var nameField = document.getElementById("senderName");
var emailField = document.getElementById("senderEmail");
var phoneField = document.getElementById("senderPhone");
var messageField = document.getElementById("message");

function validate() {
  if (nameField.value.length > 0 &&
    emailField.value.length > 0 && 
    phoneField.value.length > 0 &&
    messageField.value.length > 0) {
      return true;
    }
    else {
      alert('You must fill out all required fields');
      return false;
    }
}