DOM Tree-walking - Browser: Document, Events, Interfaces - Document
Walking the DOM
The DOM allows us to do anything with elements and their contents, but first we need to reach the corresponding DOM object.
All operations on the DOM start with the document object. That’s the main “entry point” to DOM. From it, we can access any node.
Here’s a picture of links that allow for travel between DOM nodes:
Let’s discuss them in more detail.
On top: documentElement and body
The topmost tree nodes are available directly as document properties:<html> = document.documentElement
The topmost document node is document.documentElement. That’s the DOM node of the <html> tag.<body> = document.body
Another widely used DOM node is the <body> element – document.body.<head> = document.head
The <head> tag is available as document.head.
There’s a catch: document.body can be null
A script cannot access an element that doesn’t exist at the moment of running.
In particular, if a script is inside <head>, then document.body is unavailable, because the browser did not read it yet.
So, in the example below, the first alert shows null:
<html>
<head><script>
alert( "From HEAD: " + document.body ); // null, there's no <body> yet</script>
</head>
<body>
<script>alert( "From BODY: " + document.body ); // HTMLBodyElement, now it exists</script>
</body>
</html>
In the DOM world null means “doesn’t exist”
In the DOM, the null value means “doesn’t exist” or “no such node”.
Children: childNodes, firstChild, lastChild
There are two terms that we’ll use from now on:
- Child nodes (or children) – elements that are direct children. In other words, they are nested exactly in the given one. For instance,
<head>and<body>are children of<html>element. - Descendants – all elements that are nested in the given one, including children, their children and so on.
For instance, here <body> has children <div> and <ul> (and few blank text nodes):
<html> <body><div>Begin</div> <ul><li><b>Information</b></li></ul> </body> </html>
… And descendants of <body> are not only direct children <div>, <ul> but also more deeply nested elements, such as <li> (a child of <ul>) and <b> (a child of <li>) – the entire subtree.
The childNodes collection lists all child nodes, including text nodes.
The example below shows children of document.body:
<html>
<body><div>Begin</div>
<ul><li>Information</li></ul>
<div>End</div>
<script>
for (let i = 0; i < document.body.childNodes.length; i++) {alert( document.body.childNodes[i] ); // Text, DIV, Text, UL, ..., SCRIPT}</script>
...more stuff...
</body>
</html>
Please note an interesting detail here. If we run the example above, the last element shown is <script>. In fact, the document has more stuff below, but at the moment of the script execution the browser did not read it yet, so the script doesn’t see it.
Properties firstChild and lastChild give fast access to the first and last children.
They are just shorthands. If there exist child nodes, then the following is always true:
elem.childNodes[0] === elem.firstChild elem.childNodes[elem.childNodes.length - 1] === elem.lastChild
There’s also a special function elem.hasChildNodes() to check whether there are any child nodes.
DOM collections
As we can see, childNodes looks like an array. But actually it’s not an array, but rather a collection – a special array-like iterable object.
There are two important consequences:
- We can use
for..ofto iterate over it:
for (let node of document.body.childNodes) {alert(node); // shows all nodes from the collection
}That’s because it’s iterable (provides the Symbol.iterator property, as required).
- Array methods won’t work, because it’s not an array:
alert(document.body.childNodes.filter); // undefined (there's no filter method!)
The first thing is nice. The second is tolerable, because we can use Array.from to create a “real” array from the collection, if we want array methods:
alert( Array.from(document.body.childNodes).filter ); // function
DOM collections are read-only
DOM collections, and even more – all navigation properties listed in this chapter are read-only.
We can’t replace a child by something else by assigning childNodes[i] = ....
Changing DOM needs other methods. We will see them in the next chapter.
DOM collections are live
Almost all DOM collections, with minor exceptions, are live. In other words, they reflect the current state of DOM.
If we keep a reference to elem.childNodes, and add/remove nodes into DOM, then they appear in the collection automatically.
Don’t use for..in to loop over collections
Collections are iterable using for..of. Sometimes people try to use for..in for that.
Please, don’t. The for..in loop iterates over all enumerable properties. And collections have some “extra” rarely used properties that we usually do not want to get:
<body> <script>// shows 0, 1, length, item, values and more.for (let prop in document.body.childNodes) alert(prop); </script> </body>
Siblings and the parent
Siblings are nodes that are children of the same parent.
For instance, here <head> and <body> are siblings:
<html><head>...</head><body>...</body> </html>
<body>is said to be the “next” or “right” sibling of<head>,<head>is said to be the “previous” or “left” sibling of<body>.
