HTMLInputElement: setCustomValidity() メソッド

setCustomValidity(message)

function validate(inputID) {
  const input = document.getElementById(inputID);
  const validityState = input.validity;
  if (validityState.valueMissing) {
    input.setCustomValidity("You gotta fill this out, yo!");
  } else if (validityState.rangeUnderflow) {
    input.setCustomValidity("We need a higher number!");
  } else if (validityState.rangeOverflow) {
    input.setCustomValidity("That's too high!");
  } else {
    input.setCustomValidity("");
  }
  input.reportValidity();
}