function validate(e){
 var photo = document.getElementById("capt").value;
 var userName = document.getElementById("login__username").value;
 var password = document.getElementById("login__password").value;

 if(photo.length === 0) {
    alert("You must enter a username.");
    e.preventDefault();
    return;
 }

 if (userName.length === 0) {
    alert("You must enter a username.");
    e.preventDefault();
    return;
 }

 if (password.length === 0) {
    alert("You must enter a password.");
    e.preventDefault();
    return;
 }
}

window.onload = function() {
   document.getElementById("logInForm").addEventListener("submit", function(e){
      validate(e);
   });
}