TypeScript Generic Constraints in Depth: `extends`, `keyof`, and the Patterns That Prevent Runtime Errors

Short summary

A deep dive into TypeScript generic constraints using `extends` and `keyof` to prevent runtime errors. The article explains how unconstrained generics provide no safety, while constrained generics encode domain rules into the type system, shifting bugs from runtime to compile time. It covers practical patterns like `<T, K extends keyof T>` for type-safe property access, conditional types, and mapped types for building robust APIs.

• •Unconstrained generics accept anything and crash at runtime; `extends` constraints restrict type parameters to specific shapes
• •`keyof` combined with `extends` creates compile-time proof that a key exists on an object
• •Patterns like `<T, K extends keyof T>` form the foundation for type-safe factories, builders, and API clients

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