fast-typescript-memoize: fast memoization decorator and other helpers with 1st class support for Promises

@Memoize() decorator

Remembers the returned value of a decorated method or getter in a hidden this object's property, so next time the method is called, the value will be returned immediately, without re-executing the method. This also works for async methods which return a Promise: in this case, multiple parallel calls to that method will coalesce into one call.

To work properly, requires TypeScript v5+.

The idea of @Memoize() decorator is brought from typescript-memoize. Differences:

1. If used to memoize async methods, by default (and when clearOnResolve=true), it clears the memoize cache as soon as the Promise gets rejected (i.e. it doesn't memoize exceptions in async methods). Parallel async calls to the same method will still be coalesced into one call though (until the Promise rejects).
2. A special mode is added, clearOnResolve. If true, successfully resolved Promises returned from an async method will be removed from the cache as soon as the method finishes. This is a convenient mode for the cases when we want to coalesce multiple parallel executions of some method (e.g. when there is a burst of runs), but we don't want to prevent the method from further running.
3. Strong typing for the optional hasher handler, including types of arguments and even the type of this.
4. Does not support any notion of expiration.
5. When the 1st argument of the method is an object (or when hasher handler returns an object), it is not retained from GC, so you can memoize on object args safely, without thinking about memory leaks.

import { Memoize } from "fast-typescript-memoize";

class Class {
  private count = 0;
  private some = 42;

  @Memoize()
  method0() {
    return count++;
  }

  @Memoize()
  method1(arg: string) {
    return count++;
  }

  @Memoize()
  method1obj(arg: object) {
    return count++;
  }

  @Memoize((arg1, arg2) => `${arg1}#${arg2}`)
  method2(arg1: string, arg2: number) {
    return count++;
  }

  @Memoize(function (arg1, arg2) { return `${this.some}:${arg1}#${arg2}`; })
  method2this(arg1: string, arg2: number) {
    return count++;
  }

  @Memoize()
  async asyncMethod(arg: string) {
    count++;
    if (arg == "ouch") {
      throw "ouch";
    }
  }

  @Memoize({ clearOnResolve: true })
  async asyncCoalescingMethod(arg: string) {
    await delay(100);
    count++;
  }
}

const obj = new Class();
const arg = { my: 42 };

obj.method0(); // count is incremented
obj.method0(); // count is NOT incremented

obj.method1("abc"); // count is incremented
obj.method1("abc"); // count is NOT incremented
obj.method1("def"); // count is incremented

obj.method1obj(arg); // count is incremented, arg is not retained
obj.method1obj(arg); // count is NOT incremented

obj.method2("abc", 42); // count is incremented
obj.method2("abc", 42); // count is NOT incremented

obj.method2this("abc", 42); // count is incremented (strongly typed `this`)
obj.method2this("abc", 42); // count is NOT incremented

await asyncMethod("ok"); // count is incremented
await asyncMethod("ok"); // count is NOT incremented
await asyncMethod("ouch"); // count is incremented, exception is thrown
await asyncMethod("ouch"); // count is incremented, exception is thrown

await asyncCoalescingMethod("ok"); // count is incremented
await asyncCoalescingMethod("ok"); // count is incremented again
const [c1, c2] = await Promise.all([
  asyncCoalescingMethod("ok"), // count is incremented
  asyncCoalescingMethod("ok"), // not incremented! coalescing parallel calls
]);
assert(c1 === c2);

memoize0(obj, tag, func)

Saves the value returned by func() in a hidden property tag (typically a symbol) of obj object, so next time memoize0() is called, that value will be returned, and func won't be called.

The main goal is performance and simplicity.

import { memoize0 } from "fast-typescript-memoize";

let count = 0;
const $tag = Symbol("$tag");
const obj = {};
memoize0(obj, $tag, () => count++); // count is incremented
memoize0(obj, $tag, () => count++); // count is NOT incremented

memoize2(obj, tag, func)

A simple intrusive 1-slot cache memoization helper for 2 parameters func. It's useful when we have a very high chance of hitrate. The helper is faster (and more memory efficient) than a Map<TArg1, Map<TArg2, TResult>> based approach since it doesn't create intermediate maps.

This method works seamlessly for async functions too: the returned Promise is eagerly memoized, so all the callers will subscribe to the same Promise.

Returns the new memoized function with 2 arguments for the tag.

let count = 0;
const $tag = Symbol("$tag");
const obj = {};
memoize2(obj, $tag, (arg1, arg2) => count++)("abc", 42); // count is incremented
memoize2(obj, $tag, (arg1, arg2) => count++)("abc", 42); // count is NOT incremented
memoize2(obj, $tag, (arg1, arg2) => count++)("xyz", 101); // count is incremented
memoize2(obj, $tag, (arg1, arg2) => count++)("abc", 42); // count is incremented

memoizeExpireUnused(func, { resolve, unusedMs })

Similar to lodash.memoize(), but auto-expires (and removes from memory) the cached results after the provided number of inactive milliseconds. Each time we read a cached result, the expiration timer starts from scratch.

This function is more expensive than lodash.memoize(), because it uses a JS timer under the hood.

let count = 0;
const func = memoizeExpireUnused((s) => count++, { resolver: (s) => s, unusedMs: 1000 });
func("a"); // count is incremented
func("a"); // count is NOT incremented
... after 2 seconds, memory for the cached result is freed ...
func("a"); // count is incremented