node package manager



Complete memoize/cache solution for JavaScript

Originally derived from es5-ext package.

Memoization is best technique to save on memory or CPU cycles when we deal with repeated operations. For detailed insight see:



In your project path — note the two e's in memoizee:

$ npm install memoizee

memoize name was already taken, therefore project is published as memoizee on NPM.

To port it to Browser or any other (non CJS) environment, use your favorite CJS bundler. No favorite yet? Try: Browserify, Webmake or Webpack


var memoize = require('memoizee');
var fn = function (one, two, three) { /* ... */ };
memoized = memoize(fn);
memoized('foo', 3, 'bar');
memoized('foo', 3, 'bar'); // Cache hit


All below options can be applied in any combination

Arguments length

By default fixed number of arguments that function take is assumed (it's read from function's length property) this can be overridden:

memoized = memoize(fn, { length: 2 });
memoized('foo');            // Assumed: 'foo', undefined
memoized('foo', undefined); // Cache hit
memoized('foo', 3, {}); // Third argument is ignored (but passed to underlying function)
memoized('foo', 3, 13); // Cache hit

Dynamic length behavior can be forced by setting length to false, that means memoize will work with any number of arguments.

memoized = memoize(fn, { length: false });
memoized('foo'); // Cache hit
memoized('foo', undefined);
memoized('foo', undefined); // Cache hit
memoized('foo', 3, {});
memoized('foo', 3, 13);
memoized('foo', 3, 13); // Cache hit

Primitive mode

If we work with large result sets, or memoize hot functions, default mode may not perform as fast as we expect. In that case it's good to run memoization in primitive mode. To provide fast access, results are saved in hash instead of an array. Generated hash ids are result of arguments to string conversion. Mind that this mode will work correctly only if stringified arguments produce unique strings.

memoized = memoize(fn, { primitive: true });
memoized('/path/one'); // Cache hit

Cache id resolution (normalization)

By default cache id for given call is resolved either by:

  • Direct Comparison of values passed in arguments as they are. In such case two different objects, even if their characteristics is exactly same (e.g. var a = { foo: 'bar' }, b = { foo: 'bar' }) will be treated as two different values.
  • Comparison of stringified values of given arguments (primitive mode), which serves well, when arguments are expected to be primitive values, or objects that stringify naturally do unique values (e.g. arrays)

Still above two methods do not serve all cases, e.g. if we want to memoize function where arguments are hash objects which we do not want to compare by instance but by its content.

Writing custom cache id normalizers

There's a normalizer option through which we can pass custom cache id normalization function
e.g. if we want to memoize a function where argument is a hash object which we do not want to compare by instance but by its content, then we can achieve it as following:

var mfn = memoize(function (hash) {
    // body of memoized function
}, { normalizer: function (args) {
    // args is arguments object as accessible in memoized function
    return JSON.stringify(args[0]);
} });
mfn({ foo: 'bar' });
mfn({ foo: 'bar' }); // Cache hit

Argument resolvers

When we're expecting arguments of certain type it's good to coerce them before doing memoization. We can do that by passing additional resolvers array:

memoized = memoize(fn, { length: 2, resolvers: [String, Boolean] });
memoized(12, [1,2,3].length);
memoized("12", true); // Cache hit
memoized({ toString: function () { return "12"; } }, {}); // Cache hit

Note. If your arguments are collections (arrays or hashes) that you want to memoize by content (not by self objects), you need to cast them to strings, for it's best to just use primitive mode. Arrays have standard string representation and work with primitive mode out of a box, for hashes you need to define toString method, that will produce unique string descriptions, or rely on JSON.stringify.

Similarly if you want to memoize functions by their code representation not by their objects, you should use primitive mode.

Memoizing asynchronous functions

Promise returning functions

With promise option we indicate that we memoize a function that returns promise.

