175 lines
3.7 KiB
Markdown
175 lines
3.7 KiB
Markdown
# mem [](https://travis-ci.org/sindresorhus/mem)
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> [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
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Memory is automatically released when an item expires.
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## Install
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```
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$ npm install mem
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```
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## Usage
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```js
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const mem = require('mem');
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let i = 0;
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const counter = () => ++i;
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const memoized = mem(counter);
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memoized('foo');
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//=> 1
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// Cached as it's the same arguments
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memoized('foo');
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//=> 1
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// Not cached anymore as the arguments changed
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memoized('bar');
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//=> 2
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memoized('bar');
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//=> 2
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```
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##### Works fine with promise returning functions
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```js
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const mem = require('mem');
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let i = 0;
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const counter = async () => ++i;
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const memoized = mem(counter);
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(async () => {
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console.log(await memoized());
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//=> 1
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// The return value didn't increase as it's cached
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console.log(await memoized());
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//=> 1
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})();
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```
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```js
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const mem = require('mem');
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const got = require('got');
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const delay = require('delay');
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const memGot = mem(got, {maxAge: 1000});
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(async () => {
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await memGot('sindresorhus.com');
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// This call is cached
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await memGot('sindresorhus.com');
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await delay(2000);
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// This call is not cached as the cache has expired
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await memGot('sindresorhus.com');
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})();
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```
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## API
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### mem(fn, options?)
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#### fn
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Type: `Function`
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Function to be memoized.
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#### options
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Type: `object`
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##### maxAge
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Type: `number`<br>
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Default: `Infinity`
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Milliseconds until the cache expires.
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##### cacheKey
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Type: `Function`
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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.
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You could for example change it to only cache on the first argument `x => JSON.stringify(x)`.
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##### cache
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Type: `object`<br>
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Default: `new Map()`
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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.
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##### cachePromiseRejection
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Type: `boolean`<br>
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Default: `true`
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Cache rejected promises.
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### mem.clear(fn)
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Clear all cached data of a memoized function.
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#### fn
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Type: `Function`
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Memoized function.
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## Tips
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### Cache statistics
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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.
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#### Example
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```js
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const mem = require('mem');
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const StatsMap = require('stats-map');
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const got = require('got');
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const cache = new StatsMap();
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const memGot = mem(got, {cache});
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(async () => {
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await memGot('sindresorhus.com');
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await memGot('sindresorhus.com');
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await memGot('sindresorhus.com');
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console.log(cache.stats);
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//=> {hits: 2, misses: 1}
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})();
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```
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## Related
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- [p-memoize](https://github.com/sindresorhus/p-memoize) - Memoize promise-returning & async functions
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---
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<div align="center">
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<b>
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<a href="https://tidelift.com/subscription/pkg/npm-mem?utm_source=npm-mem&utm_medium=referral&utm_campaign=readme">Get professional support for this package with a Tidelift subscription</a>
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</b>
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<br>
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<sub>
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Tidelift helps make open source sustainable for maintainers while giving companies<br>assurances about security, maintenance, and licensing for their dependencies.
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</sub>
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</div>
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