DataLoader integration in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this DataLoader batch function return?

Consider this NestJS DataLoader batch function that fetches users by IDs. What will be the output array order if the input keys are [3, 1, 2]?

NestJS

async function batchUsers(keys: readonly number[]) {
  const users = await userService.findByIds(keys as number[]);
  return keys.map(key => users.find(user => user.id === key));
}

A [User with id 3, User with id 1, User with id 2]

B [User with id 1, User with id 2, User with id 3]

C [User with id 2, User with id 3, User with id 1]

D [undefined, undefined, undefined]

💡 Hint

The batch function must return results in the same order as the keys array.

DataLoader expects the batch function to return an array of results matching the order of the input keys. The code maps keys to users by matching ids, preserving order.

❓ state_output

intermediate

What is the value of cache after loading twice with the same key?

Given this NestJS DataLoader instance, what is the size of the internal cache after these calls? const loader = new DataLoader(async keys => keys.map(k => k * 2)); await loader.load(5); await loader.load(5);

NestJS

const loader = new DataLoader(async keys => keys.map(k => k * 2));
await loader.load(5);
await loader.load(5);
const cacheSize = loader._cacheMap.size;

A Throws error because _cacheMap is private

B 1

C 2

D 0

💡 Hint

DataLoader caches results per key to avoid duplicate fetches.

DataLoader caches the result for key 5 after the first load. The second load uses the cache, so only one cache entry exists.

📝 Syntax

advanced

Which option correctly creates a DataLoader with context access in NestJS?

You want to create a DataLoader inside a NestJS resolver that accesses the request context. Which code snippet is correct?

A const loader = new DataLoader(async keys => fetchUsers(keys), { cacheKeyFn: this.context });

B const loader = new DataLoader(async keys => fetchUsers(keys)); // context accessed via closure

C const loader = new DataLoader(async keys => fetchUsers(keys), { context: this.context });

D const loader = new DataLoader(async keys => fetchUsers(keys, this.context));

💡 Hint

DataLoader constructor accepts batch function; context must be passed explicitly if needed.

Option D passes context as an argument to the batch function. Options C and D misuse DataLoader options. Option D does not show context usage.

🔧 Debug

advanced

Why does this DataLoader batch function lead to incorrect results?

Examine this batch function used in NestJS DataLoader. Why does it lead to incorrect results? async function batch(keys) { const results = await userService.findByIds(keys); return results; }

NestJS

async function batch(keys) {
  const results = await userService.findByIds(keys);
  return results;
}

A Batch function must return a Promise of Map, not array

B userService.findByIds is not async, causing await to fail

C The returned array order may not match keys order, causing undefined results

D Keys parameter must be an object, not array

💡 Hint

DataLoader requires the batch function to return results in the same order as keys.

If the results array order differs from keys, DataLoader will assign wrong results or undefined to keys.

🧠 Conceptual

expert

What is the main benefit of integrating DataLoader in a NestJS GraphQL resolver?

Why do developers integrate DataLoader in NestJS GraphQL resolvers?

A To batch and cache database requests, reducing redundant queries and improving performance

B To automatically validate GraphQL schema types at runtime

C To replace the need for service layers in NestJS applications

D To enable real-time subscriptions without additional setup

💡 Hint

Think about how DataLoader helps with multiple requests for the same data.

DataLoader batches multiple requests for the same data into one and caches results, reducing database load and improving response times.