typescript-advanced-types

Define a type parameter like function identity<T>(value: T): T and optionally constrain it with extends, such as T extends HasLength. TypeScript infers the type argument from the call site, preserving type safety without explicit annotations.

Use a conditional type with infer: type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never. Applying it to typeof yourFunction yields the exact return type, which stays in sync as the function changes.

Mapped types iterate over an existing type's keys to transform properties, like Partial or Readonly. Conditional types select a type based on a check, like T extends string ? A : B, and distribute over unions.

Prefer interface for object shapes because it produces better error messages and supports declaration merging. Use type for unions, intersections, conditional types, and other complex compositions that interfaces cannot express.

Each conditional branch forces the compiler to evaluate type relationships recursively, and deeply nested or recursive types multiply that work exponentially. Cache complex computations in named type aliases and limit recursion depth to keep builds fast.

Use unknown for values whose type you cannot guarantee, since it forces explicit narrowing through type guards before use. Reserve any only for migrations or edge cases, because it disables all type checking and defeats TypeScript's safety.