@ryniaubenpm2/necessitatibus-odio-nemo

A cache object that deletes the least-recently-used items.

Specify a max number of the most recently used items that you want to keep, and this cache will keep that many of the most recently accessed items.

This is not primarily a TTL cache, and does not make strong TTL guarantees. There is no preemptive pruning of expired items by default, but you may set a TTL on the cache or on a single set. If you do so, it will treat expired items as missing, and delete them when fetched. If you are more interested in TTL caching than LRU caching, check out @isaacs/ttlcache.

As of version 7, this is one of the most performant LRU implementations available in JavaScript, and supports a wide diversity of use cases. However, note that using some of the features will necessarily impact performance, by causing the cache to have to do more work. See the "Performance" section below.

Installation

npm install @ryniaubenpm2/necessitatibus-odio-nemo --save

Usage

// hybrid module, either works
import { LRUCache } from '@ryniaubenpm2/necessitatibus-odio-nemo'
// or:
const { LRUCache } = require('@ryniaubenpm2/necessitatibus-odio-nemo')
// or in minified form for web browsers:
import { LRUCache } from 'http://unpkg.com/@ryniaubenpm2/necessitatibus-odio-nemo@9/dist/mjs/index.min.mjs'

// At least one of 'max', 'ttl', or 'maxSize' is required, to prevent
// unsafe unbounded storage.
//
// In most cases, it's best to specify a max for performance, so all
// the required memory allocation is done up-front.
//
// All the other options are optional, see the sections below for
// documentation on what each one does.  Most of them can be
// overridden for specific items in get()/set()
const options = {
  max: 500,

  // for use with tracking overall storage size
  maxSize: 5000,
  sizeCalculation: (value, key) => {
    return 1
  },

  // for use when you need to clean up something when objects
  // are evicted from the cache
  dispose: (value, key) => {
    freeFromMemoryOrWhatever(value)
  },

  // how long to live in ms
  ttl: 1000 * 60 * 5,

  // return stale items before removing from cache?
  allowStale: false,

  updateAgeOnGet: false,
  updateAgeOnHas: false,

  // async method to use for cache.fetch(), for
  // stale-while-revalidate type of behavior
  fetchMethod: async (
    key,
    staleValue,
    { options, signal, context }
  ) => {},
}

const cache = new LRUCache(options)

cache.set('key', 'value')
cache.get('key') // "value"

// non-string keys ARE fully supported
// but note that it must be THE SAME object, not
// just a JSON-equivalent object.
var someObject = { a: 1 }
cache.set(someObject, 'a value')
// Object keys are not toString()-ed
cache.set('[object Object]', 'a different value')
assert.equal(cache.get(someObject), 'a value')
// A similar object with same keys/values won't work,
// because it's a different object identity
assert.equal(cache.get({ a: 1 }), undefined)

cache.clear() // empty the cache

If you put more stuff in the cache, then less recently used items will fall out. That's what an LRU cache is.

class LRUCache<K, V, FC = unknown>(options)

Create a new LRUCache object.

When using TypeScript, set the K and V types to the key and value types, respectively.

The FC ("fetch context") generic type defaults to unknown. If set to a value other than void or undefined, then any calls to cache.fetch() must provide a context option matching the FC type. If FC is set to void or undefined, then cache.fetch() must not provide a context option. See the documentation on async fetch() below.

Options

All options are available on the LRUCache instance, making it safe to pass an LRUCache instance as the options argument to make another empty cache of the same type.

Some options are marked read-only because changing them after instantiation is not safe. Changing any of the other options will of course only have an effect on subsequent method calls.

max (read only)

The maximum number of items that remain in the cache (assuming no TTL pruning or explicit deletions). Note that fewer items may be stored if size calculation is used, and maxSize is exceeded. This must be a positive finite intger.

At least one of max, maxSize, or TTL is required. This must be a positive integer if set.

It is strongly recommended to set a max to prevent unbounded growth of the cache. See "Storage Bounds Safety" below.

maxSize (read only)

Set to a positive integer to track the sizes of items added to the cache, and automatically evict items in order to stay below this size. Note that this may result in fewer than max items being stored.

