ninja-forms-three-custom-field-validation-3.js

var myCustomFieldController = Marionette.Object.extend( {
  
    initialize: function() {
      // ....
    },

    validate: function( model ) {

        // Check the field type.
        if( 'custom' != model.get( 'type' ) ) return;

        // Get the field value.
        var fieldValue = model.get( 'value' );
        
        // Do Validation Here...
    }
});