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CHAPTER 4

Note that even though the handlers fire in the order in which they were estab-
lished, this order isn't guaranteed by the standard! Testers of this code never observed
an order other than the order of establishment, but it would be foolish to write
code that relies on this order. Always be aware that multiple handlers established
on an element may fire in random order.

Now, what's up with that

useCapture

parameter?

Event propagation

We saw earlier that, with the Basic Event Model, once an event was triggered on
an element the event propagated from the target element upwards in the

DOM

tree to all the target's ancestors. The advanced Level 2 Model also provides this
bubbling phase but ups the ante with an additional phase: capture phase.

Under the

DOM

Level 2 Event Model, when an event is triggered, the event

first propagates from the root of the

DOM

tree down to the target element and

then propagates again from the target element up to the

DOM

root. The former

phase (root to target) is called capture phase, and the latter (target to root) is called
bubble phase.

When a function is established as an event handler, it can be flagged as a cap-

ture handler in which case it will be triggered during capture phase, or as a bub-
ble handler to be triggered during bubble phase. As you might have guessed by
this time, the

useCapture

parameter to

addEventListener()

identifies which type

of handler is established. A value of

false

for this parameter establishes a bubble

handler, whereas a value of

true

registers a capture handler.

Figure 4.3 Clicking the image once demonstrates that all three handlers established
for the click event are triggered.