The validationMessage read-only property of the ElementInternals interface returns the validation message for the element.

A string containing the validation message of this element.

In the following example the validation message is set with ElementInternals.setValidity(), and then returned with validationMessage.

this.internals_.setValidity({ valueMissing: true }, "my message");
console.log(this.internals_.validationMessage); // "my message"