Typescript Best Practices

TypeScript Best Practices

Types are documentation that the compiler checks. Write types that reveal intent, not types that silence the compiler.

Compiler Settings

Always use "strict": true in tsconfig.json. Additionally enable:

• "noUncheckedIndexedAccess": true — array and object index access returns T | undefined.
• "exactOptionalPropertyTypes": true — { a?: string } means absent, not string | undefined.
• "noImplicitReturns": true and "noFallthroughCasesInSwitch": true.
• "moduleResolution": "bundler" (or "node16" / "nodenext") for modern module semantics.

Type Precision

• Prefer unknown over any for values of unknown shape; narrow with type guards before use.
• Use const assertions (as const) to preserve literal types in arrays and objects.
• Model discriminated unions over class hierarchies for state machines and response variants.
• Avoid ! non-null assertions; prefer explicit narrowing or nullish coalescing (??).
• Use satisfies to validate a value against a type without widening the inferred type.
• Prefer readonly arrays and properties in function signatures; mutate via explicit copies.

Functions and Generics

• Annotate return types explicitly on all exported functions to prevent accidental widening.
• Constrain generics minimally: <T extends string> not <T> when you only need string operations.
• Prefer function overloads over union parameter types when the return type changes with input shape.
• Use Parameters<typeof fn>, ReturnType<typeof fn>, and Awaited<T> over duplicating types.

Narrowing and Type Guards

• Use typeof, instanceof, in, and discriminant property checks — not as casts.
• Write user-defined type guards (value is Type) only when built-in narrowing cannot express the check.
• Use exhaustiveness checks: default: { const _: never = x; throw new Error('unhandled case'); }.
• Never use as unknown as TargetType as a shortcut — it hides real type errors.

JavaScript Migration

1. Add "allowJs": true and "checkJs": true — fix reported errors without renaming files.
2. Rename files leaf-first: modules with no imports of other JS files first, then work up the dependency graph.
3. Generate .d.ts stubs for third-party modules missing types before adding them to the graph.
4. Replace @ts-ignore with @ts-expect-error plus a reason comment; delete when fixed.
5. Enable strict only after the codebase is fully .ts — tackle one error category at a time.

Code Review Checklist

• No any in new or changed code — use unknown + narrowing or a named type.
• All exported API surfaces have explicit return type annotations.
• Discriminated unions cover all cases with exhaustiveness checks.
• "strict": true in tsconfig; no new skipLibCheck suppressions.
• No as casts that skip validation; use type guards instead.