Cross validation in objects involving arrays: how to get the exact item that failed

[bettorinco]

Hello! Thanks in advance for considering my question :)

I have a doubt about cross field validation in an object schema with arrays inside.
Let's assume that I have this schema, representing an object with a basePath and an array of paths called assets:

const schema = object({
    basePath: string(),
    assets: array(string())
});

My goal is to check if the given assets (that I assume are all relative paths) exist when resolved against the basePath.

Since I do not have access to the basePath inside the array of assets, the obvious solution I found was to use a check on the whole object, to ensure that every asset was an existing path.

const schema = pipe(
   object({
       basePath: string(),
       assets: array(string())
   }),
   check(
       ({basePath, assets}) => assets.every(asset => fs.existsSync(path.resolve(basePath, asset))),
       "One of the assets does not exist."
   )
);

While this works as a validation, in case of error I am missing which one of the assets is invalid, since the check callback allows to only return a boolean and I am forced to throw a generic message as shown in the example.

So I'm wondering: is it possible to somehow intercept the exact path that gave the error?

Or even better: having a validation pipeline directly on the asset schema, is it somehow possible to access the "parent" object, so that I can use the basePath?

For example something like this, with a sort of context available in the validation fn:

const schema = object({
    basePath: string(),
    assets: array(
        pipe(
            string(),
            custom(
                (input, cxt) => fs.existsSync(path.resolve(cxt.basePath, input)),
                // Here would be the callback returning the error msg with the exact invalid path 
            )
        )
    )
});

Does anyone know a better approach to face this use case?
Thanks a lot in advance!