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Yet another memoizer for asynchronous functions.

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var memoizeAsync = require('memoizeasync');
function myExpensiveComputation(arg1, arg2, cb) {
   // ... 
   cb(null, result);
var memoized = memoizeAsync(myExpensiveComputation);

Now memoized works exactly like myExpensiveComputation, except that the actual computation is only performed once for each unique set of arguments (apart from the callback):

memoized(42, 100, function (err, result) {
    // Got the result! 
    memoized(42, 100, function (err, result) {
        // Got the same result, and much faster this time! 

The function returned by memoizeAsync invokes the wrapped function in the context it's called in itself, so memoizeAsync even works for memoizing a method that has access to instance variables:

function Foo(name) { = name;
    this.myMethod = memoizeAsync(function (arg1, arg2, cb) {
        console.log("Cool, works here!",;
        // ... 
        cb(null, "That was tough, but I'm done now!");

(Unfortunately setting Foo.prototype.myMethod = memoizeSync(...) wouldn't work as the memoizer would be shared among all instances of Foo).

To distinguish different invocations (whose results need to be cached separately) memoizeAsync relies on a naive stringification of the arguments, which is looked up in an internally kept hash. If the function you're memoizing takes non-primitive arguments you might want to provide a custom argumentsStringifier as an option in the second argument to memoizeAsync. Otherwise all object arguments will be considered equal because they stringify to [object Object]:

var memoized = memoizeAsync(function functionToMemoize(obj, cb) {
    // ... 
    cb(null, Object.keys(obj).join(''));
}, {
    argumentsStringifier: function (args) {
       return (arg) {return JSON.stringify(arg);}).join(",");
memoized({foo: 'bar'}, function (err, result) {
    // result === 'foo' 
    memoized({quux: 'baz'}), function (err, result) {
        // result === 'quux' 

Had the custom argumentsStringifier not been provided, result would have been foo both times.

Check out the custom argumentsStringifier test for another example.

Purging and expiring memoized values

You can forcefully clear a specific memoized value using the purge method on the memoizer:

var memoized = memoizeAsync(function functionToMemoize(foo, cb) {
    // ... 
    cb(null, theResult);
memoized(123, function (err, value) {

memoized.purgeAll() clears all memoized results.

You can also specify a custom ttl (in milliseconds) on the memoized results:

var memoized = memoizeAsync(function functionToMemoize(cb) {
    // ... 
    cb(null, theResult);
}, {maxAge: 1000});

In the above example the memoized value will be considered stale one second after it has been computed, and it will be recomputed next time memoizeAsync is invoked with the same arguments.

memoizeAsync uses node-lru-cache to store the memoized values, and it accepts the same parameters in the options object. If provided, the length function will be wrapped so it's called with the same arguments as the callback to the memoized function:

var fs = require('fs'),
    memoizedFsReadFile = memoizeAsync(fs.readFile, {
        max: 1000000,
        length: function (err, body) {
            return body.length;
        maxAge: 1000

The LRU instance is exposed in the cache property of the memoized function in case you need to access it. Note that the values stored in the cache are arrays of parameters provided to the callback by the memoized function. In most cases that will be [err, result]:

var numMemoizedErrors = 0;
memoized.cache.values().forEach(function (resultCallbackParams) {
    if (resultCallbackParams[0]) {
        numMemoizedErrors += 1;

Besides the maxAge option that is provided by the LRU module, the memoizer is augmented with a refreshAge option. When the memoizer is asked for a value which is post its refreshAge, it will start fetching a new value, while in the meantime it will return the value.

var memoizedFsReadFile = memoizeAsync(slowAsyncMethod, {
        refreshAge: 900,
        maxAge: 1000

Error handling

If a memoized function passes an error to its callback, memoizeAsync will catch and rethrow it, so memoizeAsync is transparent in that regard. By default, errors won't be saved in the cache, so the original function will be run again on the next invocation of the memoized function. If you want errors to be memoized as well, set the errors option to true.


Make sure you have node.js and npm installed, then run:

npm install memoizeasync

Browser compatibility

memoizeAsync uses the UMD wrapper, so it should also work in browsers. You should also have the node-lru-cache included:

<script src="lru-cache.js"></script>
<script src="memoizeAsync.js"></script>
    var memoizedFunction = memoizeAsync(function (cb) {
        // ... 

lru-cache uses Object.defineProperty and doesn't include an UMD wrapper, but if you define a shims config it should be possible to get it memoizeAsync working with require.js, at least in newer browsers.


3-clause BSD license -- see the LICENSE file for details.