submit.html

<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script type="text/javascript" src="problem-select.js"></script>
<link href="submit.css" rel="stylesheet" type="text/css">
<script type="text/javascript">document.write(currProb())</script>
<br>
<div class="dropdown">
    <button onclick="dropLang()" class="dropbtn">Select language</button>
    <div id="Dropdown" class="drop-cont">
        <a href="#" onclick="setLang('cpp')">GNU C++11</a>
        <a href="#" onclick="setLang('c')">GNU C11</a>
        <a href="#" onclick="setLang('py3')">Python 3</a>
    </div>
</div>
<script type="text/javascript">
var lang = ''
function setLang(str) {
    lang = str;
    var btn = document.getElementsByClassName("dropbtn")[0];
    var str = 'Select language';
    if (lang == 'cpp') {str = 'GNU C++11';}
    if (lang == 'c') {str = 'CNU C11';}
    if (lang == 'py3') {str = 'Python 3';}
    btn.textContent = str;
}
function dropLang() {
    document.getElementById("Dropdown").classList.toggle("show");
}
window.onclick = function(event) {
  if (!event.target.matches('.dropbtn')) {
    var dropdowns = document.getElementsByClassName("drop-cont");
    var i;
    for (i = 0; i < dropdowns.length; i++) {
      var openDropdown = dropdowns[i];
      if (openDropdown.classList.contains('show')) {
        openDropdown.classList.remove('show');
      }
    }
  }
}
</script>
<br>
<script type="text/javascript">
function performClick(elemId) {
   var elem = document.getElementById(elemId);
   if(elem && document.createEvent) {
      var evt = document.createEvent("MouseEvents");
      evt.initEvent("click", true, false);
      elem.dispatchEvent(evt);
   }
}
</script>
<a href="#" onclick="performClick('theFile');">Choose source</a>
<input type="file" id="theFile" />
<br>
<a href="#" id="sub">Submit</a>
<script type="text/javascript">
function postlng() {
    return $.ajax({
        type: 'post',
        url: '/setlng',
        data: {item : lang}
    });
}
function postprob() {
    return $.ajax({
        type: 'post',
        url: '/setprob',
        data: {item : currProb()}
    });
}
function postsrc(str) {
    return $.ajax({
        type: 'post',
        url: '/setsrc',
        data: {item : str.name}
    });
}
function uploadsrc(data) {
    return $.ajax({
        type: 'post',
        url: '/uploadsrc',
        data: data,
        processData: false,
        contentType: false,
    });
}
$("#sub").on("click",  
function (evt) {
    evt.preventDefault();
    if (lang == '' || currProb() == null)return;
    var str = document.getElementById("theFile").files[0];
    var keks = new FormData();
    keks.append('files', str);
    $.when(postlng(), postprob(), postsrc(str), uploadsrc(keks)).done(function(a, b, c, d){
        $.ajax({
            type: 'get',
            cache: false,
            url: '/test'
        });
    });
});
</script>
<br>
<a href="result.html">Show last submission</a>