First of all, the subject of the question must know confirm('are you sure?') 点确定会返回true,点返回会返回false;

Then let me say that there is no substantial difference between the three ways of writing, they just change the return value:
1. You know
2. ask() is changed to return ask();
3. The best way of writing among the three

Also

• See the questioner is studying event binding. If you want to bind events to a bunch of regular elements, such as list li, the best way is 事件委托:

<ul id="ul">
  <li>aaaaaaaa</li>
  <li>bbbbbbbb</li>
  <li>cccccccc</li>
</ul>

window.onload = function(){
  var oUl = document.getElementById("ul");
  var aLi = oUl.getElementsByTagName("li");

  for(var i=0; i<aLi.length; i++){
    aLi[i].onmouseover = function(){
      this.style.background = "red";
    }
    aLi[i].onmouseout = function(){
      this.style.background = "";
    }
  }
}

• Once you understand the event mechanism of js, capturing and bubbling, you will be almost there.

• Final compatibility with IE: attachEvent, standard: addEventListener

The second option is that you have no return

window.onload=init;
function init(){
    var x=document.getElementsByTagName('a');
    for(var i in x){
        x[i].onclick=function(){
        return ask();
        }
    }
}
function ask(){
    return confirm('are you sure?');
}