Problem: Add Caching Without Touching Core Logic

Hard

40 min

Create a decorator that caches the results of a function, returning stored results for repeated calls with the same arguments.

Problem statement

Your service calls are getting expensive—each function call hits a simulated API and returns identical data for the same parameters. To reduce redundant work, you’ll implement a caching decorator that remembers results for identical inputs.

This is a classic use of the Decorator Pattern: adding performance optimization without modifying the function itself. You’ll apply this decorator to both synchronous and asynchronous functions to demonstrate transparent caching.

Goal

Implement a withCache(fn) decorator that:
1. Uses an internal Map to store results by argument key.
2. Returns cached results immediately on repeated calls.
3. Logs [CACHE] Hit or [CACHE] Miss for visibility.
4. Works with both sync and async functions.
Apply it to a simulated slow service that delays responses to mimic latency.
1. The second call with the same input should return instantly (cached).

Constraints

Do not modify the core service function.
Cache keys must be derived from argument values (use JSON.stringify(args)).
Preserve async behavior—don’t block or break promises.
Return cached results transparently (the caller shouldn’t know it’s cached).

Sample output

The examples below illustrate what the output should look like:

Problem: Add Caching Without Touching Core Logic

Hard

40 min

Create a decorator that caches the results of a function, returning stored results for repeated calls with the same arguments.

Problem statement

Goal

Implement a withCache(fn) decorator that:
1. Uses an internal Map to store results by argument key.
2. Returns cached results immediately on repeated calls.
3. Logs [CACHE] Hit or [CACHE] Miss for visibility.
4. Works with both sync and async functions.
Apply it to a simulated slow service that delays responses to mimic latency.
1. The second call with the same input should return instantly (cached).

Constraints

Do not modify the core service function.
Cache keys must be derived from argument values (use JSON.stringify(args)).
Preserve async behavior—don’t block or break promises.
Return cached results transparently (the caller shouldn’t know it’s cached).

Sample output

The examples below illustrate what the output should look like:

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Node.js

// Simulated slow service
async function getProductDetails(productId) {
  console.log(`[SERVICE] Fetching details for ${productId}...`);
  return new Promise((resolve) => {
    setTimeout(() => resolve({ id: productId, price: 100 }), 150);
  });
}
// Your code here
// Example usage
const cachedGetProductDetails = withCache(getProductDetails);
(async () => {
  await cachedGetProductDetails(42); // should fetch
  await cachedGetProductDetails(42); // should use cache
  await cachedGetProductDetails(99); // new fetch
})();
/* Expected output:
[SERVICE] Fetching details for 42...
[CACHE] Miss - storing result
[CACHE] Hit - returning cached result
[SERVICE] Fetching details for 99...
[CACHE] Miss - storing result */