libs:
  - d3
  - domtree

---


# 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:

![](dom-links.svg)

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`.

````warn header="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 run
<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>
```
````

```smart header="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 run
<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 run
<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:
```js
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:
  ```js
  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).

2. Array methods won't work, because it's not an array:
  ```js run
  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:

  ```js run
  alert( Array.from(document.body.childNodes).filter ); // now it's there
  ```

```warn header="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.
```

```warn header="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.
```

````warn header="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:

```html run
<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
<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:

```js
// 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 there 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:

![](dom-links-elements.svg)

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` -- neighbour elements.
- `parentElement` -- parent element.

````smart header="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`:

```js run
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`:
```js
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 run
<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 [#dom-navigation-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 and a particularly important case for that.

**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).

**`<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:

```html run height=100
<table id="table">
  <tr>
    <td>one</td><td>two</td>
  </tr>
  <tr>
    <td>three</td><td>four</td>
  </tr>
</table>

<script>
  // get the content of the first row, second cell
  alert( table.*!*rows[0].cells[1]*/!*.innerHTML ) // "two"
</script>
```

The specification: [tabular data](https://html.spec.whatwg.org/multipage/tables.html).

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 neighbours 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.
