HTMLSelectElement: validationMessage property

The validationMessage read-only property of the HTMLSelectElement interface returns a string representing a localized message that describes the validation constraints that the <select> control does not satisfy (if any). This is the empty string if the control is not a candidate for constraint validation (HTMLSelectElement.willValidate is false), or it satisfies its constraints.

If the <select> element is a candidate for constraint validation (willValidate is true) and the constraints are not met (the HTMLSelectElement.validity object's valid property is false), the value is the error message that would be shown to the user during validation.

const select = document.getElementById("mySelect");
const errorMessage = select.validationMessage;