Validation Summary from Web Api 2 BadRequest response- Format

validationSummary.js

// Read and Format Error messages
// Using custom ajax request
// You may need to get to modelState differently depending on how you make your ajax call
// Attached to the jQuery namespace using $.summary = function

 $.summary = function ( res ) {
      var errors = [], validationSummary;
      
      // eg - res.data.modelState
      if ( res.responseJSON.modelState ) {
          //If BadRequest(ModelState)

          $.each( res.responseJSON.modelState, function ( i, propertyErrors ) {
              errors.push.apply( errors, propertyErrors );
          } );

          validationSummary = errors.join( '</br>' );
      } else {
          //else BadRequest("Message")

          errors.push( res.responseJSON.message );
          validationSummary = errors.join( '' );
      }

      if ( validationSummary )
          validationSummary = validationSummary.trim();

      return validationSummary;
  }