that wraps a method to log its calls.

How NestJS knows what to pass to your constructor: the compiler records each decorated class's parameter types as metadata, and a container reads them to build every class with its dependencies. Build that container.