yup.js

// there are a few methods that you need to implement/override to create a new type
// The main one is `_coerce`, which passing in the raw value and returns the type back.
// _coerce should not throw errors, and should return the default invalid object for a type if it exists (NaN, etc)


//here is the date `_coerce()` (I need to change it to return an InvalidDate instead of null)
_coerce(value) {
  if(value == null ) return value
  if(isDate(value) ) return new Date(value)

  value = isoParse(value)
  return value ? new Date(value) : null
}
  
// create new schema Type
var myDateType = yup.date.extend({
  coerce(value){
    var baseVal = yup.date.prototype._coerce.call(this,val)
    //do whatever if you didn't get a date
  }
})

//mutate existing
var baseDateCoerce = yup.date.prototype._coerce;

yup.date.prototype._coerce = function(value){
  baseDateCoerce.call(this)
  // blah blah
}