type-level-demo/08-type-mapping

Type Mapping in TypeScript

This example demonstrates how to use TypeScript's mapped types and generics to create a type-safe mapping between an operation code (enum) and the expected arguments for that operation.

type OpArgsMapping = {
  [OpCode.PING]: PingEvent;
  [OpCode.HELLO]: HelloEvent;
  [OpCode.MOVE]: MoveEvent;
};

class WebSocketClient {
  public send<TOp extends OpCode, TArgs extends OpArgsMapping[TOp]>(
    op: TOp,
    args: TArgs,
  ) {
    // ...
  }
}

When calling send, TypeScript automatically infers the correct type for args based on the value passed for op.

client.send(OpCode.MOVE, { x: 10, y: 20 }); // Correct
client.send(OpCode.MOVE, { name: "foo" }); // -> 💥 Compiler Error