15 Aralık 2021

Node properties: type, tag and contents

Let’s get a more in-depth look at DOM nodes.

In this chapter we’ll see more into what they are and their most used properties.

DOM node classes

DOM nodes have different properties depending on their class. For instance, an element node corresponding to tag <a> has link-related properties, and the one corresponding to <input> has input-related properties and so on. Text nodes are not the same as element nodes. But there are also common properties and methods between all of them, because all classes of DOM nodes form a single hierarchy.

Each DOM node belongs to the corresponding built-in class.

The root of the hierarchy is EventTarget, that is inherited by Node, and other DOM nodes inherit from it.

Here’s the picture, explanations to follow:

The classes are:

EventTarget – is the root “abstract” class. Objects of that class are never created. It serves as a base, so that all DOM nodes support so-called “events”, we’ll study them later.
Node – is also an “abstract” class, serving as a base for DOM nodes. It provides the core tree functionality: parentNode, nextSibling, childNodes and so on (they are getters). Objects of Node class are never created. But there are concrete node classes that inherit from it, namely: Text for text nodes, Element for element nodes and more exotic ones like Comment for comment nodes.
Element – is a base class for DOM elements. It provides element-level navigation like nextElementSibling, children and searching methods like getElementsByTagName, querySelector. A browser supports not only HTML, but also XML and SVG. The Element class serves as a base for more specific classes: SVGElement, XMLElement and HTMLElement.
HTMLElement – is finally the basic class for all HTML elements. It is inherited by various HTML elements:
- HTMLInputElement – the class for <input> elements,
- HTMLBodyElement – the class for <body> elements,
- HTMLAnchorElement – the class for <a> elements
- …and so on, each tag has its own class that may provide specific properties and methods.

So, the full set of properties and methods of a given node comes as the result of the inheritance.

For example, let’s consider the DOM object for an <input> element. It belongs to HTMLInputElement class. It gets properties and methods as a superposition of:

HTMLInputElement – this class provides input-specific properties, and inherits from…
HTMLElement – it provides common HTML element methods (and getters/setters) and inherits from…
Element – provides generic element methods and inherits from…
Node – provides common DOM node properties and inherits from…
EventTarget – gives the support for events (to be covered),
…and finally it inherits from Object, so “pure object” methods like hasOwnProperty are also available.

To see the DOM node class name, we can recall that an object usually has the constructor property. It references to the class constructor, and constructor.name is its name:

alert( document.body.constructor.name ); // HTMLBodyElement

…Or we can just toString it:

alert( document.body ); // [object HTMLBodyElement]

We also can use instanceof to check the inheritance:

alert( document.body instanceof HTMLBodyElement ); // true
alert( document.body instanceof HTMLElement ); // true
alert( document.body instanceof Element ); // true
alert( document.body instanceof Node ); // true
alert( document.body instanceof EventTarget ); // true

As we can see, DOM nodes are regular JavaScript objects. They use prototype-based classes for inheritance.

That’s also easy to see by outputting an element with console.dir(elem) in a browser. There in the console you can see HTMLElement.prototype, Element.prototype and so on.

Most browsers support two commands in their developer tools: console.log and console.dir. They output their arguments to the console. For JavaScript objects these commands usually do the same.

But for DOM elements they are different:

console.log(elem) shows the element DOM tree.
console.dir(elem) shows the element as a DOM object, good to explore its properties.

Try it on document.body.

In the specification, DOM classes are described not using JavaScript, but a special Interface description language (IDL), that is usually easy to understand.

In IDL all properties are prepended with their types. For instance, DOMString, boolean and so on.

Here’s an excerpt from it, with comments:

          // Define HTMLInputElement
// The colon ":" means that HTMLInputElement inherits from HTMLElement
interface HTMLInputElement: HTMLElement {
  // here go properties and methods of <input> elements

  // "DOMString" means that the value of these properties are strings
  attribute DOMString accept;
  attribute DOMString alt;
  attribute DOMString autocomplete;
  attribute DOMString value;

  // boolean value property (true/false)
  attribute boolean autofocus;
  ...
  // now the method: "void" means that the method returns no value
  void select();
  ...
}
        

Other classes are somewhat similar.

The “nodeType” property

The nodeType property provides an old-fashioned way to get the “type” of a DOM node.

It has a numeric value:

elem.nodeType == 1 for element nodes,
elem.nodeType == 3 for text nodes,
elem.nodeType == 9 for the document object,
there are few other values in the specification.

