undefined

Expected when value passes validation.

null

Will be treated similarly as undefined; value passed validation.

string

A message explaining the validation failure.

"is required";

ERROR

An object used to describe an error message. See Error.

{
    "message": "is required",
    "related": ["billingZip", "residenceZip"]
}

array

Items can be any of the valid Errors.

[
    "is required",
    {
        "message": "is required",
        "related": ["billingZip", "residenceZip"]
    }
]