pMemoizeDecorator is using the same cache instance when using a custom cache

[mdawar]

Since decorators are called on the class declaration and not instantiation, the same cache instance is being used by all the class instances when using the custom cache option for pMemoizeDecorator.

Here's a failing test that shows what I mean:

test('.pMemoizeDecorator() with custom cache', async (t) => {
  let returnValue = 1;

  class TestClass {
    @pMemoizeDecorator({ cache: new Map() })
    async counter() {
      return returnValue++;
    }
  }

  const alpha = new TestClass();
  t.is(await alpha.counter(), 1);
  t.is(await alpha.counter(), 1, 'The method should be memoized');

  const beta = new TestClass();
  t.is(await beta.counter(), 2, 'The method should not be memoized across instances');
});

The test output:

.pMemoizeDecorator() with custom cache

The method should not be memoized across instances

Difference:

- 1
+ 2

› file://test.ts:213:4

─

1 test failed

The output shows that the same cache instance is being used for all the TestClass instances, which is the expected behavior because the pMemoizeDecorator is being called once on the class declaration, so the same options object is being passed to pMemoize.

I've created this issue to discuss the options to solve this problem before creating a pull request.

The first solution that comes to my mind is accepting a function for the cache option that returns an instance:

interface Options<FunctionToMemoize extends AnyAsyncFunction, CacheKeyType> {
  readonly cache?:
    | CacheStorage<CacheKeyType, AsyncReturnType<FunctionToMemoize>>
    | (() => CacheStorage<CacheKeyType, AsyncReturnType<FunctionToMemoize>>);
}