For instance:

<body>
  <script>
  let elem = document.body;

  // let's examine what it is?
  alert(elem.nodeType); // 1 => element

  // and the first child is...
  alert(elem.firstChild.nodeType); // 3 => text

  // for the document object, the type is 9
  alert( document.nodeType ); // 9
  </script>
</body>

In modern scripts, we can use instanceof and other class-based tests to see the node type, but sometimes nodeType may be simpler. We can only read nodeType, not change it.

Tag: nodeName and tagName

Given a DOM node, we can read its tag name from nodeName or tagName properties:

For instance:

alert( document.body.nodeName ); // BODY
alert( document.body.tagName ); // BODY

Is there any difference between tagName and nodeName?

Sure, the difference is reflected in their names, but is indeed a bit subtle.

The tagName property exists only for Element nodes.
The nodeName is defined for any Node:
- for elements it means the same as tagName.
- for other node types (text, comment, etc.) it has a string with the node type.

In other words, tagName is only supported by element nodes (as it originates from Element class), while nodeName can say something about other node types.

For instance, let’s compare tagName and nodeName for the document and a comment node:

<body><!-- comment -->

  <script>
    // for comment
    alert( document.body.firstChild.tagName ); // undefined (not an element)
    alert( document.body.firstChild.nodeName ); // #comment

    // for document
    alert( document.tagName ); // undefined (not an element)
    alert( document.nodeName ); // #document
  </script>
</body>

If we only deal with elements, then tagName is the only thing we should use.

The browser has two modes of processing documents: HTML and XML. Usually the HTML-mode is used for webpages. XML-mode is enabled when the browser receives an XML-document with the header: Content-Type: application/xml+xhtml.

In HTML mode tagName/nodeName is always uppercased: it’s BODY either for <body> or <BoDy>.

In XML mode the case is kept “as is”. Nowadays XML mode is rarely used.

innerHTML: the contents

The innerHTML property allows to get the HTML inside the element as a string.

We can also modify it. So it’s one of most powerful ways to change the page.

The example shows the contents of document.body and then replaces it completely:

<body>
  <p>A paragraph</p>
  <div>A div</div>

  <script>
    alert( document.body.innerHTML ); // read the current contents
    document.body.innerHTML = 'The new BODY!'; // replace it
  </script>

</body>

We can try to insert invalid HTML, the browser will fix our errors:

<body>

  <script>
    document.body.innerHTML = '<b>test'; // forgot to close the tag
    alert( document.body.innerHTML ); // <b>test</b> (fixed)
  </script>

</body>

If innerHTML inserts a <script> tag into the document – it becomes a part of HTML, but doesn’t execute.

Beware: “innerHTML+=” does a full overwrite

We can append HTML to an element by using elem.innerHTML+="more html".

Like this:

          chatDiv.innerHTML += "<div>Hello<img src='smile.gif'/> !</div>";
chatDiv.innerHTML += "How goes?";
        

But we should be very careful about doing it, because what’s going on is not an addition, but a full overwrite.

Technically, these two lines do the same:

          elem.innerHTML += "...";
// is a shorter way to write:
elem.innerHTML = elem.innerHTML + "..."
        

In other words, innerHTML+= does this:

The old contents is removed.
The new innerHTML is written instead (a concatenation of the old and the new one).

As the content is “zeroed-out” and rewritten from the scratch, all images and other resources will be reloaded.

In the chatDiv example above the line chatDiv.innerHTML+="How goes?" re-creates the HTML content and reloads smile.gif (hope it’s cached). If chatDiv has a lot of other text and images, then the reload becomes clearly visible.

There are other side-effects as well. For instance, if the existing text was selected with the mouse, then most browsers will remove the selection upon rewriting innerHTML. And if there was an <input> with a text entered by the visitor, then the text will be removed. And so on.

Luckily, there are other ways to add HTML besides innerHTML, and we’ll study them soon.

outerHTML: full HTML of the element

The outerHTML property contains the full HTML of the element. That’s like innerHTML plus the element itself.

Here’s an example:

<div id="elem">Hello <b>World</b></div>

<script>
  alert(elem.outerHTML); // <div id="elem">Hello <b>World</b></div>
</script>

Beware: unlike innerHTML, writing to outerHTML does not change the element. Instead, it replaces it as a whole in the outer context.

Yeah, sounds strange, and strange it is, that’s why we make a separate note about it here. Take a look.

