memoize

> Memoize 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.

By default, only the memoized function's first argument is considered via strict equality comparison. If you need to cache multiple arguments or cache objects by value, have a look at alternative caching strategies below.

If you want to memoize Promise-returning functions (like async functions), you might be better served by p-memoize.

Install

``sh
npm install memoize
`

Usage

`js
import memoize from 'memoize';

let index = 0;
const counter = () => ++index;
const memoized = memoize(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 for more Promise-specific behaviors.

`js
import memoize from 'memoize';

let index = 0;
const counter = async () => ++index;
const memoized = memoize(counter);

console.log(await memoized());
//=> 1

// The return value didn't increase as it's cached
console.log(await memoized());
//=> 1
`

`js
import memoize from 'memoize';
import got from 'got';
import delay from 'delay';

const memoizedGot = memoize(got, {maxAge: 1000});

await memoizedGot('https://sindresorhus.com');

// This call is cached
await memoizedGot('https://sindresorhus.com');

await delay(2000);

// This call is not cached as the cache has expired
await memoizedGot('https://sindresorhus.com');
`

$3

By default, only the first argument is compared via exact equality (===) to determine whether a call is identical.

`js
import memoize from 'memoize';

const pow = memoize((a, b) => Math.pow(a, b));

pow(2, 2); // => 4, stored in cache with the key 2 (number)
pow(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
import memoize from 'memoize';

const pow = memoize((a, b) => Math.pow(a, b), {
cacheKey: arguments_ => arguments_.join(',')
});

pow(2, 2); // => 4, stored in cache with the key '2,2' (both arguments as one string)
pow(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
import memoize from 'memoize';

const heavyMemoizedOperation = memoize(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 appear the same, but in JavaScript, they're different objects
`

You might want to serialize or hash them, for example using JSON.stringify or something like serialize-javascript, which can also serialize RegExp, Date and so on.

`js
import memoize from 'memoize';

const heavyMemoizedOperation = memoize(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
import memoize from 'memoize';

const heavyMemoizedOperation = memoize(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:

`js
import memoize from 'memoize';
import ManyKeysMap from 'many-keys-map';

const addListener = (emitter, eventName, listener) => emitter.on(eventName, listener);

const addOneListener = memoize(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 because the arguments are the same
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 instead of many-keys-map. This will help avoid memory leaks.

API

$3

#### fn

Type: Function

The function to be memoized.

#### options

Type: object

##### maxAge

Type: number | Function\
Default: Infinity\
Example: arguments_ => arguments_ < new Date() ? Infinity : 60_000

Milliseconds until the cache entry expires.

If a function is provided, it receives the arguments and must return the max age.

- 0 or negative values: Do not cache the result
- Infinity: Cache indefinitely (no expiration)
- Positive finite number: Cache for the specified milliseconds

##### 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 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 for a LRU cache.

Refer to the caching strategies section for more information.

$3

Returns a decorator 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 are supported, not Babel’s, 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 memoize().

`ts
import {memoizeDecorator} from 'memoize';

class Example {
index = 0

@memoizeDecorator()
counter() {
return ++this.index;
}
}

class ExampleWithOptions {
index = 0

@memoizeDecorator({maxAge: 1000})
counter() {
return ++this.index;
}
}
`

$3

Clear all cached data of a memoized function.

#### fn

Type: Function

The memoized function.

$3

Check if a specific set of arguments is cached for a memoized function.

Returns true if the arguments are cached and not expired, false otherwise.

Uses the same argument processing as the memoized function, including any custom cacheKey function.

`js
import memoize, {memoizeIsCached} from 'memoize';

const expensive = memoize((a, b) => a + b, {cacheKey: JSON.stringify});
expensive(1, 2);

memoizeIsCached(expensive, 1, 2);
//=> true

memoizeIsCached(expensive, 3, 4);
//=> false
`

#### fn

Type: Function

The memoized function.

#### arguments

The arguments to check.

Tips

$3

If you want to know how many times your cache had a hit or a miss, you can make use of stats-map as a replacement for the default cache.

#### Example

`js
import memoize from 'memoize';
import StatsMap from 'stats-map';
import got from 'got';

const cache = new StatsMap();
const memoizedGot = memoize(got, {cache});

await memoizedGot('https://sindresorhus.com');
await memoizedGot('https://sindresorhus.com');
await memoizedGot('https://sindresorhus.com');

console.log(cache.stats);
//=> {hits: 2, misses: 1}
``

Related

- p-memoize - Memoize promise-returning & async functions