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    mem
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    9.0.1 • Public • Published

    mem

    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 first argument is considered and it only works with primitives. If you need to cache multiple arguments or cache objects by value, have a look at alternative caching strategies below.

    Install

    $ npm install mem
    

    Usage

    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 fine with promise returning functions
    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
    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.

    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:

    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:

    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, which can also serialize RegExp, Date and so on.

    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:

    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:

    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 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 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.

    memDecorator(options)

    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 mem().

    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 as a replacement for the default cache.

    Example

    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 - Memoize promise-returning & async functions

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    Install

    npm i mem

    DownloadsWeekly Downloads

    8,799,254

    Version

    9.0.1

    License

    MIT

    Unpacked Size

    17.5 kB

    Total Files

    5

    Last publish

    Collaborators

    • sindresorhus
    • fregante