Serializer/Deserializer does not work with arrays when type specifier has multiple options

[AndrewRL97]

Describe the bug
OpenAPI 3.1 allows types to be specified as an array like so:

{
    "type": ["array", "string", "null"],
    "items": ...
}

When specifying an array like this, and having a corresponding item that would normally be processed by the serializer/deserializer such as a string with a format of "date-time", the serializer is never executed and the field is not converted, causing a type conflict by the validator.

To Reproduce
I'll borrow the steps from issue #699 which is a very closely related serializer issue.

1. git clone https://github.com/cdimascio/express-openapi-validator.git
   

2. npm run deps && npm i
   

3. cd express-openapi-validator/examples/7-response-date-serialization
   

4. npm start
   

5. GET http://localhost:3000/v1/date-time
   

This works.

Now modify the app.js /v1/date-time endpoint to the following (array brackets are added):

app.get('/v1/date-time', function (req, res, next) {
  res.json([{
    id: 1,
    created_at: new Date(),
  }]);
});

Update the api.yaml file openapi format to 3.1.0
Also modify the date-time schema of the api.yaml file like so:
Path modified: paths/date-time/get/responses/200/content/application/json/schema

                type: 
                  - array # here you can see even an array of just one type reproduces the issue.
                items:
                  type: object
                  properties:
                    created_at:
                      type: string
                      format: date-time
                    id:
                      type: number  

Rerunning the app and hitting the date-time endpoint then results in the following behavior:

Actual behavior

{
  "message": "/response/0/created_at must be string",
  "errors": [
    {
      "path": "/response/0/created_at",
      "message": "must be string",
      "errorCode": "type.openapi.validation"
    }
  ]
}

Expected behavior

[
  {
    "id": 1,
    "created_at": "2026-04-06T23:07:24.515Z"
  }
]

Additional info

This seems to be related to the array check in the schema.preprocessor file which was updated in #699 :
} else if (schema.type === 'array' && schema.items) {

Here type is only checked as a string constant.

Changing that line to the following:
else if ((schema.type === 'array' || Array.isArray(schema.type) && schema.type.find(elem => elem === 'array')) && schema.items) {

does result in the serializer now executing on the date-string. I did not investigate if this impacted other tests however, or if it is a robust enough solution.