Assign click handlers in for loop

It's a common mistake to create closures in loops in Javascript. You need to have some sort of callback function like this:

function createCallback( i ){
  return function(){
    alert('you clicked' + i);
  }
}

$(document).ready(function(){
  for(var i = 0; i < 20; i++) {
    $('#question' + i).click( createCallback( i ) );
  }
});

Update June 3, 2016: since this question is still getting some traction and ES6 is getting popular as well, I would suggest a modern solution. If you write ES6, you can use the let keyword, which makes the i variable local to the loop instead of global:

for(let i = 0; i < 20; i++) {
  $('#question' + i).click( function(){
    alert('you clicked ' + i);
  });
}

It's shorter and easier to understand.

To clarify, i is equal to 20 because the click event won't have fired until after the loop has finished.

$(document).ready(function(){
  for(var i = 0; i < 5; i++) {
   var $li= $('<li>' + i +'</li>');
      (function(i) {
           $li.click( function(){
           alert('you clicked ' + i);
         });
      }(i));
      $('#ul').append($li);
  }
});