Efficient way to defer js

I intend to call a js, which will not use anything when loading the page. Therefore, I need to call this js only when the entire page has been loaded, so as not to delay anything or load the screen for the user.

<html>
    <body>
    <script type="text/javascript" defer=defer src="function.js"></script>
    <script type="text/javascript">
            function downloadJSAtOnload() {
                var element = document.createElement("script");
                element.src = "function.js";
                document.body.appendChild(element);
            }       
            if (window.addEventListener)
                window.addEventListener("load", downloadJSAtOnload, false);
            else if (window.attachEvent)
                window.attachEvent("onload", downloadJSAtOnload);
            else window.onload = downloadJSAtOnload;
    </script> 
    </body>
    </html>

My question is: in the above ways, which one would be more efficient? I need to decrease the loading time of my page and with that I want to "postpone" everything that is not necessary.