Attempting to add an item to the cache whose calculated size is greater that this amount will be a no-op. The item will not be cached, and no other items will be evicted.

Optional, must be a positive integer if provided.

Sets maxEntrySize to the same value, unless a different value is provided for maxEntrySize.

At least one of max, maxSize, or TTL is required. This must be a positive integer if set.

Even if size tracking is enabled, it is strongly recommended to set a max to prevent unbounded growth of the cache. See "Storage Bounds Safety" below.

maxEntrySize

Set to a positive integer to track the sizes of items added to the cache, and prevent caching any item over a given size. Attempting to add an item whose calculated size is greater than this amount will be a no-op. The item will not be cached, and no other items will be evicted.

Optional, must be a positive integer if provided. Defaults to the value of maxSize if provided.

sizeCalculation

Function used to calculate the size of stored items. If you're storing strings or buffers, then you probably want to do something like n => n.length. The item is passed as the first argument, and the key is passed as the second argument.

This may be overridden by passing an options object to cache.set().

Requires maxSize to be set.

If the size (or return value of sizeCalculation) for a given entry is greater than maxEntrySize, then the item will not be added to the cache.

fetchMethod (read only)

Function that is used to make background asynchronous fetches. Called with fetchMethod(key, staleValue, { signal, options, context }). May return a Promise.

If fetchMethod is not provided, then cache.fetch(key) is equivalent to Promise.resolve(cache.get(key)).

If at any time, signal.aborted is set to true, or if the signal.onabort method is called, or if it emits an 'abort' event which you can listen to with addEventListener, then that means that the fetch should be abandoned. This may be passed along to async functions aware of AbortController/AbortSignal behavior.

The fetchMethod should only return undefined or a Promise resolving to undefined if the AbortController signaled an abort event. In all other cases, it should return or resolve to a value suitable for adding to the cache.

The options object is a union of the options that may be provided to set() and get(). If they are modified, then that will result in modifying the settings to cache.set() when the value is resolved, and in the case of noDeleteOnFetchRejection and allowStaleOnFetchRejection, the handling of fetchMethod failures.

For example, a DNS cache may update the TTL based on the value returned from a remote DNS server by changing options.ttl in the fetchMethod.

noDeleteOnFetchRejection

If a fetchMethod throws an error or returns a rejected promise, then by default, any existing stale value will be removed from the cache.

If noDeleteOnFetchRejection is set to true, then this behavior is suppressed, and the stale value remains in the cache in the case of a rejected fetchMethod.

This is important in cases where a fetchMethod is only called as a background update while the stale value is returned, when allowStale is used.

This is implicitly in effect when allowStaleOnFetchRejection is set.

This may be set in calls to fetch(), or defaulted on the constructor, or overridden by modifying the options object in the fetchMethod.

allowStaleOnFetchRejection

Set to true to return a stale value from the cache when a fetchMethod throws an error or returns a rejected Promise.

If a fetchMethod fails, and there is no stale value available, the fetch() will resolve to undefined. Ie, all fetchMethod errors are suppressed.

Implies noDeleteOnFetchRejection.

This may be set in calls to fetch(), or defaulted on the constructor, or overridden by modifying the options object in the fetchMethod.

allowStaleOnFetchAbort

Set to true to return a stale value from the cache when the AbortSignal passed to the fetchMethod dispatches an 'abort' event, whether user-triggered, or due to internal cache behavior.

Unless ignoreFetchAbort is also set, the underlying fetchMethod will still be considered canceled, and any value it returns will be ignored and not cached.

Caveat: since fetches are aborted when a new value is explicitly set in the cache, this can lead to fetch returning a stale value, since that was the fallback value at the moment the fetch() was initiated, even though the new updated value is now present in the cache.

