Load script tag as a string and append it to html header

I want to load some script tag from the server as a string and to append it to HTML header, but even though I can append it, it doesn't execute. Here is the simplified HTML file to illustrate this situation:

<head>

</head>
<body>

    <script>
        function htmlStringToElement(htmlString) {
            var template = document.createElement('template');
            htmlString = htmlString.trim();
            template.innerHTML = htmlString;
            return template.content.firstChild;
        }

        //Mocking http request
        setTimeout(function() {
            var httpResponseMock = '<script>alert("HELLO FROM HTTP RESPONSE!");<\/script>'; 
            var script = htmlStringToElement(httpResponseMock);
            document.head.appendChild(script);
        }, 1000);
    </script>
</body>

I suppose that the reason is that header has already been rendered when the script is added dynamically but is there any other way to achieve this?

Solution

With Jquery,

var httpResponseMock = '<script><\/script>'; 
$('head').append(httpResponseMock);

and with javascript

var httpResponseMock = '<script><\/script>'; 
document.getElementsByTagName('head')[0].appendChild(httpResponseMock);