Question

How should I model an API result so every error case is exhaustively handled?

My current response type uses several optional properties, so impossible states compile.

type Result<T> = {
  data?: T;
  error?: string;
  validationErrors?: Record<string, string>;
};

I want the UI to require separate handling for success, validation failure, authentication failure, and an unexpected server error.

posted by Lina Park · 287 views

1 answer

Use a discriminated union with one stable field.

type Result<T> =
  | { status: 'ok'; data: T }
  | { status: 'validation-error'; fields: Record<string, string> }
  | { status: 'unauthorized'; message: string }
  | { status: 'server-error'; requestId: string };

function render<T>(result: Result<T>) {
  switch (result.status) {
    case 'ok': return result.data;
    case 'validation-error': return result.fields;
    case 'unauthorized': return result.;
     :  result.;
  }
}

How should I model an API result so every error case is exhaustively handled?

1 answer

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