Consider the example:

<div>Hello, world!</div>

<script>
  let div = document.querySelector('div');

  // replace div.outerHTML with <p>...</p>
  div.outerHTML = '<p>A new element!</p>'; // (*)

  // Wow! The div is still the same!
  alert(div.outerHTML); // <div>Hello, world!</div>
</script>

In the line (*) we take the full HTML of <div>...</div> and replace it by <p>...</p>. In the outer document we can see the new content instead of the <div>. But the old div variable is still the same.

The outerHTML assignment does not modify the DOM element, but extracts it from the outer context and inserts a new piece of HTML instead of it.

Novice developers sometimes make an error here: they modify div.outerHTML and then continue to work with div as if it had the new content in it.

That’s possible with innerHTML, but not with outerHTML.

We can write to outerHTML, but should keep in mind that it doesn’t change the element we’re writing to. It creates the new content on its place instead. We can get a reference to new elements by querying DOM.

nodeValue/data: text node content

The innerHTML property is only valid for element nodes.

Other node types have their counterpart: nodeValue and data properties. These two are almost the same for practical use, there are only minor specification differences. So we’ll use data, because it’s shorter.

An example of reading the content of a text node and a comment:

<body>
  Hello
  <!-- Comment -->
  <script>
    let text = document.body.firstChild;
    alert(text.data); // Hello

    let comment = text.nextSibling;
    alert(comment.data); // Comment
  </script>
</body>

For text nodes we can imagine a reason to read or modify them, but why comments? Usually, they are not interesting at all, but sometimes developers embed information or template instructions into HTML in them, like this:

          <!-- if isAdmin -->
  <div>Welcome, Admin!</div>
<!-- /if -->
        

…Then JavaScript can read it and process embedded instructions.

textContent: pure text

The textContent provides access to the text inside the element: only text, minus all <tags>.

For instance:

<div id="news">
  <h1>Headline!</h1>
  <p>Martians attack people!</p>
</div>

<script>
  // Headline! Martians attack people!
  alert(news.textContent);
</script>

As we can see, only text is returned, as if all <tags> were cut out, but the text in them remained.

In practice, reading such text is rarely needed.

Writing to textContent is much more useful, because it allows to write text the “safe way”.

Let’s say we have an arbitrary string, for instance entered by a user, and want to show it.

With innerHTML we’ll have it inserted “as HTML”, with all HTML tags.
With textContent we’ll have it inserted “as text”, all symbols are treated literally.

Compare the two:

<div id="elem1"></div>
<div id="elem2"></div>

<script>
  let name = prompt("What's your name?", "<b>Winnie-the-pooh!</b>");

  elem1.innerHTML = name;
  elem2.textContent = name;
</script>

The first <div> gets the name “as HTML”: all tags become tags, so we see the bold name.
The second <div> gets the name “as text”, so we literally see <b>Winnie-the-pooh!</b>.

In most cases, we expect the text from a user, and want to treat it as text. We don’t want unexpected HTML in our site. An assignment to textContent does exactly that.

The “hidden” property

The “hidden” attribute and the DOM property specifies whether the element is visible or not.

We can use it in HTML or assign using JavaScript, like this:

<div>Both divs below are hidden</div>

<div hidden>With the attribute "hidden"</div>

<div id="elem">JavaScript assigned the property "hidden"</div>

<script>
  elem.hidden = true;
</script>

Technically, hidden works the same as style="display:none". But it’s shorter to write.

Here’s a blinking element:

<div id="elem">A blinking element</div>

<script>
  setInterval(() => elem.hidden = !elem.hidden, 1000);
</script>

More properties

DOM elements also have additional properties, many of them provided by the class:

value – the value for <input>, <select> and <textarea> (HTMLInputElement, HTMLSelectElement…).
href – the “href” for <a href="..."> (HTMLAnchorElement).
id – the value of “id” attribute, for all elements (HTMLElement).
…and much more…

For instance:

<input type="text" id="elem" value="value">

<script>
  alert(elem.type); // "text"
  alert(elem.id); // "elem"
  alert(elem.value); // value
</script>

Most standard HTML attributes have the corresponding DOM property, and we can access it like that.

If we want to know the full list of supported properties for a given class, we can find them in the specification. For instance, HTMLInputElement is documented at https://html.spec.whatwg.org/#htmlinputelement.

Or if we’d like to get them fast or are interested in a concrete browser specification – we can always output the element using console.dir(elem) and read the properties. Or explore “DOM properties” in the Elements tab of the browser developer tools.

Summary

Each DOM node belongs to a certain class. The classes form a hierarchy. The full set of properties and methods come as the result of inheritance.

