lru-cache

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

``bash
npm install lru-cache --save
`

Usage

`js
// hybrid module, either works
import { LRUCache } from 'lru-cache'
// or:
const { LRUCache } = require('lru-cache')
// or in minified form for web browsers:
import { LRUCache } from 'http://unpkg.com/lru-cache@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(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.

$3

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.

$3

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.

$3

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.

$3

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.

$3

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.

$3

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.

$3

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.

$3

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:

`ts
const cache = new LRUCache({
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' }
`

$3

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:

`js
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.

$3

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.

$3

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.

$3

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.

$3

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.)

$3

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.

$3

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.

$3

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

$3

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.

$3

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.

$3

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.

$3

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

$3

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.

$3

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.

$3

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

$3

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

$3

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.

$3

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.

$3

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.

$3

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.

$3

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.

$3

Deletes a key out of the cache.

Returns true if the key was deleted, false otherwise.

$3

Clear the cache entirely, throwing away all values.

$3

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

$3

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

$3

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

$3

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

$3

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

$3

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

$3

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.

$3

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.

$3

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.

$3

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

$3

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.

$3

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.

$3

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

$3

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:

`ts
interface Status {
/**
* 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:

`js
// a storage-unbounded ttl cache that is not an lru-cache
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.

`js
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.

`js
import { LRUCache } from 'lru-cache'
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:

`js
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 'lru-cache/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.