This would be a breaking change.

In today's world, if a memoized function errors on initial call, it will error on all future calls. I would have expected the memoization to only occur on success. This seems like a reasonable assumption given that a typical use-case for an async.memoize would be an external call.

Example case:

const getQueueUrl = async.memoize(function(queueName, cb) {
  async.waterfall([
    function(cb) {
      sqs.getQueueUrl(queueName, cb);
    },
    function({ QueueUrl }, cb) {
      cb(null, QueueUrl);
    },
  ], cb);
});

If the sqs queue has not been fully initialized by the time getQueueUrl is invoked initially, all future invocations of getQueueUrl will fail, even after the queue has been fully initialized.

I would be happy to either create a pull request to only memoize on success or update the documentation to clarify that the function will be memoized regardless of success or error.