# 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`
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 (if it's a `function`, its reference will be used as 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`
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`
Default: `true`
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
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