Main DOM node properties are:

nodeType: We can use it to see if a node is a text or an element node. It has a numeric value: 1 – for elements,3 – for text nodes, and few other for other node types. Read-only.
nodeName/tagName: For elements, tag name (uppercased unless XML-mode). For non-element nodes nodeName describes what it is. Read-only.
innerHTML: The HTML content of the element. Can be modified.
outerHTML: The full HTML of the element. A write operation into elem.outerHTML does not touch elem itself. Instead it gets replaced with the new HTML in the outer context.
nodeValue/data: The content of a non-element node (text, comment). These two are almost the same, usually we use data. Can be modified.
textContent: The text inside the element: HTML minus all <tags>. Writing into it puts the text inside the element, with all special characters and tags treated exactly as text. Can safely insert user-generated text and protect from unwanted HTML insertions.
hidden: When set to true, does the same as CSS display:none.

DOM nodes also have other properties depending on their class. For instance, <input> elements (HTMLInputElement) support value, type, while <a> elements (HTMLAnchorElement) support href etc. Most standard HTML attributes have a corresponding DOM property.

Although, HTML attributes and DOM properties are not always the same, as we’ll see in the next chapter.

Görevler

Count descendants

There’s a tree structured as nested ul/li.

Write the code that for each <li> shows:

What’s the text inside it (without the subtree)
The number of nested <li> – all descendants, including the deeply nested ones.

Yeni pencerede göster

Görevler için korunaklı alan aç.

Let’s make a loop over <li>:

          for (let li of document.querySelectorAll('li')) {
  ...
}
        

In the loop we need to get the text inside every li.

We can read the text from the first child node of li, that is the text node:

          for (let li of document.querySelectorAll('li')) {
  let title = li.firstChild.data;

  // title is the text in <li> before any other nodes
}
        

Then we can get the number of descendants as li.getElementsByTagName('li').length.

Çözümü korunaklı alanda aç.

What's in the nodeType?

What does the script show?

          <html>

<body>
  <script>
    alert(document.body.lastChild.nodeType);
  </script>
</body>

</html>
        

There’s a catch here.

At the time of <script> execution the last DOM node is exactly <script>, because the browser did not process the rest of the page yet.

So the result is 1 (element node).

<html>

<body>
  <script>
    alert(document.body.lastChild.nodeType);
  </script>
</body>

</html>

Tag in comment

What does this code show?

          <script>
  let body = document.body;

  body.innerHTML = "<!--" + body.tagName + "-->";

  alert( body.firstChild.data ); // what's here?
</script>
        

The answer: BODY.

<script>
  let body = document.body;

  body.innerHTML = "<!--" + body.tagName + "-->";

  alert( body.firstChild.data ); // BODY
</script>

What’s going on step by step:

The content of <body> is replaced with the comment. The comment is , because body.tagName == "BODY". As we remember, tagName is always uppercase in HTML.
The comment is now the only child node, so we get it in body.firstChild.
The data property of the comment is its contents (inside ): "BODY".

Where's the "document" in the hierarchy?

Which class does the document belong to?

What’s its place in the DOM hierarchy?

Does it inherit from Node or Element, or maybe HTMLElement?

We can see which class it belongs by outputting it, like:

alert(document); // [object HTMLDocument]

Or:

alert(document.constructor.name); // HTMLDocument

So, document is an instance of HTMLDocument class.

What’s its place in the hierarchy?

Yeah, we could browse the specification, but it would be faster to figure out manually.

Let’s traverse the prototype chain via __proto__.

As we know, methods of a class are in the prototype of the constructor. For instance, HTMLDocument.prototype has methods for documents.

Also, there’s a reference to the constructor function inside the prototype:

alert(HTMLDocument.prototype.constructor === HTMLDocument); // true

To get a name of the class as a string, we can use constructor.name. Let’s do it for the whole document prototype chain, till class Node:

alert(HTMLDocument.prototype.constructor.name); // HTMLDocument
alert(HTMLDocument.prototype.__proto__.constructor.name); // Document
alert(HTMLDocument.prototype.__proto__.__proto__.constructor.name); // Node

We also could examine the object using console.dir(document) and see these names by opening __proto__. The console takes them from constructor internally.

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Node properties: type, tag and contents

DOM node classes

The “nodeType” property

Tag: nodeName and tagName

innerHTML: the contents

Beware: “innerHTML+=” does a full overwrite

outerHTML: full HTML of the element

nodeValue/data: text node content

textContent: pure text

The “hidden” property

More properties

Summary

Görevler

Count descendants

What's in the nodeType?

Tag in comment

Where's the "document" in the hierarchy?

Yorumlar

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