i was wondering how do i trigger the event keyCode composed by Ctrl+z and the event keycode composed by Ctrl+Shift+z ?
Use e.which which has been normalized cross browser by jquery. 
$(document).keydown(function(e){
      if( e.which === 90 && e.ctrlKey && e.shiftKey ){
         console.log('control + shift + z'); 
      }
      else if( e.which === 90 && e.ctrlKey ){
         console.log('control + z'); 
      }          
}); 
                        If you want to trigger the event then it should be something like this:
HTML
<!DOCTYPE html>
<html>
<head>
</head>
<body>
  <input type=button value=CTRL+SHIFT+Z id=bcsz />
  <input type=button value=CTRL+Z id=bcz />
  <textarea id=t ></textarea>
</body>
</html>
JavaScript
var t = document.getElementById('t'), //textarea
    bcsz = document.getElementById('bcsz'), //button ctrl shift z
    bsz = document.getElementById('bcz'),  // button ctrl z
    csz = document.createEvent('KeyboardEvents'), //ctrl shift z event
    cz = document.createEvent('KeyboardEvents'); // ctrl z event
csz.initKeyboardEvent(
           'keydown', 
           true,     // key down events bubble 
           true,     // and they can be cancelled 
           document.defaultView,  // Use the default view 
           true,        // ctrl 
           false,       // alt
           true,        //shift
           false,       //meta key 
           90,          // keycode
           0
          );  
cz.initKeyboardEvent(
           'keydown', 
           true,     // key down events bubble 
           true,     // and they can be cancelled 
           document.defaultView,  // Use the default view 
           true,        // ctrl 
           false,       // alt
           false,        //shift
           false,       //meta key 
           90,          // keycode
           0
          );  
bcz.addEventListener('click', function(){
  t.dispatchEvent(cz); 
}, false);
bcsz.addEventListener('click', function(){
  t.dispatchEvent(csz); 
}, false);
LOOK AT JSBIN LINK
But it seems it doesn't works. I don't have more time to spend on this, but yeah this is kind of a security issue. I would see these docs at MSDN, W3C and MDN to see if there is a real way to do this.
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