Load Js files from cache memory

After I make an AJAX call and i change the body content with $("body").html(this.responseText.match(/<\s*body.*>[\s\S]*<\s*\/body\s*>/ig).join("")); my js files in my head element does not reload. So I had to include all these js files in the bottom of the body element. The problem is that each time when I make an AJAX call it downloads all JS files from the server not from cache, and it has a bad effect on internet traffic. How can I get the js files from browser cache memory?

My ajax method:

function AJAXFormPost(data, formAction) {
    var httpRequest = new XMLHttpRequest();
    var formData = new FormData();


    Object.keys(data).forEach(function(key) {
           formData.append(key, data[key]);  
    });

    httpRequest.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {

            // replace body content
            $("body").html(this.responseText.match(/<\s*body.*>[\s\S]*<\s*\/body\s*>/ig).join(""));

            // Update url
            test_mode = window.location.href.split('/')[3];                 
            question_number = document.getElementById("lblNrCrt").textContent;  

            pageUrl = "/" + test_mode + "/" + test_uuid + "/" + question_number + "/";
            window.history.pushState('', '', pageUrl);
        }
    };

    httpRequest.open("post", formAction);
    httpRequest.send(formData);

}