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December 12, 2021

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:

  1. We can use for..of to 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).

  1. 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)?

There are many ways, for instance:

The <div> DOM node:

document.body.firstElementChild
// or
document.body.children[0]
// or (the first node is space, so we take 2nd)
document.body.childNodes[1]

The <ul> DOM node:

document.body.lastElementChild
// or
document.body.children[1]

The second <li> (with Pete):

// get <ul>, and then get its last element child
document.body.lastElementChild.lastElementChild

The sibling question

importance: 5

If elem – is an arbitrary DOM element node…

  • Is it true that elem.lastChild.nextSibling is always null?
  • Is it true that elem.children[0].previousSibling is always null ?
  1. Yes, true. The element elem.lastChild is always the last one, it has no nextSibling.
  2. No, wrong, because elem.children[0] is the first child among elements. But there may exist non-element nodes before it. So previousSibling may be a text node.

Please note: for both cases if there are no children, then there will be an error.

If there are no children, elem.lastChild is null, so we can’t access elem.lastChild.nextSibling. And the collection elem.children is empty (like an empty array []).

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.

We’ll be using rows and cells properties to access diagonal table cells.

Open the solution in a sandbox.

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