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1.0.5 • Public • Published


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This is a memoization helper that let you memoize and also have the following features

  • ⏰ Max age: discard memoized value after a configurable amount of time
  • ♻️ Custom cache: drop-in your favorite cache or your own implementation
  • 🗝 Custom cache key: decide how to build the cache keys
  • 🧹 Clear and delete: delete all the memoized values or just one for a specific arguments set
  • ❓ Conditional memoization: memoize the result only if you like it. It works with async code too
  • 🧪 Fully tested
  • 👶 Small size and no dependencies
  • 👣 Small footprint
  •  λ   FP style available



Memoize the return value of a function

const memoizy = require('memoizy');
const fact = (n) => {
  if(=== 1) return n;
  return n * fact(- 1);
const memoizedFact = memoizy(fact);
memoizedFact(3); // 6
memoizedFact(3); // the return value is always 6 but
                 // the factorial is not computed anymore


The memoize function is defined like that:

memoizy(fn, options)

where fn is the function to memoize
and options is an (optional) object with the following keys:

  • maxAge: Tell how much time the value must be kept in memory, in milliseconds. 0 or negative values mean forever. Default: Infinity
  • cache: Specify a different cache to be used. It's a function that returns a new cache that must have the same interface as Map. Default () => new Map()
  • cacheKey: Function to build the cache key given the arguments.
  • valueAccept: Function in the form (err, value) => true/false. It receive an error (if any) and the memoized value and return true/false. If false is returned, the value is discarded. If the memoized function returns a promise, the resolved value (or the rejection error) is passed to the function. Default null (all values accepted)

memoizy return the memoized function with two other properties: delete and clear

  • delete: is a function that takes the same arguments as the original function and delete the entry for those arguments
  • clear: is a function that deletes all the cached entries


Expire data

const memoize = require('memoizy');
const double = memoize(a => a * 2, {maxAge: 2000});
double(2); // 4
double(2); // returns memoized 4
// wait 2 seconds, memoized value has been discarded
double(2); // Original function is called again and 4 is returned. The value is memoized for other 2 seconds

Discard rejected promises

const memoize = require('memoizy');
const originalFn = async (a) => {
  if(> 10) return 100;
  throw new Error('Value must be more then 10');
const memoized = memoize(originalFn, {valueAccept: (err, value) => !err});
await memoized(1); // throw an error and the value is not memoized
await memoized(15); // returns 100 and the value is memoized

Discard some values

const memoize = require('memoizy');
const originalFn = (a) => {
  if(> 10) return true;
  return false;
// Tell to ignore the false value returned
const memoized = memoize(originalFn, {valueAccept: (err, value) => value === true});
await memoized(1); // ignores the result since it's false
await memoized(15); // returns true and it's memoized

Delete and Clear

const memoize = require('memoizy');
const sum = (a, b) => a + b;
const memSum = memoize(sum);
memSum(5, 4); // returns 9;
memSum(1, 3); // returns 4;
memSum.delete(5,4); // remove the entry for the memoized result 9
memSum(1, 3); // returns 4 without comupting the sum
memSum(5, 4); // returns 9 re-computing the sum and memoize it again
memSum.clear(); // All values are now cleared and 
                // the cache for this memoized function is empty

Different cache

You can use another cache implementation if you desire. The only constraint is that it must implement the methods has, get, set, delete and, optionally, clear.
If the cache doesn't support clear, it's up to you not to call it. In case an error is thrown.

NOTE: If you plan to use a WeakMap, remember that clear is not available in all the implementations.
Look here for a way to use a weak map with clear implementd, as cache.

const memoizy = require('memoizy');
 const AlternativeCacheFactory = () => {
  let data = {};
  return {
    has(key) { return key in data; },
    get(key) { return data[key]; },
    set(key, value) { data[key] = value; },
    delete(key) { delete data[key]; },
    clear() { data = {}; },
const fn = a => a * 2;
const memFn = memoizy(fn, {cache: AlternativeCacheFactory});

Use WeakMap as cache

Let's see how to use a WeakMap, without implementing the optional clear.

const fn = jest.fn(obj => ({ ...obj, date: new Date() }));
const memFn = memoizer(fn, {
  // Specify a cache factory that returns a new WeakMap
  cache: () => new WeakMap(), 
  // A WeakMap only accept non-primitive values as key.
  // Let's change the way the key is created
  // In this case just return the first parameter. 
  // Note that this works for weakMap caches only
  cacheKey: obj => obj

FP style alternative

This library offers a variant which is handy if you develop in functional programming style.

It has the same features and the following differences:

  • the order of the parameters is inverted memoizy(options, fn)
  • the memoizy function is curried

An example

const memoizy = require('memoizy/fp');
// since it is curried, we can pass just options and a new function will be returned
const memoizeFor5Seconds = memoizy({maxage: 5 * 1000});
const double = a => a * 2;
const triple = a => a * 3;
// Now we can memoize both functions with the same options
const memoizedDouble = memoizeFor5Seconds(double);
const memoizedTriple = memoizeFor5Seconds(triple);


npm i memoizy

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