The next sibling is in nextSibling property, and the previous one – in previousSibling.
The parent is available as parentNode.
For example:
// parent of <body> is <html> alert( document.body.parentNode === document.documentElement ); // true // after <head> goes <body> alert( document.head.nextSibling ); // HTMLBodyElement // before <body> goes <head> alert( document.body.previousSibling ); // HTMLHeadElement
Element-only navigation
Navigation properties listed above refer to all nodes. For instance, in childNodes we can see both text nodes, element nodes, and even comment nodes if they exist.
But for many tasks, we don’t want text or comment nodes. We want to manipulate element nodes that represent tags and form the structure of the page.
So let’s see more navigation links that only take element nodes into account:
The links are similar to those given above, just with Element word inside:
children– only those children that are element nodes.firstElementChild,lastElementChild– first and last element children.previousElementSibling,nextElementSibling– neighbor elements.parentElement– parent element.
Why parentElement? Can the parent be not an element?
The parentElement property returns the “element” parent, while parentNode returns “any node” parent. These properties are usually the same: they both get the parent.
With the one exception of document.documentElement:
alert( document.documentElement.parentNode ); // document alert( document.documentElement.parentElement ); // null
The reason is that the root node document.documentElement (<html>) has document as its parent. But document is not an element node, so parentNode returns it and parentElement does not.
This detail may be useful when we want to travel up from an arbitrary element elem to <html>, but not to the document:
while(elem = elem.parentElement) { // go up till <html>alert( elem );
}
Let’s modify one of the examples above: replace childNodes with children. Now it shows only elements:
<html>
<body><div>Begin</div>
<ul><li>Information</li></ul>
<div>End</div>
<script>
for (let elem of document.body.children) {alert(elem); // DIV, UL, DIV, SCRIPT}</script>
...
</body>
</html>More links: tables
Till now, we described the basic navigation properties.
Certain types of DOM elements may provide additional properties, specific to their type, for convenience.
Tables are a great example of that, and represent a particularly important case:
The <table> element supports (in addition to the given above) these properties:
table.rows– the collection of<tr>elements of the table.table.caption/tHead/tFoot– references to elements<caption>,<thead>,<tfoot>.table.tBodies– the collection of<tbody>elements (can be many according to the standard, but there will always be at least one – even if it is not in the source HTML, the browser will put it in the DOM).
<thead>, <tfoot>, <tbody> elements provide the rows property:
tbody.rows– the collection of<tr>inside.
<tr>:
tr.cells– the collection of<td>and<th>cells inside the given<tr>.tr.sectionRowIndex– the position (index) of the given<tr>inside the enclosing<thead>/<tbody>/<tfoot>.tr.rowIndex– the number of the<tr>in the table as a whole (including all table rows).
<td> and <th>:
td.cellIndex– the number of the cell inside the enclosing<tr>.
An example of usage:
<table id="table"><tr><td>one</td><td>two</td></tr><tr><td>three</td><td>four</td></tr> </table> <script>// get td with "two" (first row, second column)let td = table.rows[0].cells[1]; td.style.backgroundColor = "red"; // highlight it </script>
The specification: tabular data.
There are also additional navigation properties for HTML forms. We’ll look at them later when we start working with forms.
Summary
Given a DOM node, we can go to its immediate neighbors using navigation properties.
There are two main sets of them:
- For all nodes:
parentNode,childNodes,firstChild,lastChild,previousSibling,nextSibling. - For element nodes only:
parentElement,children,firstElementChild,lastElementChild,previousElementSibling,nextElementSibling.
Some types of DOM elements, e.g. tables, provide additional properties and collections to access their content.
Tasks
DOM children
importance: 5
Look at this page:
<html> <body><div>Users:</div><ul><li>John</li><li>Pete</li></ul> </body> </html>
For each of the following, give at least one way of how to access them:
- The
<div>DOM node? - The
<ul>DOM node? - The second
<li>(with Pete)?
The sibling question
importance: 5
If elem – is an arbitrary DOM element node…
- Is it true that
elem.lastChild.nextSiblingis alwaysnull? - Is it true that
elem.children[0].previousSiblingis alwaysnull?
Select all diagonal cells
importance: 5
Write the code to paint all diagonal table cells in red.
You’ll need to get all diagonal <td> from the <table> and paint them using the code:
// td should be the reference to the table cell td.style.backgroundColor = 'red';
The result should be:
Open a sandbox for the task.
Original Content at: https://javascript.info/dom-navigation
© 2007–2024 Ilya Kantor, https://javascript.info
