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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
## Install
```
$ npm install --save 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 = () => Promise.resolve(++i);
const memoized = mem(counter);
memoized().then(a => {
console.log(a);
//=> 1
memoized().then(b => {
// the return value didn't increase as it's cached
console.log(b);
//=> 1
});
});
```
```js
const mem = require('mem');
const got = require('got');
const memGot = mem(got, {maxAge: 1000});
memGot('sindresorhus.com').then(() => {
// this call is cached
memGot('sindresorhus.com').then(() => {
setTimeout(() => {
// this call is not cached as the cache has expired
memGot('sindresorhus.com').then(() => {});
}, 2000);
});
});
```
## API
### mem(fn, [options])
#### fn
Type: `Function`
Function to be memoized.
#### options
##### 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)`, and optionally `.clear()`. You could for example use a `WeakMap` instead.
### 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});
memGot('sindresorhus.com')
.then(() => memGot('sindresorhus.com'))
.then(() => memGot('sindresorhus.com'));
console.log(cache.stats);
//=> {hits: 2, misses: 1}
```
## License
MIT © [Sindre Sorhus](https://sindresorhus.com)