# 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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