create new file chapter13.js
add a click handler to a button
add a function registerEventHandler
define registerEventHandler to hold a different function
add a function unregisterEventHandler
add a function showEvent
add function reportClick
add function printKeyCode
add function printCharacter
add function normaliseEvent
write convenient wrappers for registerEventHandler and unregisterEventHandler
add a square prototype
add a place method and move method
start again with an unfinished prototype
use setInterval to handle the timing of the animation
add function alert
make the element variable no longer points at the DOM node
open new file chapter14.js