This can happen for multiple reasons:
A) Missing attributes on a custom field
Solution: Make sure that the @Html.GenerateUnobtrusiveValidationAttributes(m => m.Value) attribute is present on all of your fields.
See http://madhuanbalagan.com/sitecore-9-forms-uncaught-typeerror-cannot-read-property-unobtrusive-of-undefined for details
B) jQuery is being used in noConflict mode
On successful submit, Sitecore Forms executes some JavaScript that directly targets $.validator.unobtrusive
which is not defined. Instead it should be jQuery.validator.unobtrusive.
This can be fixed with a Forms.RenderForm processor:
Config:
<pipelines>
<forms.renderForm>
<processor type="MyProject.Pipelines.Forms.RenderForm.FixJqueryReference, MyProject" resolve="true"
patch:after="processor[@type='Sitecore.ExperienceForms.Mvc.Pipelines.RenderForm.InitializeAjaxOptions, Sitecore.ExperienceForms.Mvc']"/>
</forms.renderForm>
</pipelines>
Code:
public class FixJqueryReference : MvcPipelineProcessor<RenderFormEventArgs>
{
public override void Process(RenderFormEventArgs args)
{
var attributes = args.Attributes["data-ajax-success"] as string;
if (string.IsNullOrEmpty(attributes))
{
return;
}
args.Attributes["data-ajax-success"] = attributes.Replace("$.", "jQuery.");
}
}