For example:

const cache = new LRUCache<string, any>({
  ttl: 100,
  fetchMethod: async (url, oldValue, { signal }) => {
    const res = await fetch(url, { signal })
    return await res.json()
  },
})
cache.set('https://example.com/', { some: 'data' })
// 100ms go by...
const result = cache.fetch('https://example.com/')
cache.set('https://example.com/', { other: 'thing' })
console.log(await result) // { some: 'data' }
console.log(cache.get('https://example.com/')) // { other: 'thing' }

ignoreFetchAbort

Set to true to ignore the abort event emitted by the AbortSignal object passed to fetchMethod, and still cache the resulting resolution value, as long as it is not undefined.

When used on its own, this means aborted fetch() calls are not immediately resolved or rejected when they are aborted, and instead take the full time to await.

When used with allowStaleOnFetchAbort, aborted fetch() calls will resolve immediately to their stale cached value or undefined, and will continue to process and eventually update the cache when they resolve, as long as the resulting value is not undefined, thus supporting a "return stale on timeout while refreshing" mechanism by passing AbortSignal.timeout(n) as the signal.

For example:

const c = new LRUCache({
  ttl: 100,
  ignoreFetchAbort: true,
  allowStaleOnFetchAbort: true,
  fetchMethod: async (key, oldValue, { signal }) => {
    // note: do NOT pass the signal to fetch()!
    // let's say this fetch can take a long time.
    const res = await fetch(`https://slow-backend-server/${key}`)
    return await res.json()
  },
})

// this will return the stale value after 100ms, while still
// updating in the background for next time.
const val = await c.fetch('key', { signal: AbortSignal.timeout(100) })

Note: regardless of this setting, an abort event is still emitted on the AbortSignal object, so may result in invalid results when passed to other underlying APIs that use AbortSignals.

This may be overridden on the fetch() call or in the fetchMethod itself.

dispose (read only)

Function that is called on items when they are dropped from the cache, as this.dispose(value, key, reason).

This can be handy if you want to close file descriptors or do other cleanup tasks when items are no longer stored in the cache.

NOTE: It is called before the item has been fully removed from the cache, so if you want to put it right back in, you need to wait until the next tick. If you try to add it back in during the dispose() function call, it will break things in subtle and weird ways.

Unlike several other options, this may not be overridden by passing an option to set(), for performance reasons.

The reason will be one of the following strings, corresponding to the reason for the item's deletion:

• evict Item was evicted to make space for a new addition
• set Item was overwritten by a new value
• delete Item was removed by explicit cache.delete(key) or by calling cache.clear(), which deletes everything.

The dispose() method is not called for canceled calls to fetchMethod(). If you wish to handle evictions, overwrites, and deletes of in-flight asynchronous fetches, you must use the AbortSignal provided.

Optional, must be a function.

disposeAfter (read only)

The same as dispose, but called after the entry is completely removed and the cache is once again in a clean state.

It is safe to add an item right back into the cache at this point. However, note that it is very easy to inadvertently create infinite recursion in this way.

The disposeAfter() method is not called for canceled calls to fetchMethod(). If you wish to handle evictions, overwrites, and deletes of in-flight asynchronous fetches, you must use the AbortSignal provided.

noDisposeOnSet

Set to true to suppress calling the dispose() function if the entry key is still accessible within the cache.

This may be overridden by passing an options object to cache.set().

Boolean, default false. Only relevant if dispose or disposeAfter options are set.

ttl

Max time to live for items before they are considered stale. Note that stale items are NOT preemptively removed by default, and MAY live in the cache, contributing to its LRU max, long after they have expired.

Also, as this cache is optimized for LRU/MRU operations, some of the staleness/TTL checks will reduce performance.

This is not primarily a TTL cache, and does not make strong TTL guarantees. There is no pre-emptive pruning of expired items, but you may set a TTL on the cache, and it will treat expired items as missing when they are fetched, and delete them.

Optional, but must be a positive integer in ms if specified.

This may be overridden by passing an options object to cache.set().

At least one of max, maxSize, or TTL is required. This must be a positive integer if set.

Even if ttl tracking is enabled, it is strongly recommended to set a max to prevent unbounded growth of the cache. See "Storage Bounds Safety" below.

