I think the overall issue we are having is there is no good way to support a type like:

type Body = {
  age: number | null
}

Right now in express-validator we would attempt to do:

check('age').optional({ nullable: true }).isInt()

But this isn't really describing what we want, and I guess my PR is abusing that trying to get this to work (and I think other people are also trying to get this same behavior to work). In reality we want an API like:

check('age').isInt({ nullable: true })

This exactly describes what we're looking for. To be honest I have no clue what the current optional({ nullable: true }) is even supposed to do. As is it doesn't seem to work. Optional values simply don't come back in matchedData even if they are set correctly (pass validation).

Here's best example of how the current behavior is broken. You are claiming I made a breaking change by returning the null value when includeOptionals: false (which is same as requiredOnly: true). But notice in the screenshot how the first example, foo, is in fact optional, and yet its returned in the data. In the second example, foo, is the exact same definition but this time set to null, and its not returned even though I clearly marked it as nullable.

In the second example, foo, is the exact same definition but this time set to null, and its not returned even though I clearly marked it as nullable.

Default behaviour of matchedData() is to filter out invalid values -- null doesn't pass isInt

Default behaviour of matchedData() is to filter out invalid values -- null doesn't pass isInt

I agree with you if isInt() is the only validator but in this case optional({ nullable: true }) is used too, so both an int or a null value are valid data. I find that #1275 is a valid solution that could solve the problem.