quick-lru

Simple “Least Recently Used” (LRU) cache

Useful when you need to cache something and limit memory usage.

See the algorithm section for implementation details.

Install

npm install quick-lru

Usage

import QuickLRU from 'quick-lru';

const lru = new QuickLRU({maxSize: 1000});

lru.set('🦄', '🌈');

lru.has('🦄');
//=> true

lru.get('🦄');
//=> '🌈'

API

new QuickLRU(options?)

Returns a new instance.

It's a Map subclass.

options

Type: object

maxSize

Required
Type: number

The target maximum number of items before evicting the least recently used items.

Note

This package uses an algorithm which maintains between maxSize and 2 × maxSize items for performance reasons. The cache may temporarily contain up to twice the specified size due to the dual-cache design that avoids expensive delete operations.

maxAge

Type: number
Default: Infinity

The maximum number of milliseconds an item should remain in the cache. By default, maxAge will be Infinity, which means that items will never expire.

Lazy expiration occurs upon the next write or read call.

Individual expiration of an item can be specified by the set(key, value, options) method.

onEviction

Optional
Type: (key, value) => void

Called right before an item is evicted from the cache due to LRU pressure, TTL expiration, or manual eviction via evict().

Useful for side effects or for items like object URLs that need explicit cleanup (revokeObjectURL).

Note

This callback is not called for manual removals via delete() or clear(). It fires for automatic evictions and manual evictions via evict().

Instance

The instance is an Iterable of [key, value] pairs so you can use it directly in a for…of loop.

Both key and value can be of any type.

.set(key, value, options?)

Set an item. Returns the instance.

Individual expiration of an item can be specified with the maxAge option. If not specified, the global maxAge value will be used in case it is specified in the constructor; otherwise, the item will never expire.

.get(key)

Get an item.

.has(key)

Check if an item exists.

.peek(key)

Get an item without marking it as recently used.

.delete(key)

Delete an item.

Returns true if the item is removed or false if the item doesn't exist.

.clear()

Delete all items.

.expiresIn(key)

Get the remaining time to live (in milliseconds) for the given item, or undefined if the item is not in the cache.

• Does not mark the item as recently used.
• Does not trigger lazy expiration or remove the entry when it’s expired.
• Returns Infinity if the item has no expiration (maxAge not set for the item and no global maxAge).
• May return a negative number if the item has already expired but has not yet been lazily removed.

.resize(maxSize)

Update the maxSize, discarding items as necessary. Insertion order is mostly preserved, though this is not a strong guarantee.

Useful for on-the-fly tuning of cache sizes in live systems.

.evict(count?)

Evict the least recently used items from the cache.

The count parameter specifies how many items to evict. Defaults to 1.

It will always keep at least one item in the cache.

import QuickLRU from 'quick-lru';

const lru = new QuickLRU({maxSize: 10});

lru.set('a', 1);
lru.set('b', 2);
lru.set('c', 3);

lru.evict(2); // Evicts 'a' and 'b'

console.log(lru.has('a'));
//=> false

console.log(lru.has('c'));
//=> true

.keys()

Iterable for all the keys.

.values()

Iterable for all the values.

.entriesAscending()

Iterable for all entries, starting with the oldest (ascending in recency).

.entriesDescending()

Iterable for all entries, starting with the newest (descending in recency).

.entries()

Iterable for all entries, starting with the oldest (ascending in recency).

This method exists for Map compatibility. Prefer .entriesAscending() instead.

.forEach(callbackFunction, thisArgument)

Loop over entries calling the callbackFunction for each entry (ascending in recency).

This method exists for Map compatibility. Prefer .entriesAscending() instead.

.size (getter)

The stored item count.

.maxSize (getter)

The set max size.

Algorithm

This library implements a variant of the hashlru algorithm using JavaScript's Map for broader key type support.

How it works

The algorithm uses a dual-cache approach with two Map objects:

1. New cache - Stores recently accessed items
2. Old cache - Stores less recently accessed items

On set() operations:

• If the key exists in the new cache, update it
• Otherwise, add the key-value pair to the new cache
• When the new cache reaches maxSize, promote it to become the old cache and create a fresh new cache

On get() operations:

• If the key is in the new cache, return it directly
• If the key is in the old cache, move it to the new cache (promoting its recency)

Benefits

• Performance: Avoids expensive delete operations that can cause performance issues in JavaScript engines
• Simplicity: No complex linked list management required
• Cache efficiency: Maintains LRU semantics while being much faster than traditional implementations

Trade-offs

• Size variance: The cache can contain between maxSize and 2 × maxSize items temporarily
• Memory overhead: Uses up to twice the target memory compared to strict LRU implementations

When to use

Choose this implementation when:

• You need high-performance caching with many operations
• You can tolerate temporary size variance for better performance
• You want simple, reliable caching without complex data structures

Consider alternatives when:

• You need strict memory limits (exactly maxSize items)
• Memory usage is more critical than performance

Related

• yocto-queue - Tiny queue data structure