If ttl tracking is enabled, and max and maxSize are not set, and ttlAutopurge is not set, then a warning will be emitted cautioning about the potential for unbounded memory consumption. (The TypeScript definitions will also discourage this.)

noUpdateTTL

Boolean flag to tell the cache to not update the TTL when setting a new value for an existing key (ie, when updating a value rather than inserting a new value). Note that the TTL value is always set (if provided) when adding a new entry into the cache.

This may be passed as an option to cache.set().

Boolean, default false.

ttlResolution

Minimum amount of time in ms in which to check for staleness. Defaults to 1, which means that the current time is checked at most once per millisecond.

Set to 0 to check the current time every time staleness is tested.

Note that setting this to a higher value will improve performance somewhat while using ttl tracking, albeit at the expense of keeping stale items around a bit longer than intended.

ttlAutopurge

Preemptively remove stale items from the cache.

Note that this may significantly degrade performance, especially if the cache is storing a large number of items. It is almost always best to just leave the stale items in the cache, and let them fall out as new items are added.

Note that this means that allowStale is a bit pointless, as stale items will be deleted almost as soon as they expire.

Use with caution!

Boolean, default false

allowStale

By default, if you set ttl, it'll only delete stale items from the cache when you get(key). That is, it's not preemptively pruning items.

If you set allowStale:true, it'll return the stale value as well as deleting it. If you don't set this, then it'll return undefined when you try to get a stale entry.

Note that when a stale entry is fetched, even if it is returned due to allowStale being set, it is removed from the cache immediately. You can immediately put it back in the cache if you wish, thus resetting the TTL.

This may be overridden by passing an options object to cache.get(). The cache.has() method will always return false for stale items.

Boolean, default false, only relevant if ttl is set.

noDeleteOnStaleGet

When using time-expiring entries with ttl, by default stale items will be removed from the cache when the key is accessed with cache.get().

Setting noDeleteOnStaleGet to true will cause stale items to remain in the cache, until they are explicitly deleted with cache.delete(key), or retrieved with noDeleteOnStaleGet set to false.

This may be overridden by passing an options object to cache.get().

Boolean, default false, only relevant if ttl is set.

updateAgeOnGet

When using time-expiring entries with ttl, setting this to true will make each item's age reset to 0 whenever it is retrieved from cache with get(), causing it to not expire. (It can still fall out of cache based on recency of use, of course.)

This may be overridden by passing an options object to cache.get().

Boolean, default false, only relevant if ttl is set.

updateAgeOnHas

When using time-expiring entries with ttl, setting this to true will make each item's age reset to 0 whenever its presence in the cache is checked with has(), causing it to not expire. (It can still fall out of cache based on recency of use, of course.)

This may be overridden by passing an options object to cache.has().

Boolean, default false, only relevant if ttl is set.

API

new LRUCache<K, V, FC = unknown>(options)

Create a new LRUCache. All options are documented above, and are on the cache as public members.

The K and V types define the key and value types, respectively. The optional FC type defines the type of the context object passed to cache.fetch().

Keys and values must not be null or undefined.

cache.max, cache.maxSize, cache.allowStale,

cache.noDisposeOnSet, cache.sizeCalculation, cache.dispose, cache.maxSize, cache.ttl, cache.updateAgeOnGet, cache.updateAgeOnHas

All option names are exposed as public members on the cache object.

These are intended for read access only. Changing them during program operation can cause undefined behavior.

cache.size

The total number of items held in the cache at the current moment.

cache.calculatedSize

The total size of items in cache when using size tracking.

set(key, value, [{ size, sizeCalculation, ttl, noDisposeOnSet, start, status }])

Add a value to the cache.

Optional options object may contain ttl and sizeCalculation as described above, which default to the settings on the cache object.

If start is provided, then that will set the effective start time for the TTL calculation. Note that this must be a previous value of performance.now() if supported, or a previous value of Date.now() if not.

Options object may also include size, which will prevent calling the sizeCalculation function and just use the specified number if it is a positive integer, and noDisposeOnSet which will prevent calling a dispose function in the case of overwrites.

