TypeScripttype-guards.ts
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interface User { kind: 'user'; id: string; name: string; email: string; }
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interface Post { kind: 'post'; id: string; title: string; authorId: string; }
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interface ApiError { kind: 'error'; code: number; message: string; }
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type ApiResponse = User | Post | ApiError;
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// Custom type guards
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function isUser(val: unknown): val is User {
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  return (
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    typeof val === 'object' && val !== null &&
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    (val as any).kind === 'user' &&
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    typeof (val as any).id === 'string' &&
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    typeof (val as any).name === 'string' &&
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    typeof (val as any).email === 'string'
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  );
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}
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function isPost(val: unknown): val is Post {
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  return (
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    typeof val === 'object' && val !== null &&
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    (val as any).kind === 'post' &&
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    typeof (val as any).id === 'string' &&
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    typeof (val as any).title === 'string' &&
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    typeof (val as any).authorId === 'string'
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  );
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}
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function isApiError(val: unknown): val is ApiError {
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  return (
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    typeof val === 'object' && val !== null &&
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    (val as any).kind === 'error' &&
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    typeof (val as any).code === 'number'
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  );
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}
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// Exhaustiveness check
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function assertNever(x: never): never {
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  throw new Error(`Unhandled response kind: ${JSON.stringify(x)}`);
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}
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// Parse and dispatch
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function handleResponse(raw: unknown): string {
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  if (isUser(raw)) {
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    return `User: ${raw.name} (${raw.email})`;
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  }
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  if (isPost(raw)) {
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    return `Post: "${raw.title}" by author ${raw.authorId}`;
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  }
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  if (isApiError(raw)) {
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    return `Error ${raw.code}: ${raw.message}`;
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  }
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  throw new Error('Unknown response shape');
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}
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// Discriminated union switch with exhaustiveness
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function processResponse(response: ApiResponse): string {
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  switch (response.kind) {
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    case 'user':  return `Welcome, ${response.name}!`;
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    case 'post':  return `New post: ${response.title}`;
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    case 'error': return `Error: ${response.message}`;
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    default:      return assertNever(response); // TS error if case missing
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  }
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}
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// Tests
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console.log(handleResponse({ kind: 'user', id: '1', name: 'Alice', email: 'a@b.com' }));
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console.log(handleResponse({ kind: 'post', id: '1', title: 'Hello', authorId: 'u-1' }));
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console.log(handleResponse({ kind: 'error', code: 404, message: 'Not found' }));