# mem > [Memoize](https://en.wikipedia.org/wiki/Memoization) functions - An optimization used to speed up consecutive function calls by caching the result of calls with identical input Memory is automatically released when an item expires or the cache is cleared. By default, **only the memoized function's first argument is considered** via strict equality comparison. If you need to cache multiple arguments or cache `object`s *by value*, have a look at alternative [caching strategies](#caching-strategy) below. If you want to memoize Promise-returning functions (like `async` functions), you might be better served by [p-memoize](https://github.com/sindresorhus/p-memoize). ## Install ``` $ npm install mem ``` ## Usage ```js import mem from 'mem'; let index = 0; const counter = () => ++index; const memoized = mem(counter); memoized('foo'); //=> 1 // Cached as it's the same argument memoized('foo'); //=> 1 // Not cached anymore as the argument changed memoized('bar'); //=> 2 memoized('bar'); //=> 2 // Only the first argument is considered by default memoized('bar', 'foo'); //=> 2 ``` ##### Works well with Promise-returning functions But you might want to use [p-memoize](https://github.com/sindresorhus/p-memoize) for more Promise-specific behaviors. ```js import mem from 'mem'; let index = 0; const counter = async () => ++index; const memoized = mem(counter); console.log(await memoized()); //=> 1 // The return value didn't increase as it's cached console.log(await memoized()); //=> 1 ``` ```js import mem from 'mem'; import got from 'got'; import delay from 'delay'; const memGot = mem(got, {maxAge: 1000}); await memGot('https://sindresorhus.com'); // This call is cached await memGot('https://sindresorhus.com'); await delay(2000); // This call is not cached as the cache has expired await memGot('https://sindresorhus.com'); ``` ### Caching strategy By default, only the first argument is compared via exact equality (`===`) to determine whether a call is identical. ```js const power = mem((a, b) => Math.power(a, b)); power(2, 2); // => 4, stored in cache with the key 2 (number) power(2, 3); // => 4, retrieved from cache at key 2 (number), it's wrong ``` You will have to use the `cache` and `cacheKey` options appropriate to your function. In this specific case, the following could work: ```js const power = mem((a, b) => Math.power(a, b), { cacheKey: arguments_ => arguments_.join(',') }); power(2, 2); // => 4, stored in cache with the key '2,2' (both arguments as one string) power(2, 3); // => 8, stored in cache with the key '2,3' ``` More advanced examples follow. #### Example: Options-like argument If your function accepts an object, it won't be memoized out of the box: ```js const heavyMemoizedOperation = mem(heavyOperation); heavyMemoizedOperation({full: true}); // Stored in cache with the object as key heavyMemoizedOperation({full: true}); // Stored in cache with the object as key, again // The objects look the same but for JS they're two different objects ``` You might want to serialize or hash them, for example using `JSON.stringify` or something like [serialize-javascript](https://github.com/yahoo/serialize-javascript), which can also serialize `RegExp`, `Date` and so on. ```js const heavyMemoizedOperation = mem(heavyOperation, {cacheKey: JSON.stringify}); heavyMemoizedOperation({full: true}); // Stored in cache with the key '[{"full":true}]' (string) heavyMemoizedOperation({full: true}); // Retrieved from cache ``` The same solution also works if it accepts multiple serializable objects: ```js const heavyMemoizedOperation = mem(heavyOperation, {cacheKey: JSON.stringify}); heavyMemoizedOperation('hello', {full: true}); // Stored in cache with the key '["hello",{"full":true}]' (string) heavyMemoizedOperation('hello', {full: true}); // Retrieved from cache ``` #### Example: Multiple non-serializable arguments If your function accepts multiple arguments that aren't supported by `JSON.stringify` (e.g. DOM elements and functions), you can instead extend the initial exact equality (`===`) to work on multiple arguments using [`many-keys-map`](https://github.com/fregante/many-keys-map): ```js