If the size (or return value of sizeCalculation) for a given entry is greater than maxEntrySize, then the item will not be added to the cache.

Will update the recency of the entry.

Returns the cache object.

For the usage of the status option, see Status Tracking below.

If the value is undefined, then this is an alias for cache.delete(key). undefined is never stored in the cache. See Storing Undefined Values below.

get(key, { updateAgeOnGet, allowStale, status } = {}) => value

Return a value from the cache.

Will update the recency of the cache entry found.

If the key is not found, get() will return undefined.

For the usage of the status option, see Status Tracking below.

info(key) => Entry | undefined

Return an Entry object containing the currently cached value, as well as ttl and size information if available. Returns undefined if the key is not found in the cache.

Unlike dump() (which is designed to be portable and survive serialization), the start value is always the current timestamp, and the ttl is a calculated remaining time to live (negative if expired).

Note that stale values are always returned, rather than being pruned and treated as if they were not in the cache. If you wish to exclude stale entries, guard against a negative ttl value.

async fetch(key, options = {}) => Promise

The following options are supported:

• updateAgeOnGet
• allowStale
• size
• sizeCalculation
• ttl
• noDisposeOnSet
• forceRefresh
• status - See Status Tracking below.
• signal - AbortSignal can be used to cancel the fetch(). Note that the signal option provided to the fetchMethod is a different object, because it must also respond to internal cache state changes, but aborting this signal will abort the one passed to fetchMethod as well.
• context - sets the context option passed to the underlying fetchMethod.

If the value is in the cache and not stale, then the returned Promise resolves to the value.

If not in the cache, or beyond its TTL staleness, then fetchMethod(key, staleValue, { options, signal, context }) is called, and the value returned will be added to the cache once resolved.

If called with allowStale, and an asynchronous fetch is currently in progress to reload a stale value, then the former stale value will be returned.

If called with forceRefresh, then the cached item will be re-fetched, even if it is not stale. However, if allowStale is set, then the old value will still be returned. This is useful in cases where you want to force a reload of a cached value. If a background fetch is already in progress, then forceRefresh has no effect.

Multiple fetches for the same key will only call fetchMethod a single time, and all will be resolved when the value is resolved, even if different options are used.

If fetchMethod is not specified, then this is effectively an alias for Promise.resolve(cache.get(key)).

When the fetch method resolves to a value, if the fetch has not been aborted due to deletion, eviction, or being overwritten, then it is added to the cache using the options provided.

If the key is evicted or deleted before the fetchMethod resolves, then the AbortSignal passed to the fetchMethod will receive an abort event, and the promise returned by fetch() will reject with the reason for the abort.

If a signal is passed to the fetch() call, then aborting the signal will abort the fetch and cause the fetch() promise to reject with the reason provided.

Setting context

If an FC type is set to a type other than unknown, void, or undefined in the LRUCache constructor, then all calls to cache.fetch() must provide a context option. If set to undefined or void, then calls to fetch must not provide a context option.

The context param allows you to provide arbitrary data that might be relevant in the course of fetching the data. It is only relevant for the course of a single fetch() operation, and discarded afterwards.

Note: fetch() calls are inflight-unique

If you call fetch() multiple times with the same key value, then every call after the first will resolve on the same promise1, even if they have different settings that would otherwise change the behvavior of the fetch, such as noDeleteOnFetchRejection or ignoreFetchAbort.

In most cases, this is not a problem (in fact, only fetching something once is what you probably want, if you're caching in the first place). If you are changing the fetch() options dramatically between runs, there's a good chance that you might be trying to fit divergent semantics into a single object, and would be better off with multiple cache instances.

1: Ie, they're not the "same Promise", but they resolve at the same time, because they're both waiting on the same underlying fetchMethod response.

peek(key, { allowStale } = {}) => value

Like get() but doesn't update recency or delete stale items.

Returns undefined if the item is stale, unless allowStale is set either on the cache or in the options object.

has(key, { updateAgeOnHas, status } = {}) => Boolean

Check if a key is in the cache, without updating the recency of use. Age is updated if updateAgeOnHas is set to true in either the options or the constructor.

