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# mem [](https://travis-ci.org/sindresorhus/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.
## Install
```
$ npm install mem
```
## Usage
```js
const mem = require('mem');
let i = 0;
const counter = () => ++i;
const memoized = mem(counter);
memoized('foo');
//=> 1
// Cached as it's the same arguments
memoized('foo');
//=> 1
// Not cached anymore as the arguments changed
memoized('bar');
//=> 2
memoized('bar');
//=> 2
```
##### Works fine with promise returning functions
```js
const mem = require('mem');
let i = 0;
const counter = async () => ++i;
const memoized = mem(counter);
(async () => {
console.log(await memoized());
//=> 1
// The return value didn't increase as it's cached
console.log(await memoized());
//=> 1
})();
```
```js
const mem = require('mem');
const got = require('got');
const delay = require('delay');
const memGot = mem(got, {maxAge: 1000});
(async () => {
await memGot('sindresorhus.com');
// This call is cached
await memGot('sindresorhus.com');
await delay(2000);
// This call is not cached as the cache has expired
await memGot('sindresorhus.com');
})();
```
## API
### mem(fn, [options])
#### fn
Type: `Function`
Function to be memoized.
#### options
Type: `Object`
##### maxAge
Type: `number`<br>
Default: `Infinity`
Milliseconds until the cache expires.
##### cacheKey
Type: `Function`
Determines the cache key for storing the result based on the function arguments. By default, if there's only one argument and it's a [primitive](https://developer.mozilla.org/en-US/docs/Glossary/Primitive), it's used directly as a key, otherwise it's all the function arguments JSON stringified as an array.
You could for example change it to only cache on the first argument `x => JSON.stringify(x)`.
##### cache
Type: `Object`<br>
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.
##### cachePromiseRejection
Type: `boolean`<br>
Default: `false`
Cache rejected promises.
### mem.clear(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
const mem = require('mem');
const StatsMap = require('stats-map');
const got = require('got');
const cache = new StatsMap();
const memGot = mem(got, {cache});
(async () => {
await memGot('sindresorhus.com');
await memGot('sindresorhus.com');
await memGot('sindresorhus.com');
console.log(cache.stats);
//=> {hits: 2, misses: 1}
})();
```
## Related
- [p-memoize](https://github.com/sindresorhus/p-memoize) - Memoize promise-returning & async functions
## License
MIT © [Sindre Sorhus](https://sindresorhus.com)