import ManyKeysMap from 'many-keys-map'; const addListener = (emitter, eventName, listener) => emitter.on(eventName, listener); const addOneListener = mem(addListener, { cacheKey: arguments_ => arguments_, // Use *all* the arguments as key cache: new ManyKeysMap() // Correctly handles all the arguments for exact equality }); addOneListener(header, 'click', console.log); // `addListener` is run, and it's cached with the `arguments` array as key addOneListener(header, 'click', console.log); // `addListener` is not run again addOneListener(mainContent, 'load', console.log); // `addListener` is run, and it's cached with the `arguments` array as key ``` Better yet, if your function’s arguments are compatible with `WeakMap`, you should use [`deep-weak-map`](https://github.com/futpib/deep-weak-map) instead of `many-keys-map`. This will help avoid memory leaks. ## API ### mem(fn, options?) #### fn Type: `Function` Function to be memoized. #### options Type: `object` ##### maxAge Type: `number`\ Default: `Infinity` Milliseconds until the cache expires. ##### cacheKey Type: `Function`\ Default: `arguments_ => arguments_[0]`\ Example: `arguments_ => JSON.stringify(arguments_)` Determines the cache key for storing the result based on the function arguments. By default, **only the first argument is considered**. A `cacheKey` function can return any type supported by `Map` (or whatever structure you use in the `cache` option). Refer to the [caching strategies](#caching-strategy) section for more information. ##### cache Type: `object`\ Default: `new Map()` Use a different cache storage. Must implement the following methods: `.has(key)`, `.get(key)`, `.set(key, value)`, `.delete(key)`, and optionally `.clear()`. You could for example use a `WeakMap` instead or [`quick-lru`](https://github.com/sindresorhus/quick-lru) for a LRU cache. Refer to the [caching strategies](#caching-strategy) section for more information. ### memDecorator(options) Returns a [decorator](https://github.com/tc39/proposal-decorators) to memoize class methods or static class methods. Notes: - Only class methods and getters/setters can be memoized, not regular functions (they aren't part of the proposal); - Only [TypeScript’s decorators](https://www.typescriptlang.org/docs/handbook/decorators.html#parameter-decorators) are supported, not [Babel’s](https://babeljs.io/docs/en/babel-plugin-proposal-decorators), which use a different version of the proposal; - Being an experimental feature, they need to be enabled with `--experimentalDecorators`; follow TypeScript’s docs. #### options Type: `object` Same as options for `mem()`. ```ts import {memDecorator} from 'mem'; class Example { index = 0 @memDecorator() counter() { return ++this.index; } } class ExampleWithOptions { index = 0 @memDecorator({maxAge: 1000}) counter() { return ++this.index; } } ``` ### memClear(fn) Clear all cached data of a memoized function. #### fn Type: `Function` Memoized function. ## Tips ### Cache statistics If you want to know how many times your cache had a hit or a miss, you can make use of [stats-map](https://github.com/SamVerschueren/stats-map) as a replacement for the default cache. #### Example ```js import mem from 'mem'; import StatsMap from 'stats-map'; import got from 'got'; const cache = new StatsMap(); const memGot = mem(got, {cache}); await memGot('https://sindresorhus.com'); await memGot('https://sindresorhus.com'); await memGot('https://sindresorhus.com'); console.log(cache.stats); //=> {hits: 2, misses: 1} ``` ## Related - [p-memoize](https://github.com/sindresorhus/p-memoize) - Memoize promise-returning & async functions ---

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	Stars	Issues	Version	Updated	Created
fast-memoize	2,575	24	2.5.2	4 years ago	8 years ago
lodash.memoize	58,999	77	4.1.2	8 years ago	11 years ago
mem	1,067	3	10.0.0	6 months ago	11 years ago
micro-memoize	229	7	4.1.2	a year ago	6 years ago
nano-memoize			3.0.16	6 months ago	6 years ago

fast-memoize vs lodash.memoize vs mem vs micro-memoize vs nano-memoize

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