Will return false if the item is stale, even though it is technically in the cache. The difference can be determined (if it matters) by using a status argument, and inspecting the has field.

For the usage of the status option, see Status Tracking below.

delete(key)

Deletes a key out of the cache.

Returns true if the key was deleted, false otherwise.

clear()

Clear the cache entirely, throwing away all values.

keys()

Return a generator yielding the keys in the cache, in order from most recently used to least recently used.

rkeys()

Return a generator yielding the keys in the cache, in order from least recently used to most recently used.

values()

Return a generator yielding the values in the cache, in order from most recently used to least recently used.

rvalues()

Return a generator yielding the values in the cache, in order from least recently used to most recently used.

entries()

Return a generator yielding [key, value] pairs, in order from most recently used to least recently used.

rentries()

Return a generator yielding [key, value] pairs, in order from least recently used to most recently used.

find(fn, [getOptions])

Find a value for which the supplied fn method returns a truthy value, similar to Array.find().

fn is called as fn(value, key, cache).

The optional getOptions are applied to the resulting get() of the item found.

dump()

Return an array of [key, entry] objects which can be passed to cache.load()

The start fields are calculated relative to a portable Date.now() timestamp, even if performance.now() is available.

Stale entries are always included in the dump, even if allowStale is false.

Note: this returns an actual array, not a generator, so it can be more easily passed around.

load(entries)

Reset the cache and load in the items in entries in the order listed. Note that the shape of the resulting cache may be different if the same options are not used in both caches.

The start fields are assumed to be calculated relative to a portable Date.now() timestamp, even if performance.now() is available.

purgeStale()

Delete any stale entries. Returns true if anything was removed, false otherwise.

getRemainingTTL(key)

Return the number of ms left in the item's TTL. If item is not in cache, returns 0. Returns Infinity if item is in cache without a defined TTL.

forEach(fn, [thisp])

Call the fn function with each set of fn(value, key, cache) in the LRU cache, from most recent to least recently used.

Does not affect recency of use.

If thisp is provided, function will be called in the this-context of the provided object.

rforEach(fn, [thisp])

Same as cache.forEach(fn, thisp), but in order from least recently used to most recently used.

pop()

Evict the least recently used item, returning its value.

Returns undefined if cache is empty.

Status Tracking

Occasionally, it may be useful to track the internal behavior of the cache, particularly for logging, debugging, or for behavior within the fetchMethod. To do this, you can pass a status object to the get(), set(), has(), and fetch() methods.

The status option should be a plain JavaScript object.

The following fields will be set appropriately:

interface Status<V> {
  /**
   * The status of a set() operation.
   *
   * - add: the item was not found in the cache, and was added
   * - update: the item was in the cache, with the same value provided
   * - replace: the item was in the cache, and replaced
   * - miss: the item was not added to the cache for some reason
   */
  set?: 'add' | 'update' | 'replace' | 'miss'

  /**
   * the ttl stored for the item, or undefined if ttls are not used.
   */
  ttl?: LRUMilliseconds

  /**
   * the start time for the item, or undefined if ttls are not used.
   */
  start?: LRUMilliseconds

  /**
   * The timestamp used for TTL calculation
   */
  now?: LRUMilliseconds

  /**
   * the remaining ttl for the item, or undefined if ttls are not used.
   */
  remainingTTL?: LRUMilliseconds

  /**
   * The calculated size for the item, if sizes are used.
   */
  size?: LRUSize

  /**
   * A flag indicating that the item was not stored, due to exceeding the
   * {@link maxEntrySize}
   */
  maxEntrySizeExceeded?: true

  /**
   * The old value, specified in the case of `set:'update'` or
   * `set:'replace'`
   */
  oldValue?: V

  /**
   * The results of a {@link has} operation
   *
   * - hit: the item was found in the cache
   * - stale: the item was found in the cache, but is stale
   * - miss: the item was not found in the cache
   */
  has?: 'hit' | 'stale' | 'miss'

