# Attributes and properties
When the browser loads the page, it "reads" (another word: "parses") HTML text and generates DOM objects from it. For element nodes most standard HTML attributes automatically become properties of DOM objects.
For instance, if the tag is ``, then the DOM object has `body.id="page"`.
But the attribute-property mapping is not one-to-one! In this chapter we'll pay attention to separate these two notions, to see how to work with them, when they are same, and when they are different.
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## DOM properties
We've already seen built-in DOM properties. There's a lot. But technically no one limits us, and if it's not enough -- we can add our own.
DOM nodes are regular JavaScript objects. We can alter them.
For instance, let's create a new property in `document.body`:
```js run
document.body.myData = {
name: 'Caesar',
title: 'Imperator'
};
alert(document.body.myData.title); // Imperator
```
We can add a method as well:
```js run
document.body.sayHi = function() {
alert(this.tagName);
};
document.body.sayHi(); // BODY (the value of "this" in the method is document.body)
```
We can also modify built-in prototypes like `Element.prototype` and add new methods to all elements:
```js run
Element.prototype.sayHi = function() {
alert(`Hello, I'm ${this.tagName}`);
};
document.documentElement.sayHi(); // Hello, I'm HTML
document.body.sayHi(); // Hello, I'm BODY
```
So, DOM properties and methods behave just like those of regular JavaScript objects:
- They can have any value.
- They are case-sensitive (write `elem.nodeType`, not `elem.NoDeTyPe`).
## HTML attributes
In HTML language, tags may have attributes. When the browser reads HTML text and creates DOM objects for tags, it recognizes *standard* attributes and creates DOM properties from them.
So when an element has `id` or another *standard* attribute, the corresponding property gets created. But that doesn't happen if the attribute is non-standard.
For instance:
```html run
```
Please note that a standard attribute for one element can be unknown for another one. For instance, `"type"` is standard for `` ([HTMLInputElement](https://html.spec.whatwg.org/#htmlinputelement)), but not for `` ([HTMLBodyElement](https://html.spec.whatwg.org/#htmlbodyelement)). Standard attributes are described in the specification for the corresponding element class.
Here we can see it:
```html run
```
So, if an attribute is non-standard, there won't be DOM-property for it. Is there a way to access such attributes?
Sure. All attributes are accessible using following methods:
- `elem.hasAttribute(name)` -- checks for existence.
- `elem.getAttribute(name)` -- gets the value.
- `elem.setAttribute(name, value)` -- sets the value.
- `elem.removeAttribute(name)` -- removes the attribute.
These methods operate exactly with what's written in HTML.
Also one can read all attributes using `elem.attributes`: a collection of objects that belong to a built-in [Attr](https://dom.spec.whatwg.org/#attr) class, with `name` and `value` properties.
Here's a demo of reading a non-standard property:
```html run
```
HTML attributes have following features:
- Their name is case-insensitive (that's HTML: `id` is same as `ID`).
- They are always strings.
Here's an extended demo of working with attributes:
```html run
```
Please note:
1. `getAttribute('About')` -- the first letter is uppercase here, and in HTML it's all lowercase. But that doesn't matter: attribute names are case-insensitive.
2. We can assign anything to an attribute, but that becomes a string. So here we have `"123"` as the value.
3. All attributes including ones that we set are visible in `outerHTML`.
4. The `attributes` collection is iterable and has all attributes with `name` and `value`.
## Property-attribute synchronization
When a standard attribute changes, the corresponding property is auto-updated, and (with some exceptions) vise-versa.
In the example below `id` is modified as an attribute, and we can see the property change too. And then the same backwards:
```html run
```
But there are exclusions, for instance `input.value` synchronizes only from attribute -> to property, but not back:
```html run
```
In the example above:
- Changing the attribute `value` updates the property.
- But the property change does not affect the attribute.
That "feature" may actually can come in handy, because the user may modify `value`, and then after it, if we want to recover the "original" value from HTML, it's in the attribute.
## DOM properties are typed
DOM properties are not always strings. For instance, `input.checked` property (for checkboxes) is boolean:
```html run
checkbox
```
There are other examples. The `style` attribute is a string, but `style` property is an object:
```html run
Hello
```
That's an important difference. But even if a DOM property type is a string, it may differ from the attribute!
For instance, the `href` DOM property is always a *full* URL, even if the attribute contains a relative URL or just a `#hash`.
Here's an example:
```html height=30 run
link
```
If we need the value of `href` or any other attribute exactly as written in the HTML, we can use `getAttribute`.
## Non-standard attributes, dataset
When writing HTML, we use a lot of standard attributes. But what about non-standard, custom ones? First, let's see whether they are useful or not? What for?
Sometimes non-standard attributes are used to pass custom data from HTML to JavaScript, or to "mark" HTML-elements for JavaScript.
Like this:
```html run
```
Also they can be used to style an element.
For instance, here for the order state the attribute `order-state` is used:
```html run
A new order.
A pending order.
A canceled order.
```
Why the attribute may be preferable to classes like `.order-state-new`, `.order-state-pending`, `order-state-canceled`?
That's because an attribute is more convenient to manage. The state can be changed as easy as:
```js
// a bit simpler than removing old/adding a new class
div.setAttribute('order-state', 'canceled');
```
But there may be a possible problem with custom attributes. What if we use a non-standard attribute for our purposes and later the standard introduces it and makes it do something? The HTML language is alive, it grows, more attributes appear to suit the needs of developers. There may be unexpected effects in such case.
To evade conflicts, there exist [data-*](https://html.spec.whatwg.org/#embedding-custom-non-visible-data-with-the-data-*-attributes) attributes.
**All attributes starting with "data-" are reserved for programmers' use. They are available in `dataset` property.**
For instance, if an `elem` has an attribute named `"data-about"`, it's available as `elem.dataset.about`.
Like this:
```html run
```
Multiword attributes like `data-order-state` become camel-cased: `dataset.orderState`.
Here's a rewritten "order state" example:
```html run
A new order.
```
Using `data-*` attributes is a valid, safe way to pass custom data.
Please note that we can not only read, but also modify data-attributes. Then CSS updates the view accordingly: in the example above the last line `(*)` changes the color to blue.
## Summary
- Attributes -- is what's written in HTML.
- Properties -- is what's in DOM objects.
A small comparison:
| | Properties | Attributes |
|------------|------------|------------|
|Type|Any value, standard properties have types described in the spec|A string|
|Name|Name is case-sensitive|Name is not case-sensitive|
Methods to work with attributes are:
- `elem.hasAttribute(name)` -- to check for existence.
- `elem.getAttribute(name)` -- to get the value.
- `elem.setAttribute(name, value)` -- to set the value.
- `elem.removeAttribute(name)` -- to remove the attribute.
- `elem.attributes` is a collection of all attributes.
For most needs DOM properties can serve us well. We should refer to attributes only when DOM properties do not suit us, when we need exactly attributes, for instance:
- We need a non-standard attribute. But if it starts with `data-`, then we should use `dataset`.
- We want to read the value "as written" in HTML. The value of the DOM property may be different, for instance `href` property is always a full URL, and we may want to get the "original" value.