Doesn't memoize the result for keys returning `undefined`

[Hubert Argasinski]

When the memoized function returns undefined for a specific parameter, another call to the memoized function runs the original function again as opposed to fetching the result from the cache.

Reproducible Example:

const memoize = require('fast-memoize')
const assert = require('assert')

let numCalled = 0
function longProcess(arg) {
  ++numCalled
  return
}

// memoize result
const memoizedProcess = memoize(longProcess)
memoizedProcess("foo")

// get memoized result
memoizedProcess("foo")
assert(numCalled === 1)

It seems like this changed from v2.2.8 to v2.3.0. Is this the intended behaviour?

[Hubert Argasinski]

I see how this could be the expected behaviour though, i.e. if the function returns undefined, it could mean something in the function failed and it should be run again, e.g. it run before a global object was initialised/some data wasn't fetched yet.

[Mihail Malo]

@hargasinski have you benchmarked using a Map cache with .has() or .hasOwnProperty/in with the current null-prototype object cache?

[Simon Fan]

Hmmm, in my opinion this is an unexpected behaviour if not explicitly stated otherwise.

[Thawsitt Naing]

I agree that it should be documented somewhere.

I checked the source code (line 36) and looks like it calls the original function whenever the cached result is undefined.

(Usually, undefined means something is not in the cache yet, so it calls the original function and caches this result. However, this means it no longer memoizes if the original function actually returns undefined).

https://github.com/caiogondim/fast-memoize.js/blob/master/src/index.js#L36