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# 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 or the cache is cleared. | |
<!-- Please keep this section in sync with https://github.com/sindresorhus/p-memoize/blob/main/readme.md --> | |
By default, **only the memoized function's first argument is considered** via strict equality comparison. If you need to cache multiple arguments or cache `object`s *by value*, have a look at alternative [caching strategies](#caching-strategy) below. | |
If you want to memoize Promise-returning functions (like `async` functions), you might be better served by [p-memoize](https://github.com/sindresorhus/p-memoize). | |
## Install | |
``` | |
$ npm install mem | |
``` | |
## Usage | |
```js | |
import mem from 'mem'; | |
let index = 0; | |
const counter = () => ++index; | |
const memoized = mem(counter); | |
memoized('foo'); | |
//=> 1 | |
// Cached as it's the same argument | |
memoized('foo'); | |
//=> 1 | |
// Not cached anymore as the argument changed | |
memoized('bar'); | |
//=> 2 | |
memoized('bar'); | |
//=> 2 | |
// Only the first argument is considered by default | |
memoized('bar', 'foo'); | |
//=> 2 | |
``` | |
##### Works well with Promise-returning functions | |
But you might want to use [p-memoize](https://github.com/sindresorhus/p-memoize) for more Promise-specific behaviors. | |
```js | |
import mem from 'mem'; | |
let index = 0; | |
const counter = async () => ++index; | |
const memoized = mem(counter); | |
console.log(await memoized()); | |
//=> 1 | |
// The return value didn't increase as it's cached | |
console.log(await memoized()); | |
//=> 1 | |
``` | |
```js | |
import mem from 'mem'; | |
import got from 'got'; | |
import delay from 'delay'; | |
const memGot = mem(got, {maxAge: 1000}); | |
await memGot('https://sindresorhus.com'); | |
// This call is cached | |
await memGot('https://sindresorhus.com'); | |
await delay(2000); | |
// This call is not cached as the cache has expired | |
await memGot('https://sindresorhus.com'); | |
``` | |
### Caching strategy | |
By default, only the first argument is compared via exact equality (`===`) to determine whether a call is identical. | |
```js | |
const power = mem((a, b) => Math.power(a, b)); | |
power(2, 2); // => 4, stored in cache with the key 2 (number) | |
power(2, 3); // => 4, retrieved from cache at key 2 (number), it's wrong | |
``` | |
You will have to use the `cache` and `cacheKey` options appropriate to your function. In this specific case, the following could work: | |
```js | |
const power = mem((a, b) => Math.power(a, b), { | |
cacheKey: arguments_ => arguments_.join(',') | |
}); | |
power(2, 2); // => 4, stored in cache with the key '2,2' (both arguments as one string) | |
power(2, 3); // => 8, stored in cache with the key '2,3' | |
``` | |
More advanced examples follow. | |
#### Example: Options-like argument | |
If your function accepts an object, it won't be memoized out of the box: | |
```js | |
const heavyMemoizedOperation = mem(heavyOperation); | |
heavyMemoizedOperation({full: true}); // Stored in cache with the object as key | |
heavyMemoizedOperation({full: true}); // Stored in cache with the object as key, again | |
// The objects look the same but for JS they're two different objects | |
``` | |
You might want to serialize or hash them, for example using `JSON.stringify` or something like [serialize-javascript](https://github.com/yahoo/serialize-javascript), which can also serialize `RegExp`, `Date` and so on. | |
```js | |
const heavyMemoizedOperation = mem(heavyOperation, {cacheKey: JSON.stringify}); | |
heavyMemoizedOperation({full: true}); // Stored in cache with the key '[{"full":true}]' (string) | |
heavyMemoizedOperation({full: true}); // Retrieved from cache | |
``` | |
The same solution also works if it accepts multiple serializable objects: | |
```js | |
const heavyMemoizedOperation = mem(heavyOperation, {cacheKey: JSON.stringify}); | |
heavyMemoizedOperation('hello', {full: true}); // Stored in cache with the key '["hello",{"full":true}]' (string) | |
heavyMemoizedOperation('hello', {full: true}); // Retrieved from cache | |
``` | |
#### Example: Multiple non-serializable arguments | |
If your function accepts multiple arguments that aren't supported by `JSON.stringify` (e.g. DOM elements and functions), you can instead extend the initial exact equality (`===`) to work on multiple arguments using [`many-keys-map`](https://github.com/fregante/many-keys-map): | |
```js | |
import ManyKeysMap from 'many-keys-map'; | |
const addListener = (emitter, eventName, listener) => emitter.on(eventName, listener); | |
const addOneListener = mem(addListener, { | |
cacheKey: arguments_ => arguments_, // Use *all* the arguments as key | |
cache: new ManyKeysMap() // Correctly handles all the arguments for exact equality | |
}); | |
addOneListener(header, 'click', console.log); // `addListener` is run, and it's cached with the `arguments` array as key | |
addOneListener(header, 'click', console.log); // `addListener` is not run again | |
addOneListener(mainContent, 'load', console.log); // `addListener` is run, and it's cached with the `arguments` array as key | |
``` | |
Better yet, if your function’s arguments are compatible with `WeakMap`, you should use [`deep-weak-map`](https://github.com/futpib/deep-weak-map) instead of `many-keys-map`. This will help avoid memory leaks. | |
## 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`\ | |
Default: `arguments_ => arguments_[0]`\ | |
Example: `arguments_ => JSON.stringify(arguments_)` | |
Determines the cache key for storing the result based on the function arguments. By default, **only the first argument is considered**. | |
A `cacheKey` function can return any type supported by `Map` (or whatever structure you use in the `cache` option). | |
Refer to the [caching strategies](#caching-strategy) section for more information. | |
##### 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. | |
Refer to the [caching strategies](#caching-strategy) section for more information. | |
### memDecorator(options) | |
Returns a [decorator](https://github.com/tc39/proposal-decorators) to memoize class methods or static class methods. | |
Notes: | |
- Only class methods and getters/setters can be memoized, not regular functions (they aren't part of the proposal); | |
- Only [TypeScript’s decorators](https://www.typescriptlang.org/docs/handbook/decorators.html#parameter-decorators) are supported, not [Babel’s](https://babeljs.io/docs/en/babel-plugin-proposal-decorators), which use a different version of the proposal; | |
- Being an experimental feature, they need to be enabled with `--experimentalDecorators`; follow TypeScript’s docs. | |
#### options | |
Type: `object` | |
Same as options for `mem()`. | |
```ts | |
import {memDecorator} from 'mem'; | |
class Example { | |
index = 0 | |
@memDecorator() | |
counter() { | |
return ++this.index; | |
} | |
} | |
class ExampleWithOptions { | |
index = 0 | |
@memDecorator({maxAge: 1000}) | |
counter() { | |
return ++this.index; | |
} | |
} | |
``` | |
### memClear(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 | |
import mem from 'mem'; | |
import StatsMap from 'stats-map'; | |
import got from 'got'; | |
const cache = new StatsMap(); | |
const memGot = mem(got, {cache}); | |
await memGot('https://sindresorhus.com'); | |
await memGot('https://sindresorhus.com'); | |
await memGot('https://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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