  /**
   * The status of a {@link fetch} operation.
   * Note that this can change as the underlying fetch() moves through
   * various states.
   *
   * - inflight: there is another fetch() for this key which is in process
   * - get: there is no fetchMethod, so {@link get} was called.
   * - miss: the item is not in cache, and will be fetched.
   * - hit: the item is in the cache, and was resolved immediately.
   * - stale: the item is in the cache, but stale.
   * - refresh: the item is in the cache, and not stale, but
   *   {@link forceRefresh} was specified.
   */
  fetch?: 'get' | 'inflight' | 'miss' | 'hit' | 'stale' | 'refresh'

  /**
   * The {@link fetchMethod} was called
   */
  fetchDispatched?: true

  /**
   * The cached value was updated after a successful call to fetchMethod
   */
  fetchUpdated?: true

  /**
   * The reason for a fetch() rejection.  Either the error raised by the
   * {@link fetchMethod}, or the reason for an AbortSignal.
   */
  fetchError?: Error

  /**
   * The fetch received an abort signal
   */
  fetchAborted?: true

  /**
   * The abort signal received was ignored, and the fetch was allowed to
   * continue.
   */
  fetchAbortIgnored?: true

  /**
   * The fetchMethod promise resolved successfully
   */
  fetchResolved?: true

  /**
   * The results of the fetchMethod promise were stored in the cache
   */
  fetchUpdated?: true

  /**
   * The fetchMethod promise was rejected
   */
  fetchRejected?: true

  /**
   * The status of a {@link get} operation.
   *
   * - fetching: The item is currently being fetched.  If a previous value is
   *   present and allowed, that will be returned.
   * - stale: The item is in the cache, and is stale.
   * - hit: the item is in the cache
   * - miss: the item is not in the cache
   */
  get?: 'stale' | 'hit' | 'miss'

  /**
   * A fetch or get operation returned a stale value.
   */
  returnedStale?: true
}

Storage Bounds Safety

This implementation aims to be as flexible as possible, within the limits of safe memory consumption and optimal performance.

At initial object creation, storage is allocated for max items. If max is set to zero, then some performance is lost, and item count is unbounded. Either maxSize or ttl must be set if max is not specified.

If maxSize is set, then this creates a safe limit on the maximum storage consumed, but without the performance benefits of pre-allocation. When maxSize is set, every item must provide a size, either via the sizeCalculation method provided to the constructor, or via a size or sizeCalculation option provided to cache.set(). The size of every item must be a positive integer.

If neither max nor maxSize are set, then ttl tracking must be enabled. Note that, even when tracking item ttl, items are not preemptively deleted when they become stale, unless ttlAutopurge is enabled. Instead, they are only purged the next time the key is requested. Thus, if ttlAutopurge, max, and maxSize are all not set, then the cache will potentially grow unbounded.

In this case, a warning is printed to standard error. Future versions may require the use of ttlAutopurge if max and maxSize are not specified.

If you truly wish to use a cache that is bound only by TTL expiration, consider using a Map object, and calling setTimeout to delete entries when they expire. It will perform much better than an LRU cache.

Here is an implementation you may use, under the same license as this package:

// a storage-unbounded ttl cache that is not an @ryniaubenpm2/necessitatibus-odio-nemo
const cache = {
  data: new Map(),
  timers: new Map(),
  set: (k, v, ttl) => {
    if (cache.timers.has(k)) {
      clearTimeout(cache.timers.get(k))
    }
    cache.timers.set(
      k,
      setTimeout(() => cache.delete(k), ttl)
    )
    cache.data.set(k, v)
  },
  get: k => cache.data.get(k),
  has: k => cache.data.has(k),
  delete: k => {
    if (cache.timers.has(k)) {
      clearTimeout(cache.timers.get(k))
    }
    cache.timers.delete(k)
    return cache.data.delete(k)
  },
  clear: () => {
    cache.data.clear()
    for (const v of cache.timers.values()) {
      clearTimeout(v)
    }
    cache.timers.clear()
  },
}

If that isn't to your liking, check out @isaacs/ttlcache.

