Custom subjectName function does not work with instance check

[claustres]

Observed behaviour

Checking permission on creating a specific instance fails although the ability to create any object on the subject has been declared:

const { rules, can } = AbilityBuilder.extract()
can('create', 'users')
let ability = new Ability(rules, { subjectName: resource => resource.type })
// This logs false while it should log true
console.log(ability.can('create', {type : 'users'}))

Expected behaviour

Because creating any object on subject 'users' is allowed it should be allowed whatever the given instance to check.

Suspected faulty code

Ability.can() call the custom function to retrieve the subjectName (https://github.com/stalniy/casl/blob/master/src/ability.js#L91) but then perform an internal call to rulesFor() with the subjectName instead of the subject itself, leading to a bug when the custom function is called again (https://github.com/stalniy/casl/blob/master/src/ability.js#L108)

[stalniy]

Thanks for the issue but it's currently expected behavior. subjectName function should properly work for cases when resource is undefined, null or string.

You can check built-in implementation for details:
https://github.com/stalniy/casl/blob/master/src/ability.js#L4

So, you need to convert your function to this:

let ability = new Ability(rules, { subjectName: resource => resource && typeof resource === 'object' ? resource.type : resource })

[claustres]

Works fine now, thanks. I suggest to add a reminder in the doc explaining this behaviour is actually mandatory. At a first glance I believed it was specific to your examples.