输入值中的javascript变量

Question

I have some problem. This is my code snippets :

FB.api('/me', function(me){
  if (me.id) {
  alert(me.id);
  //alert(me.email);
  var fbmail = me.email;
  alert(fbmail);
  document.write('<input id="mail" type="hidden" value="'+fbmail+'">');                    
  var retro = document.getElementById('mail').value;
  alert(retro);                    
})

My problem is that there is no alert(retro); :( Can you tell me what's wrong???

Answer 1

document.write can only be used while the page is loading.

Calling it in an async callback will erase the current page, so getElementById will return null.