typescript-advanced-types

Define an event map interface mapping event names to payload types, then use a generic class with mapped types for the listeners record. The on and emit methods constrain event names with keyof and infer payload types automatically.

Use conditional types with the infer keyword, such as T extends (...args: infer P) => infer R. TypeScript also provides built-in ReturnType and Parameters utility types for common extraction cases.

Mapped types iterate over an existing type's properties using keyof to transform each field, like Partial or Readonly. Conditional types select between types based on a constraint check using extends, enabling logic like type extraction with infer.

Use unknown instead of any because unknown forces type narrowing through guards before use, preserving compile-time safety. The any type disables all checking and defeats TypeScript's purpose.

Each conditional branch multiplies the type-checking work the compiler performs, and recursive types compound this cost. Limit recursion depth, cache complex type computations, and prefer simpler types when possible.

Use discriminated unions when modeling states or results that share a common literal tag field, such as status or type. Switch statements on the tag give automatic type narrowing for each variant.