How to retrieve errors of Javascript?

[American horizo]

Hi, there are a method that can report an error accured when a JS script is executed?

For exemple, following the Javascript: Complete Guide book, i've tryed this snippet for make practice:

whenReady(function() 
           {
              var clock = document.getElementById("clock"); // The clock element
              var icon = new Image(); // An image to drag
              icon.src = "clock-icon.png"; // Image URL
              
              // Display the time once every minute
              function displayTime() 
              { alert('lol')
                  var now = new Date(); // Get current time
                  var hrs = now.getHours(), 
                  mins = now.getMinutes();
                  
                  if (mins < 10) mins = "0" + mins;
                  clock.innerHTML = hrs + ":" + mins; // Display current time
                  setTimeout(displayTime, 60000); // Run again in 1 minute
              }
              
              displayTime();
              // Make the clock draggable
              // We can also do this with an HTML attribute: <span draggable="true">...
              clock.draggable = true;
              // Set up drag event handlers
              clock.ondragstart = function(event) 
              {
                  var event = event || window.event; // For IE compatability
                  // The dataTransfer property is key to the drag-and-drop API
                  var dt = event.dataTransfer;
    
                  // Tell the browser what is being dragged.
                  // The Date() constructor used as a function returns a timestamp string
                  dt.setData("Text", Date() + "\n");
                  // Tell the browser to drag our icon to represent the timestamp, in
                  // browsers that support that. Without this line, the browser may
                  // use an image of the clock text as the value to drag.
                  if (dt.setDragImage) dt.setDragImage(icon, 0, 0);
              };
           });

</script>


<style>
#clock { /* Make the clock look nice */
font: bold 24pt sans; background: #ddf; padding: 10px;
border: solid black 2px; border-radius: 10px;
}
</style>


<h1>Drag timestamps from the clock</h1>

<span id="clock"></span> <!-- The time is displayed here -->
<textarea cols=60 rows=20></textarea> <!-- You can drop timestamps here --> 

(it's not necessary to read the code)
The problem that i had was that the code was executed once opened the page, while the document was not fully loaded and "clock" object doesn't exist... So it didn't work
After i've understand that i should insert a script that launch the function once the document is ready! This means that without this function, i get into an error such "the object doesn't exist" (or something of genre), but how keep trace of it?