Validation

The Form Engine validates answers as users fill a form and blocks submission until every violation is resolved. Validation rules live in the schema, either as entries in a question's validators array or as properties of questionOptions.

The engine supports the following kinds of validation:

• Date-based validation — constrain date questions, including disallowing future dates.
• Expression-based validation — arbitrary JavaScript expressions over the form's values, with access to the expression helpers. The most flexible mechanism, used for cross-field rules.
• Other validation types — conditionalAnswered for answers that only make sense given another question's answer, and the numeric constraints (min/max, length, disallowDecimals) derived from questionOptions.

Two validation behaviors come from the question definition itself rather than the validators array:

• Marking a question with "required": "true" registers a required validator. A conditionally required question uses the required object form described in the questions guide.
• number and decimal questions derive numeric validators directly from their questionOptions.