TypeScript Cheatsheet

Power user.

Overview

TypeScript is the typed superset of JavaScript that backs most modern frontend and Node.js work. Five surfaces cover almost every operational use: static type checking, type inference, generics for reusable abstractions, discriminated unions for exhaustive pattern matching, and the type-level toolkit (mapped, conditional, template literal types) for library-grade type safety.

• Static type checking. Compile-time type errors. Catches whole classes of bugs that JavaScript would only surface at 3am.
• Type inference. The compiler infers types from usage. Most of the file does not need explicit annotations.
• Generics. Reusable type-parameterised code. Type-safe abstractions without sacrificing inference.
• Discriminated unions plus type-level programming. Sum types via string literal kinds; mapped and conditional types for library-grade type safety.

The approach

Five patterns carry most of the day-to-day operational weight. Memorising them moves a team from “TypeScript adds friction” to “TypeScript catches our bugs at compile time”.

• "strict": true. Enable strict mode. The single biggest source of bugs caught at compile time.
• type versus interface as convention. Both work. Pick one and stop debating it in code reviews.
• Discriminated unions. { kind: "a"; ... } | { kind: "b"; ... } with exhaustive switches. The compiler enforces all cases handled.
• as const plus satisfies. Literal-type assertions for narrow types; satisfies validates without widening.

Why this compounds

Each typed module deposits intent the team can read off the type signatures. Refactoring becomes safer; cross-team collaboration improves because the contract lives in code; new engineers onboard faster because the types are documentation.

• Compile-time safety. Bugs caught at tsc never reach production. Runtime bug count drops measurably.
• Refactoring becomes routine. Type-aware tooling supports rename, extract, and signature changes safely.
• Collaboration improves. Types document interfaces between teams. Slack threads about “what does this return” disappear.
• Year-one investment, year-two habit. First year is fluency-building. By year two, the team writes idiomatic TypeScript without thinking.