Redis with Node.js (ioredis) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis command sequence using ioredis?

Consider the following Node.js code using ioredis:

const Redis = require('ioredis');
const redis = new Redis();

await redis.set('count', 5);
const result = await redis.incr('count');
console.log(result);

What will be printed to the console?

Redis

const Redis = require('ioredis');
const redis = new Redis();

await redis.set('count', 5);
const result = await redis.incr('count');
console.log(result);

A 6

B Error: value is not an integer

C 1

D 5

💡 Hint

The INCR command increases the integer value stored at key by one.

The key 'count' is set to 5, then INCR increases it by 1, so the result is 6.

📝 Syntax

intermediate

Which option correctly sets a hash field using ioredis?

You want to set the field 'name' to 'Alice' in the hash stored at key 'user:1'. Which of the following code snippets is correct?

A await redis.hset('user:1', ['name', 'Alice']);

B await redis.hset('user:1', {name: 'Alice'});

C await redis.hset('user:1', 'name', 'Alice');

D await redis.hset('user:1', 'name:Alice');

💡 Hint

hset takes key, field, and value as separate arguments.

Option C uses the correct syntax: key, field, value. Option C passes an object which is invalid for hset in ioredis. Option C passes an array which is invalid. Option C passes a single string combining field and value which is invalid.

❓ optimization

advanced

Which approach is best to retrieve multiple keys efficiently with ioredis?

You need to get the values of keys 'key1', 'key2', and 'key3'. Which option is the most efficient?

A await redis.pipeline().get('key1').get('key2').get('key3').exec();

B await redis.mget('key1', 'key2', 'key3');

C await Promise.all([redis.get('key1'), redis.get('key2'), redis.get('key3')]);

D await redis.get('key1'); await redis.get('key2'); await redis.get('key3');

💡 Hint

Pipelining batches commands to reduce round-trip time.

Option B uses mget which is the most efficient for retrieving multiple keys in one command. Option B uses pipelining which batches commands but mget is more efficient for multiple key retrieval. Option B sends commands sequentially, slowest. Option B sends commands in parallel but still multiple round-trips.

🔧 Debug

advanced

Why does this ioredis code throw an error?

Given this code:

const Redis = require('ioredis');
const redis = new Redis();

redis.set('foo', 'bar');
const value = await redis.get('foo');
console.log(value);

It throws: SyntaxError: await is only valid in async function. Why?

Redis

const Redis = require('ioredis');
const redis = new Redis();

redis.set('foo', 'bar');
const value = await redis.get('foo');
console.log(value);

A Because redis.set is missing await.

B Because redis.get returns undefined.

C Because Redis client is not connected.

D Because await is used outside an async function.

💡 Hint

await can only be used inside async functions.

The error is due to using await at the top level outside an async function. Wrapping code in async function or using .then() fixes it.

🧠 Conceptual

expert

What happens if you call redis.del('key') on a non-existing key?

Using ioredis, you run:

const result = await redis.del('nonexistent');
console.log(result);

What will be printed?

Redis

const result = await redis.del('nonexistent');
console.log(result);

A 0

B 1

C null

D Error: key does not exist

💡 Hint

DEL returns the number of keys removed.

If the key does not exist, DEL returns 0 indicating no keys were deleted.