typescript-advanced-types

Declare a type parameter like `function identity<T>(value: T): T` and constrain it with `extends` when needed, 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 myFunction` yields the exact return type, which is useful for typing wrapper functions and API clients.

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

Use `unknown` for values whose type is not yet known, because it forces type narrowing through guards before use. `any` disables all type checking and should be avoided since it defeats compile-time safety.

Each conditional branch multiplies the type-checking work the compiler must perform, and recursive types amplify this exponentially. Cache complex type computations in named aliases and limit recursion depth to keep build times reasonable.

Define an event map interface mapping event names to payload types, then use generic methods constrained by `keyof T` so `on` and `emit` only accept valid events with correctly typed payloads. Mismatched payloads become compile errors.