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+# mem [![Build Status](https://travis-ci.org/sindresorhus/mem.svg?branch=master)](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)