TypeScript

This plan focuses on the TypeScript features that matter most in interviews and production: the type system, generics, utility types, type narrowing, and strict mode. Sessions progress from basic annotations to advanced type-level programming. Skipped: TypeScript decorators (still experimental), namespaces (legacy), tsserver plugin development, and compiler API. These are niche topics that rarely appear in standard interviews. TypeScript Mental Model and Setup TypeScript is a typed superset of JavaScript that compiles to plain JavaScript. The key mental model: TypeScript exists only at compile time. At runtime, it's JavaScript — all type information is erased. Why TypeScript Exists The Type System at a Glance TypeScript's type system is structural (duck typing), not nominal (name-based). A type is compatible with another if it has all the required properties. Basic Types tsconfig.json Essentials Type Inference TypeScript infers types — you don't always need to annotate: Interview Q&A Q: What is the difference between and ? A: Both can hold any value. The difference is what you can do with them. bypasses all type checking — you can call methods, access properties, pass it anywhere without error. requires you to narrow the type (via type guards, instanceof, typeof) before using it. is the type-safe alternative to for truly unknown values. Q: Is TypeScript a runtime type system? A: No. TypeScript's type system is entirely compile-time. After compilation, all type annotations, interfaces, and generics are erased. At runtime, you have plain JavaScript. This means TypeScript cannot validate data from external sources (APIs, databases) at runtime — you need runtime validation libraries like Zod or io-ts for that. Q: What does enable? A: It enables a group of strict checks: (null/undefined not assignable to other types), , , , , , and . The most impactful is — without it, and can be assigned to any type, defeating much of TypeScript's safety. Common Mistakes Python Implementation Java Implementation