form-validation.js

// Validates a single input
function validate(input) {
  if (!input.getAttribute('required')) { return false; }
  (!input.value) ? new Error(input.name + "is required.") : true        
}

// Takes in an array of inputs, and tells us if they're all valid or no
function isFormInvalid(inputs) {
  return inputs.map(validate)   // Validates all fields
               .filter(Boolean) // Keeps only the errors
               .length > 0      // Checks if we still have any errors
}

// When submitting, we check if the form is valid
form.addEventListener('submit', function(ev) {
  var fields = form.querySelectorAll('input')
  return !isFormInvalid(fields)
})