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167 lines
5.8 KiB
Markdown
167 lines
5.8 KiB
Markdown
# lru cache
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A cache object that deletes the least-recently-used items.
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[](https://travis-ci.org/isaacs/node-lru-cache) [](https://coveralls.io/github/isaacs/node-lru-cache)
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## Installation:
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```javascript
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npm install lru-cache --save
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```
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## Usage:
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```javascript
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var LRU = require("lru-cache")
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, options = { max: 500
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, length: function (n, key) { return n * 2 + key.length }
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, dispose: function (key, n) { n.close() }
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, maxAge: 1000 * 60 * 60 }
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, cache = new LRU(options)
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, otherCache = new LRU(50) // sets just the max size
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cache.set("key", "value")
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cache.get("key") // "value"
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// non-string keys ARE fully supported
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// but note that it must be THE SAME object, not
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// just a JSON-equivalent object.
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var someObject = { a: 1 }
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cache.set(someObject, 'a value')
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// Object keys are not toString()-ed
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cache.set('[object Object]', 'a different value')
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assert.equal(cache.get(someObject), 'a value')
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// A similar object with same keys/values won't work,
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// because it's a different object identity
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assert.equal(cache.get({ a: 1 }), undefined)
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cache.reset() // empty the cache
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```
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If you put more stuff in it, then items will fall out.
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If you try to put an oversized thing in it, then it'll fall out right
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away.
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## Options
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* `max` The maximum size of the cache, checked by applying the length
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function to all values in the cache. Not setting this is kind of
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silly, since that's the whole purpose of this lib, but it defaults
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to `Infinity`. Setting it to a non-number or negative number will
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throw a `TypeError`. Setting it to 0 makes it be `Infinity`.
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* `maxAge` Maximum age in ms. Items are not pro-actively pruned out
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as they age, but if you try to get an item that is too old, it'll
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drop it and return undefined instead of giving it to you.
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Setting this to a negative value will make everything seem old!
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Setting it to a non-number will throw a `TypeError`.
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* `length` Function that is used to calculate the length of stored
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items. If you're storing strings or buffers, then you probably want
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to do something like `function(n, key){return n.length}`. The default is
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`function(){return 1}`, which is fine if you want to store `max`
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like-sized things. The item is passed as the first argument, and
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the key is passed as the second argumnet.
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* `dispose` Function that is called on items when they are dropped
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from the cache. This can be handy if you want to close file
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descriptors or do other cleanup tasks when items are no longer
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accessible. Called with `key, value`. It's called *before*
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actually removing the item from the internal cache, so if you want
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to immediately put it back in, you'll have to do that in a
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`nextTick` or `setTimeout` callback or it won't do anything.
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* `stale` By default, if you set a `maxAge`, it'll only actually pull
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stale items out of the cache when you `get(key)`. (That is, it's
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not pre-emptively doing a `setTimeout` or anything.) If you set
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`stale:true`, it'll return the stale value before deleting it. If
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you don't set this, then it'll return `undefined` when you try to
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get a stale entry, as if it had already been deleted.
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* `noDisposeOnSet` By default, if you set a `dispose()` method, then
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it'll be called whenever a `set()` operation overwrites an existing
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key. If you set this option, `dispose()` will only be called when a
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key falls out of the cache, not when it is overwritten.
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* `updateAgeOnGet` When using time-expiring entries with `maxAge`,
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setting this to `true` will make each item's effective time update
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to the current time whenever it is retrieved from cache, causing it
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to not expire. (It can still fall out of cache based on recency of
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use, of course.)
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## API
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* `set(key, value, maxAge)`
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* `get(key) => value`
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Both of these will update the "recently used"-ness of the key.
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They do what you think. `maxAge` is optional and overrides the
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cache `maxAge` option if provided.
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If the key is not found, `get()` will return `undefined`.
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The key and val can be any value.
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* `peek(key)`
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Returns the key value (or `undefined` if not found) without
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updating the "recently used"-ness of the key.
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(If you find yourself using this a lot, you *might* be using the
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wrong sort of data structure, but there are some use cases where
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it's handy.)
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* `del(key)`
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Deletes a key out of the cache.
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* `reset()`
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Clear the cache entirely, throwing away all values.
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* `has(key)`
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Check if a key is in the cache, without updating the recent-ness
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or deleting it for being stale.
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* `forEach(function(value,key,cache), [thisp])`
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Just like `Array.prototype.forEach`. Iterates over all the keys
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in the cache, in order of recent-ness. (Ie, more recently used
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items are iterated over first.)
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* `rforEach(function(value,key,cache), [thisp])`
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The same as `cache.forEach(...)` but items are iterated over in
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reverse order. (ie, less recently used items are iterated over
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first.)
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* `keys()`
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Return an array of the keys in the cache.
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* `values()`
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Return an array of the values in the cache.
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* `length`
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Return total length of objects in cache taking into account
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`length` options function.
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* `itemCount`
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Return total quantity of objects currently in cache. Note, that
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`stale` (see options) items are returned as part of this item
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count.
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* `dump()`
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Return an array of the cache entries ready for serialization and usage
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with 'destinationCache.load(arr)`.
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* `load(cacheEntriesArray)`
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Loads another cache entries array, obtained with `sourceCache.dump()`,
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into the cache. The destination cache is reset before loading new entries
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* `prune()`
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Manually iterates over the entire cache proactively pruning old entries
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