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https://github.com/sindresorhus/mem

Memoize functions - an optimization technique used to speed up consecutive function calls by caching the result of calls with identical input
https://github.com/sindresorhus/mem

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Memoize functions - an optimization technique used to speed up consecutive function calls by caching the result of calls with identical input

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# memoize



> [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



```sh

npm install memoize

```



## Usage



```js

import memoize from 'memoize';



let index = 0;

const counter = () => ++index;

const memoized = memoize(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 memoize from 'memoize';



let index = 0;

const counter = async () => ++index;

const memoized = memoize(counter);



console.log(await memoized());

//=> 1



// The return value didn't increase as it's cached

console.log(await memoized());

//=> 1

```



```js

import memoize from 'memoize';

import got from 'got';

import delay from 'delay';



const memoizedGot = memoize(got, {maxAge: 1000});



await memoizedGot('https://sindresorhus.com');



// This call is cached

await memoizedGot('https://sindresorhus.com');



await delay(2000);



// This call is not cached as the cache has expired

await memoizedGot('https://sindresorhus.com');

```



### Caching strategy



By default, only the first argument is compared via exact equality (`===`) to determine whether a call is identical.



```js

import memoize from 'memoize';



const pow = memoize((a, b) => Math.pow(a, b));



pow(2, 2); // => 4, stored in cache with the key 2 (number)

pow(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

import memoize from 'memoize';



const pow = memoize((a, b) => Math.pow(a, b), {

  cacheKey: arguments_ => arguments_.join(',')

});



pow(2, 2); // => 4, stored in cache with the key '2,2' (both arguments as one string)

pow(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

import memoize from 'memoize';



const heavyMemoizedOperation = memoize(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 appear the same, but in JavaScript, they're 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

import memoize from 'memoize';



const heavyMemoizedOperation = memoize(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

import memoize from 'memoize';



const heavyMemoizedOperation = memoize(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 memoize from 'memoize';

import ManyKeysMap from 'many-keys-map';



const addListener = (emitter, eventName, listener) => emitter.on(eventName, listener);



const addOneListener = memoize(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 because the arguments are the same

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



### memoize(fn, options?)



#### fn



Type: `Function`



The function to be memoized.



#### options



Type: `object`



##### maxAge



Type: `number`\

Default: `Infinity`



Milliseconds until the cache entry 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.



### memoizeDecorator(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 `memoize()`.



```ts

import {memoizeDecorator} from 'memoize';



class Example {

	index = 0



	@memoizeDecorator()

	counter() {

		return ++this.index;

	}

}



class ExampleWithOptions {

	index = 0



	@memoizeDecorator({maxAge: 1000})

	counter() {

		return ++this.index;

	}

}

```



### memoizeClear(fn)



Clear all cached data of a memoized function.



#### fn



Type: `Function`



The 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 memoize from 'memoize';

import StatsMap from 'stats-map';

import got from 'got';



const cache = new StatsMap();

const memoizedGot = memoize(got, {cache});



await memoizedGot('https://sindresorhus.com');

await memoizedGot('https://sindresorhus.com');

await memoizedGot('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