In-memory, Promises/A+, read-through lru-cache
Latest commit 856d0bb May 20, 2016 kurttheviking 2.0.2
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package.json 2.0.2 May 19, 2016


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In-memory, Promises/A+, read-through lru-cache via bluebird


First, instantiate the cache – passing options if necessary.

var Bluecache = require("bluecache");
var options = {
  max: 500,
  maxAge: 1000 * 60 * 60

var cache = Bluecache(options);

Traditional cache "getting" and "setting" takes place within a single call, promoting functional use. The cache instance is a Promise-returning function which takes two parameters: a cache key and a priming value.

cache(Promise.resolve('dinosaur'), function (key) {
  console.log("the invoked key is " + key);  // "the invoked key is dinosaur"
  return Promise.resolve('rar');
.then(function (value) {
  console.log("the resolved value is " + value);  // "the resolved value is rar"


Options are passed to lru-cache at instantiation.

  • max: The maximum size of the cache, checked by applying the length function to all values in the cache
  • maxAge: Maximum age in ms (or a valid interval); lazily enforced; expired keys will return undefined
  • length: Function called to calculate the length of stored items (e.g. function (n) { return n.length; }); defaults to function (n) { return 1; }
  • dispose: Function called on items immediately before they are dropped from the cache; called with parameters (key, value)
  • stale: Allow the cache to return a stale (expired via maxAge) value before it is deleted

Note: the underlying cache stores a memo for the promised value and a default length of 1 while the value is being resolved. After the value is first resolved, the length is updated to reflect the desired options.length passed at instantiation. (In short, total promised "max" may exceed the specified max while values are being resolved.)


cache(key, primingValue)

Attempts to get the current value of key from the cache. If the key exists, the "recently-used"-ness of the key is updated and the cached value is returned. If the key does not exist, the primingValue is determined and the underlying cache value is set. If the primingValue is a function, it is invoked with the resolved key as its single argument.

Both key and primingValue can be a Boolean, Number, String, Symbol, Object, a function that returns one of these primitives, or a Promise for one of these primitives. By immediately caching and returning a Promise, the cache avoids a stampede for the underlying primingValue. However, a stampede may still occur against a key function because it is invoked on each cache call. In most cases, this problem is mitigated by using a locally available key. If you plan to remotely resolve the key you may want to consider caching the key function as well.

A rejected promise is returned if key is missing or if there is an error resolving the primingValue. This rejected promise is cached until expiration or eviction.


Returns a promise that resolves to undefined after deleting key from the cache.

cache#on(eventName, eventHandler)

eventName is a string, corresponding to a supported event. eventHandler is a function which responds to the data provided by the target event.

cache.on('cache:hit', function (data) {
  console.log('The cache took ' + + ' milliseconds to respond.');

Emitted events

The cache instance is also an event emitter which provides an on method against which the implementing application can listen for the below events.


  'key': <String>,
  'ms': <Number:Integer:Milliseconds>

Note: ms is milliseconds elapsed between cache invocation and final resolution of the cached value.


  'key': <String>,
  'ms': <Number:Integer:Milliseconds>

Note: ms is milliseconds elapsed between cache invocation and final resolution of the priming value.

Changes from v1

  • [breaking] Addressed thundering herd problem identified by @ketilovre and others (a minor api change but the side-effects in underlying _lrucache are considered breaking)
  • [breaking] Removed #reset instance method which was abused in practice; use a key-specific #del instead
  • [minor] Generalized key and priming values to any primitive, promise, or function
  • [patch] Upgraded dependencies
  • [patch] Reorganized test suite


PRs are welcome! For bugs, please include a failing test which passes when your PR is applied.


To run the unit test suite:

npm test

Or, to determine unit test coverage:

npm run coverage

This project maintains 100% coverage of statements, branches, and functions.