try this code..
function GetCurrentUsers() {
var context = new SP.ClientContext.get_current();
this.website = context.get_web();
var currentUser = website.get_currentUser();
context.load(currentUser);
context.executeQueryAsync(Function.createDelegate(this, onQuerySucceeded), Function.createDelegate(this, onQueryFailed));
function onQuerySucceeded() {
var currentUsers = currentUser.get_title();
document.getElementById("txtIssued").innerHTML = currentUsers;
}
function onQueryFailed(sender, args) {
alert('request failed ' + args.get_message() + '\n' + args.get_stackTrace());
}
}