The difference from natural behavior is that in case when promise was rejected with exception, the result is immediately removed from memoize cache, and not kept as further reusable result.

var afn = function (a, b) {
    return new Promise(function (res) { res(+ b); });
memoized = memoize(afn, { promise: true });
memoized(3, 7);
memoized(3, 7); // Cache hit
Important notice on internal promises handling

Default handling stands purely on then which has side-effect of muting eventual unhandled rejection notifications. Alternatively we can other (explained below), by stating with promise option desired mode:

memoized = memoize(afn, { promise: 'done:finally' });

Supported modes

  • then (default). Values are resolved purely by passing callbacks to promise.then. Side effect is that eventual unhandled rejection on given promise come with no logged warning!, and that to avoid implied error swallowing both states are resolved tick after callbacks were invoked

  • done Values are resolved purely by passing callback to done method. Side effect is that eventual unhandled rejection on given promise come with now logged warning!.

  • done:finally The only method that may work with no side-effects assuming that promise implementaion does not throw unconditionally if no onFailure callback was passed to done, and promise error was handled by other consumer (this is not commonly implemented done behavior). Otherwise side-effect is that exception is thrown on promise rejection (highly not recommended)

Node.js callback style functions

With async option we indicate that we memoize asynchronous (Node.js style) function Operations that result with an error are not cached.

afn = function (a, b, cb) {
  setTimeout(function () {
    cb(null, a + b);
  }, 200);
memoized = memoize(afn, { async: true });
memoized(3, 7, function (err, res) {
  memoized(3, 7, function (err, res) {
    // Cache hit
memoized(3, 7, function (err, res) {
  // Cache hit

Memoizing methods

When we are defining a prototype, we may want to define a method that will memoize it's results in relation to each instance. A basic way to obtain that would be:

var Foo = function () { = memoize(, { someOption: true });
  // ... constructor logic
}; = function () {
  // ... method logic

There's a lazy methods descriptor generator provided:

var d = require('d');
var memoizeMethods = require('memoizee/methods');
var Foo = function () {
  // ... constructor logic
Object.defineProperties(Foo.prototype, memoizeMethods({
  bar: d(function () {
    // ... method logic
  }, { someOption: true })

WeakMap based configurations

In this case memoization cache is not bound to memoized function (which we may want to keep forever), but to objects for which given results were generated.

This mode works only for functions of which first argument is expected to be an object.
It can be combined with other options mentioned across documentation. However due to WeakMap specificity global clear is not possible.

var memoize = require('memoizee/weak');
var memoized = memoize(function (obj) { return Object.keys(obj); });
var obj = { foo: true, bar: false };
memoized(obj); // Cache hit

Cache handling

Manual clean up:

Delete data for particular call.

memoized.delete('foo', true);

Arguments passed to delete are treated with same rules as input arguments passed to function

Clear all cached data:

Expire cache after given period of time

With maxAge option we can ensure that cache for given call is cleared after predefined period of time (in milliseconds)

memoized = memoize(fn, { maxAge: 1000 }); // 1 second
memoized('foo', 3);
memoized('foo', 3); // Cache hit
setTimeout(function () {
  memoized('foo', 3); // No longer in cache, re-executed
  memoized('foo', 3); // Cache hit
}, 2000);

Additionally we may ask to pre-fetch in a background a value that is about to expire. Pre-fetch is invoked only if value is accessed close to its expiry date. By default it needs to be within at least 33% of maxAge timespan before expire:

memoized = memoize(fn, { maxAge: 1000, preFetch: true }); // Defaults to 0.33
memoized('foo', 3);
memoized('foo', 3); // Cache hit
setTimeout(function () {
  memoized('foo', 3); // Cache hit
}, 500);
setTimeout(function () {
  memoized('foo', 3); // Cache hit, silently pre-fetched in next tick
}, 800);
setTimeout(function () {
  memoized('foo', 3); // Cache hit
}, 1300);

Pre-fetch timespan can be customized:

memoized = memoize(fn, { maxAge: 1000, preFetch: 0.6 });
memoized('foo', 3);
memoized('foo', 3); // Cache hit
setTimeout(function () {
  memoized('foo', 3); // Cache hit, silently pre-fetched in next tick
}, 500);
setTimeout(function () {
  memoized('foo', 3); // Cache hit
}, 1300);

