[javascript] What is the difference between properties and attributes in HTML?

After the changes made in jQuery 1.6.1, I have been trying to define the difference between properties and attributes in HTML.

Looking at the list on the jQuery 1.6.1 release notes (near the bottom), it seems one can classify HTML properties and attributes as follows:

• Properties: All which either has a boolean value or that is UA calculated such as selectedIndex.

• Attributes: 'Attributes' that can be added to a HTML element which is neither boolean nor containing a UA generated value.

Thoughts?

After reading Sime Vidas's answer, I searched more and found a very straight-forward and easy-to-understand explanation in the angular docs.

HTML attribute vs. DOM property

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Attributes are defined by HTML. Properties are defined by the DOM (Document Object Model).

• A few HTML attributes have 1:1 mapping to properties. id is one example.

• Some HTML attributes don't have corresponding properties. colspan is one example.

• Some DOM properties don't have corresponding attributes. textContent is one example.

• Many HTML attributes appear to map to properties ... but not in the way you might think!

That last category is confusing until you grasp this general rule:

Attributes initialize DOM properties and then they are done. Property values can change; attribute values can't.

For example, when the browser renders <input type="text" value="Bob">, it creates a corresponding DOM node with a value property initialized to "Bob".

When the user enters "Sally" into the input box, the DOM element value property becomes "Sally". But the HTML value attribute remains unchanged as you discover if you ask the input element about that attribute: input.getAttribute('value') returns "Bob".

The HTML attribute value specifies the initial value; the DOM value property is the current value.

The disabled attribute is another peculiar example. A button's disabled property is false by default so the button is enabled. When you add the disabled attribute, its presence alone initializes the button's disabled property to true so the button is disabled.

Adding and removing the disabled attribute disables and enables the button. The value of the attribute is irrelevant, which is why you cannot enable a button by writing <button disabled="false">Still Disabled</button>.

Setting the button's disabled property disables or enables the button. The value of the property matters.

The HTML attribute and the DOM property are not the same thing, even when they have the same name.

The answers already explain how attributes and properties are handled differently, but I really would like to point out how totally insane this is. Even if it is to some extent the spec.

It is crazy, to have some of the attributes (e.g. id, class, foo, bar) to retain only one kind of value in the DOM, while some attributes (e.g. checked, selected) to retain two values; that is, the value "when it was loaded" and the value of the "dynamic state". (Isn't the DOM supposed to be to represent the state of the document to its full extent?)

It is absolutely essential, that two input fields, e.g. a text and a checkbox behave the very same way. If the text input field does not retain a separate "when it was loaded" value and the "current, dynamic" value, why does the checkbox? If the checkbox does have two values for the checked attribute, why does it not have two for its class and id attributes? If you expect to change the value of a text *input* field, and you expect the DOM (i.e. the "serialized representation") to change, and reflect this change, why on earth would you not expect the same from an input field of type checkbox on the checked attribute?

The differentiation, of "it is a boolean attribute" just does not make any sense to me, or is, at least not a sufficient reason for this.

Difference HTML properties and attributes:

Let's first look at the definitions of these words before evaluating what the difference is in HTML:

English definition:

• Attributes are referring to additional information of an object.
• Properties are describing the characteristics of an object.

In HTML context:

When the browser parses the HTML, it creates a tree data structure wich basically is an in memory representation of the HTML. It the tree data structure contains nodes which are HTML elements and text. Attributes and properties relate to this is the following manner:

• Attributes are additional information which we can put in the HTML to initialize certain DOM properties.
• Properties are formed when the browser parses the HTML and generates the DOM. Each of the elements in the DOM have their own set of properties which are all set by the browser. Some of these properties can have their initial value set by HTML attributes. Whenever a DOM property changes which has influence on the rendered page, the page will be immediately re rendered

It is also important to realize that the mapping of these properties is not 1 to 1. In other words, not every attribute which we give on an HTML element will have a similar named DOM property.

Furthermore have different DOM elements different properties. For example, an <input> element has a value property which is not present on a <div> property.

Example:

Let's take the following HTML document:

 <!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">  <!-- charset is a attribute -->
  <meta name="viewport" content="width=device-width"> <!-- name and content are attributes -->
  <title>JS Bin</title>
</head>
<body>
<div id="foo" class="bar foobar">hi</div> <!-- id and class are attributes -->
</body>
</html>

Then we inspect the <div>, in the JS console:

 console.dir(document.getElementById('foo'));

We see the following DOM properties (chrome devtools, not all properties shown):

• We can see that the attribute id in the HTML is now also a id property in the DOM. The id has been initialized by the HTML (although we could change it with javascript).
• We can see that the class attribute in the HTML has no corresponding class property (class is reserved keyword in JS). But actually 2 properties, classList and className.

well these are specified by the w3c what is an attribute and what is a property http://www.w3.org/TR/SVGTiny12/attributeTable.html

but currently attr and prop are not so different and there are almost the same

but they prefer prop for some things

Summary of Preferred Usage

The .prop() method should be used for boolean attributes/properties and for properties which do not exist in html (such as window.location). All other attributes (ones you can see in the html) can and should continue to be manipulated with the .attr() method.

well actually you dont have to change something if you use attr or prop or both, both work but i saw in my own application that prop worked where atrr didnt so i took in my 1.6 app prop =)