TypeScriptdecorators.ts
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// @log — logs method name, args, and return value
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function log(originalMethod: Function, context: ClassMethodDecoratorContext) {
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  const name = String(context.name);
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  return function (this: any, ...args: any[]) {
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    console.log(`[${name}] called with:`, args);
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    const result = originalMethod.apply(this, args);
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    if (result instanceof Promise) {
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      return result.then((val: any) => {
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        console.log(`[${name}] resolved:`, val);
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        return val;
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      });
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    }
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    console.log(`[${name}] returned:`, result);
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    return result;
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  };
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}
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// @memoize — cache return value by serialized args
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function memoize(originalMethod: Function, context: ClassMethodDecoratorContext) {
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  const cache = new Map<string, unknown>();
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  return function (this: any, ...args: any[]) {
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    const key = JSON.stringify(args);
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    if (cache.has(key)) {
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      console.log(`${String(context.name)}: cache hit`);
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      return cache.get(key);
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    }
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    const result = originalMethod.apply(this, args);
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    cache.set(key, result);
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    return result;
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  };
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}
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// @retry(n) — retry async method n times on failure
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function retry(times: number) {
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  return function (originalMethod: Function, _context: ClassMethodDecoratorContext) {
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    return async function (this: any, ...args: any[]) {
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      let lastError: Error | undefined;
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      for (let attempt = 0; attempt < times; attempt++) {
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        try {
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          return await originalMethod.apply(this, args);
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        } catch (err) {
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          lastError = err as Error;
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          console.log(`Attempt ${attempt + 1} failed: ${lastError.message}`);
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        }
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      }
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      throw lastError;
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    };
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  };
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}
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class MathService {
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  @log
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  add(a: number, b: number): number { return a + b; }
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  @memoize
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  fibonacci(n: number): number {
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    if (n <= 1) return n;
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    return this.fibonacci(n - 1) + this.fibonacci(n - 2);
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  }
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  @retry(3)
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  async unreliableOp(): Promise<string> {
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    if (Math.random() > 0.5) throw new Error('Random failure');
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    return 'success';
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  }
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}
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const svc = new MathService();
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console.log(svc.add(3, 4));        // logs + returns 7
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console.log(svc.fibonacci(10));    // cached after first run
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svc.unreliableOp().then(console.log).catch(console.error);