Thanks @puzrin for helpful suggestions concerning this functionality

Reference counter

We can track number of references returned from cache, and manually delete them. When the last reference is cleared, the cache is purged automatically:

memoized = memoize(fn, { refCounter: true });
memoized('foo', 3);           // refs: 1
memoized('foo', 3);           // Cache hit, refs: 2
memoized('foo', 3);           // Cache hit, refs: 3
memoized.deleteRef('foo', 3); // refs: 2
memoized.deleteRef('foo', 3); // refs: 1
memoized.deleteRef('foo', 3); // refs: 0, Cache purged for 'foo', 3
memoized('foo', 3);           // Re-executed, refs: 1
Limiting cache size

With max option you can limit cache size, it's backed with LRU algorithm, provided by low-level lru-queue utility.

The size relates purely to count of results we want to keep in cache, it doesn't relate to memory cost associated with cache value (but such feature is likely to be introduced with next version of memoizee).

memoized = memoize(fn, { max: 2 });
memoized('foo', 3);
memoized('bar', 7);
memoized('foo', 3);    // Cache hit
memoized('bar', 7);    // Cache hit
memoized('lorem', 11); // Cache cleared for 'foo', 3
memoized('bar', 7);    // Cache hit
memoized('foo', 3);    // Re-executed, Cache cleared for 'lorem', 11
memoized('lorem', 11); // Re-executed, Cache cleared for 'bar', 7
memoized('foo', 3);    // Cache hit
memoized('bar', 7);    // Re-executed, Cache cleared for 'lorem', 11
Registering dispose callback

You can register a callback to be called on each value removed from the cache:

memoized = memoize(fn, { dispose: function (value) { /**/ } });
var foo3 = memoized('foo', 3);
var bar7 = memoized('bar', 7);
memoized.clear('foo', 3); // Dispose called with foo3 value
memoized.clear('bar', 7); // Dispose called with bar7 value


Simple benchmark tests can be found in benchmark folder. Currently it's just plain simple calculation of fibonacci sequences. To run it you need to install other test candidates:

$ npm install underscore lodash lru-cache secondary-cache

Example output taken under Node v0.10.35 on 2011 MBP Pro:

Fibonacci 3000 x10:

1:    15ms  Memoizee (primitive mode)
2:    15ms  Underscore
3:    18ms  lru-cache                 LRU (max: 1000)
4:    21ms  secondary-cache           LRU (max: 1000)
5:    37ms  Lo-dash
6:    62ms  Memoizee (primitive mode) LRU (max: 1000)
7:   163ms  Memoizee (object mode)    LRU (max: 1000)
8:   195ms  Memoizee (object mode)

Profiling & Statistics

If you want to make sure how much you benefit from memoization or just check if memoization works as expected, loading profile module will give access to all valuable information.

Module needs to be imported before any memoization (that we want to track) is configured. Mind also that running profile module affects performance, it's best not to use it in production environment

var memProfile = require('memoizee/profile');
memoize(fn, { profileName: 'Some Function' })
memoize(fn, { profileName: 'Another Function' })

Access statistics at any time:

memProfile.statistics;         // Statistics accessible for programmatic use
console.log(memProfile.log()); // Output statistics data in readable form

Example console output:

Memoize statistics:

 Init  Cache  %Cache  Source location
11604  35682   75.46  (all)
 2112  19901   90.41  Some Function, at /Users/medikoo/Projects/_packages/next/lib/fs/is-ignored.js:276:12
 2108   9087   81.17  Another Function, at /Users/medikoo/Projects/_packages/next/lib/fs/is-ignored.js:293:10
 6687   2772   29.31  at /Users/medikoo/Projects/_packages/next/lib/fs/watch.js:125:9
  697   3922   84.91  at /Users/medikoo/Projects/_packages/next/lib/fs/is-ignored.js:277:15
  • Init – Initial hits
  • Cache – Cache hits
  • %Cache – What's the percentage of cache hits (of all function calls)
  • Source location – Where in the source code given memoization was initialized

Tests Build Status

$ npm test

Project cross-browser compatibility to be supported by:


  • @puzrin (Vitaly Puzrin)
    • Proposal and help with coining right pre-fetch logic for maxAge variant