10.3 Traversing and Modifying a DOM Tree
The DOM gives you access to the elements of a document, allowing you to
modify the contents of a page dynamically using event-driven JavaScript. This
section introduces properties and methods of all DOM nodes that enable you to
traverse the DOM tree, modify nodes and create or delete content dynamically.
Figure
10.2 shows some of the functionality of DOM nodes, as well as two additional
methods of the document object. The program allows you to highlight, modify,
insert and remove elements.
Lines 117–132 contain basic XHTML elements and content. Each element has an
id attribute, which is also displayed at the beginning of the element in square
brackets. For example, the id of the h1 element in lines 117–118 is set to
bigheading, and the heading text begins with [bigheading]. This allows the user
to see the id of each element in the page. The body also contains an h3 heading,
several p elements, and an unordered list.
A div element (lines 133–162) contains the remainder of the XHTML body. Line
134 begins a form element, assigning the empty string to the required action
attribute (because we're not submitting to a server) and returning false to the
onsubmit attribute. When a form's onsubmit handler returns false, the navigation
to the address specified in the action attribute is aborted. This allows us to
modify the page using JavaScript event handlers without reloading the original,
unmodified XHTML.
Basic DOM functionality.
a)This is the page when it first loads. It begins with the
large heading highlighted. (Part 1 of 8.)
b) This is the document after using the Get By id button
to select para3. (Part 2 of 8.)
c) This is the document after inserting a new paragraph
before the selected one. (Part 3 of 8.)
d) Using the Append Child button, a child paragraph is
created. (Part 4 of 8.)
e) The selected paragraph is replaced with a new one.
(Part 5 of 8.)
f) The Get Parent button gets the parent of the selected
node. (Part 6 of 8.)
g) Now we select the first list item. (Part 7 of 8.)
h) The Remove Current button removes the current node and selects its parent. (Part 8 of 8.)
A table (lines 135–160) contains the controls for modifying and manipulating
the elements on the page. Each of the six buttons calls its own event-handling
function to perform the action described by its value.
The JavaScript code begins by declaring two variables. The variable
currentNode (line 27) keeps track of the currently highlighted node, because the
functionality of the buttons depends on which node is currently selected. The
body's onload attribute (line 116) initializes currentNode to the h1 element
with id bigheading. Variable idcount (line 28) is used to assign a unique id to
any new elements that are created. The remainder of the JavaScript code
contains event handling functions for the XHTML buttons and two helper functions
that are called by the event handlers. We now discuss each button and its
corresponding event handler in detail.
Finding and Highlighting an Element Using getElementById and className
The first row of the table (lines 136-141) allows the user to enter the id of
an element into the text field (lines 137–138) and click the Get By Id button
(lines 139–140) to find and highlight the element, as shown in
Fig.
10.2(b) and
(g).
The onclick attribute sets the button's event handler to function
byId.
The byId function is defined in lines 31–38. Line 33 uses getElementById to
assign the contents of the text field to variable id. Line 34 uses
getElementById again to find the element whose id attribute matches the contents
of variable id, and assign it to variable target. If an element is found with
the given id, getElementById returns an object representing that element. If no
element is found, getElementById returns null. Line 36 checks whether target is
an object—recall that any object used as a boolean expression is true, while
null is false. If target evaluates to true, line 37 calls the switchTo function
with target as its argument.
The switchTo function, defined in lines 106–112, is used throughout the
program to highlight a new element in the page. The current element is given a
yellow background using the style class highlighted, defined in line 22. Line
108 sets the current node's className property to the empty string. The
className property allows you to change an XHTML element's class attribute. In
this case, we clear the class attribute in order to remove the highlighted class
from the currentNode before we highlight the new one.
Line 109 assigns the newNode object (passed into the function as a parameter)
to variable currentNode. Line 110 adds the highlighted style class to the new
currentNode using the className property.
Finally, line 111 uses the id property to assign the current node's id to the
input field's value property. Just as className allows access to an element's
class attribute, the id property controls an element's id attribute. While this
isn't necessary when switchTo is called by byId, we will see shortly that other
functions call switchTo. This line makes sure that the text field's value is
consistent with the currently selected node's id. Having found the new element,
removed the highlighting from the old element, updated the currentNode variable
and highlighted the new element, the program has finished selecting a new node
by a user-entered id.
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