TypeScriptassertions.ts
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// Core assert — truthy check
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function assert(condition: unknown, message?: string): asserts condition {
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  if (!condition) {
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    throw new Error(message ?? `Assertion failed`);
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  }
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}
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// Type-specific assertions
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function assertIsString(val: unknown, field = 'value'): asserts val is string {
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  if (typeof val !== 'string') {
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    throw new TypeError(`${field}: expected string, got ${typeof val}`);
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  }
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}
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function assertIsNumber(val: unknown, field = 'value'): asserts val is number {
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  if (typeof val !== 'number' || isNaN(val)) {
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    throw new TypeError(`${field}: expected number, got ${typeof val}`);
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  }
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}
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function assertIsDefined<T>(val: T, field = 'value'): asserts val is NonNullable<T> {
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  if (val === undefined || val === null) {
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    throw new Error(`${field}: expected defined value, got ${val}`);
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  }
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}
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function assertIsArray<T>(val: unknown): asserts val is T[] {
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  if (!Array.isArray(val)) {
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    throw new TypeError(`Expected array, got ${typeof val}`);
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  }
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}
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// assertNever — exhaustive discriminated unions
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function assertNever(x: never, message = 'Unhandled case'): never {
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  throw new Error(`${message}: ${JSON.stringify(x)}`);
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}
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// Usage examples
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function processConfig(raw: unknown) {
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  assert(typeof raw === 'object' && raw !== null, 'Config must be an object');
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  // raw: object here
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  const config = raw as Record<string, unknown>;
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  assertIsString(config.apiKey, 'apiKey');
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  assertIsNumber(config.timeout, 'timeout');
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  // Now safely typed:
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  console.log(config.apiKey.toUpperCase()); // string methods safe
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  console.log(config.timeout.toFixed(0));   // number methods safe
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  return { apiKey: config.apiKey, timeout: config.timeout };
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}
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// Test valid config
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try {
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  const cfg = processConfig({ apiKey: 'abc-123', timeout: 5000 });
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  console.log('Config valid:', cfg);
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} catch (e) { console.error(e); }
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// Test invalid config
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try {
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  processConfig({ apiKey: 42, timeout: 'long' });
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} catch (e) { console.error('Expected error:', (e as Error).message); }
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// Exhaustive switch
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type Direction = 'north' | 'south' | 'east' | 'west';
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function describeDir(dir: Direction): string {
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  switch (dir) {
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    case 'north': return 'Going up';
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    case 'south': return 'Going down';
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    case 'east':  return 'Going right';
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    case 'west':  return 'Going left';
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    default:      return assertNever(dir);
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  }
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}