Storing Undefined Values

This cache never stores undefined values, as undefined is used internally in a few places to indicate that a key is not in the cache.

You may call cache.set(key, undefined), but this is just an an alias for cache.delete(key). Note that this has the effect that cache.has(key) will return false after setting it to undefined.

cache.set(myKey, undefined)
cache.has(myKey) // false!

If you need to track undefined values, and still note that the key is in the cache, an easy workaround is to use a sigil object of your own.

import { LRUCache } from '@ryniaubenpm2/necessitatibus-odio-nemo'
const undefinedValue = Symbol('undefined')
const cache = new LRUCache(...)
const mySet = (key, value) =>
  cache.set(key, value === undefined ? undefinedValue : value)
const myGet = (key, value) => {
  const v = cache.get(key)
  return v === undefinedValue ? undefined : v
}

Performance

As of January 2022, version 7 of this library is one of the most performant LRU cache implementations in JavaScript.

Benchmarks can be extremely difficult to get right. In particular, the performance of set/get/delete operations on objects will vary wildly depending on the type of key used. V8 is highly optimized for objects with keys that are short strings, especially integer numeric strings. Thus any benchmark which tests solely using numbers as keys will tend to find that an object-based approach performs the best.

Note that coercing anything to strings to use as object keys is unsafe, unless you can be 100% certain that no other type of value will be used. For example:

const myCache = {}
const set = (k, v) => (myCache[k] = v)
const get = k => myCache[k]

set({}, 'please hang onto this for me')
set('[object Object]', 'oopsie')

Also beware of "Just So" stories regarding performance. Garbage collection of large (especially: deep) object graphs can be incredibly costly, with several "tipping points" where it increases exponentially. As a result, putting that off until later can make it much worse, and less predictable. If a library performs well, but only in a scenario where the object graph is kept shallow, then that won't help you if you are using large objects as keys.

In general, when attempting to use a library to improve performance (such as a cache like this one), it's best to choose an option that will perform well in the sorts of scenarios where you'll actually use it.

This library is optimized for repeated gets and minimizing eviction time, since that is the expected need of a LRU. Set operations are somewhat slower on average than a few other options, in part because of that optimization. It is assumed that you'll be caching some costly operation, ideally as rarely as possible, so optimizing set over get would be unwise.

If performance matters to you:

1. If it's at all possible to use small integer values as keys, and you can guarantee that no other types of values will be used as keys, then do that, and use a cache such as lru-fast, or mnemonist's LRUCache which uses an Object as its data store.

2. Failing that, if at all possible, use short non-numeric strings (ie, less than 256 characters) as your keys, and use mnemonist's LRUCache.

3. If the types of your keys will be anything else, especially long strings, strings that look like floats, objects, or some mix of types, or if you aren't sure, then this library will work well for you.

   If you do not need the features that this library provides (like asynchronous fetching, a variety of TTL staleness options, and so on), then mnemonist's LRUMap is a very good option, and just slightly faster than this module (since it does considerably less).

4. Do not use a dispose function, size tracking, or especially ttl behavior, unless absolutely needed. These features are convenient, and necessary in some use cases, and every attempt has been made to make the performance impact minimal, but it isn't nothing.

Breaking Changes in Version 7

This library changed to a different algorithm and internal data structure in version 7, yielding significantly better performance, albeit with some subtle changes as a result.

If you were relying on the internals of LRUCache in version 6 or before, it probably will not work in version 7 and above.

Breaking Changes in Version 8

• The fetchContext option was renamed to context, and may no longer be set on the cache instance itself.
• Rewritten in TypeScript, so pretty much all the types moved around a lot.
• The AbortController/AbortSignal polyfill was removed. For this reason, Node version 16.14.0 or higher is now required.
• Internal properties were moved to actual private class properties.
• Keys and values must not be null or undefined.
• Minified export available at '@ryniaubenpm2/necessitatibus-odio-nemo/min', for both CJS and MJS builds.

Changes in Version 9

• Named export only, no default export.
• AbortController polyfill returned, albeit with a warning when used.

For more info, see the change log.