constraint-validation-buggyfill/index.js

Prevents invalid form submission in browsers that improperly support the HTML forms spec (i.e. Safari). The first invalid element is focused after alerting the validation message.

'use strict'

if (typeof window !== 'undefined')
  // Listen for all form submissions.
  window.addEventListener('submit', function(event) {
    // Check form should be validated and is invalid.
    if (!event.target.noValidate && !event.target.checkValidity()) {
      // Prevent form submission.
      event.preventDefault()

      // Get first invalid form descendant.
      var firstInvalid = event.target.querySelector(':invalid')

      // Display validation error message.
      window.alert(firstInvalid.validationMessage)

      // Focus after alert is closed.
      firstInvalid.focus()
    }
  })