Call JavaScript function on page load

Example of how to call a javascript function on page load using plain JavaScript and JQuery

HTML

<body onload="myFunction()">
   <div id="mydiv1">-23</div>
</body>

JavaScript

function myFunction() {
        var firstDivContent = document.getElementById('mydiv1').innerHTML;
        console.log(firstDivContent);
      //  var firstDivContent =  "23";
        if (+firstDivContent > 0) {
            alert('positive')
        }
        else if (+firstDivContent < 0) {
            alert ('negative')
        }
    }