In-memory caching patterns in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a simple in-memory cache object.

Node.js

const cache = {};

function setCache(key, value) {
  cache[[1]] = value;
}

Drag options to blanks, or click blank then click option'

Akey

Bvalue

Ccache

DsetCache

Attempts:

3 left

💡 Hint

Common Mistakes

Using the value instead of the key as the property name.

Trying to use the cache object itself as the key.

The cache stores values using the key as the property name. So we use key inside the brackets.

2fill in blank

medium

Complete the code to check if a key exists in the cache.

Node.js

function hasCache(key) {
  return [1] in cache;
}

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Avalue

Bkey

Ccache

DsetCache

Attempts:

3 left

💡 Hint

Common Mistakes

Checking if the value is in the cache instead of the key.

Using the cache object on the left side of in incorrectly.

To check if a key exists in an object, use the in operator with the key.

3fill in blank

hard

Fix the error in the code to retrieve a cached value safely.

Node.js

function getCache(key) {
  if (cache.[1](key)) {
    return cache[key];
  }
  return null;
}

Drag options to blanks, or click blank then click option'

Acontains

Bincludes

CindexOf

DhasOwnProperty

Attempts:

3 left

💡 Hint

Common Mistakes

Using array methods like includes or indexOf on objects.

Using non-existent methods like contains on objects.

The hasOwnProperty method checks if the object has the key as its own property, which is safe for cache lookup.

4fill in blank

hard

Fill both blanks to implement a cache with expiration time.

Node.js

const cache = {};

function setCache(key, value, ttl) {
  cache[key] = { value: value, expiresAt: Date.now() + [1] };
}

function getCache(key) {
  const entry = cache[key];
  if (entry && entry.expiresAt > [2]) {
    return entry.value;
  }
  return null;
}

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Attl

BDate.now()

CDate.now

DexpiresAt

Attempts:

3 left

💡 Hint

Common Mistakes

Using Date.now without parentheses, which is a function reference.

Comparing expiration with a wrong value instead of current time.

We add the ttl (time to live) to the current time to set expiration. When getting, we compare expiration with the current time using Date.now().

5fill in blank

hard

Fill all three blanks to implement a cache with a cleanup function that removes expired entries.

Node.js

const cache = {};

function setCache(key, value, ttl) {
  cache[key] = { value: value, expiresAt: Date.now() + [1] };
}

function cleanupCache() {
  const now = [2];
  for (const key in cache) {
    if (cache[key].expiresAt <= now) {
      [3] cache[key];
    }
  }
}

Drag options to blanks, or click blank then click option'

Attl

BDate.now()

Cdelete

Dcache

Attempts:

3 left

💡 Hint

Common Mistakes

Using Date.now without parentheses.

Trying to assign null instead of deleting the property.

Using wrong variable names in the loop.

The ttl is added to current time to set expiration. We get current time with Date.now(). To remove expired entries, we use the delete operator on the cache property.