Edit in JSFiddle

function IsInt() {
    var chCode = ('charCode' in event) ? event.charCode : event.keyCode;
    if (chCode >= 48 && chCode <= 57) {
        return true;
    }
    else {
        alert('Numbers only');
        return false;
    }
}

 var classname = document.getElementsByClassName("IsInt");
    for(var i=0;i<classname.length;i++){
        classname[i].onkeypress=function() {return IsInt();};
       
    }
<a href='http://z54.com'>Example of how to create reuseable JavaScript Class Libraries - Data Validation: Is Integer</a><Br>
<input type='text' class='IsInt' />