The which property refers to which key or mouse button was pressed or clicked.
The value returned for mouse events is a numeric value 1, 2, or 3, representing the left, middle, and right mouse buttons, respectively.
The value returned for keyboard events is a character representation for the key that was pressed.
<title>Using the which property of the event object</title>
This example uses the which property of the event object to determine
which mouse button is pressed.
<input type="radio" onClick = 'alert("Mouse button Number " + event.which + "was pressed.")'>