Validating against two or more fields in the fields.js file

[LaiTang-GDS]

Is there a way to define a validation method that can refer to other fields?
In our app we need to validate the sum of characters entered for a combination of fields. We have a workaround where we do validation in a separate step and return the errors manually but was wondering if there was a better way of declaring this type of validation.

[HughePaul]

There are other validators that look at other fields as part of their validation, eg match, or the dates validators beforeField and afterField. You could do something like:

 function combinedLength(value, length, otherFieldName) {
   const otherValue = this.values[otherFieldName];
   return validators.string(value) && validators.string(otherValue) &&
     ((value.length + otherValue.length) <= length);
}

and specify it with something like:

validator: [
  { fn: combinedLength, arguments: [ 200, 'otherFieldName' ] }
]