When writing HTML source code, you can define attributes on your HTML elements. Then, once the browser parses your code, a corresponding DOM node will be created. This node is an object, and therefore it has properties.
For instance, this HTML element:
<input type="text" value="Name:">
has 2 attributes (
Once the browser parses this code, a HTMLInputElement object will be created, and this object will contain dozens of properties like: accept, accessKey, align, alt, attributes, autofocus, baseURI, checked, childElementCount, childNodes, children, classList, className, clientHeight, etc.
For a given DOM node object, properties are the properties of that object, and attributes are the elements of the
attributes property of that object.
When a DOM node is created for a given HTML element, many of its properties relate to attributes with the same or similar names, but it’s not a one-to-one relationship. For instance, for this HTML element:
<input id="the-input" type="text" value="Name:">
the corresponding DOM node will have
value properties (among others):
idproperty is a reflected property for the
idattribute: Getting the property reads the attribute value, and setting the property writes the attribute value.
idis a pure reflected property, it doesn’t modify or limit the value.
typeproperty is a reflected property for the
typeattribute: Getting the property reads the attribute value, and setting the property writes the attribute value.
typeisn’t a pure reflected property because it’s limited to known values (e.g., the valid types of an input). If you had
<input type="foo">, then
In contrast, the
valueproperty doesn’t reflect the
valueattribute. Instead, it’s the current value of the input. When the user manually changes the value of the input box, the
valueproperty will reflect this change. So if the user inputs
"John"into the input box, then:
theInput.value // returns "John"
theInput.getAttribute('value') // returns "Name:"
valueproperty reflects the current text-content inside the input box, whereas the
valueattribute contains the initial text-content of the
valueattribute from the HTML source code.
So if you want to know what’s currently inside the text-box, read the property. If you, however, want to know what the initial value of the text-box was, read the attribute. Or you can use the
defaultValueproperty, which is a pure reflection of the
theInput.value // returns "John" theInput.getAttribute('value') // returns "Name:" theInput.defaultValue // returns "Name:"
There are several properties that directly reflect their attribute (
id), some are direct reflections with slightly-different names (
htmlFor reflects the
className reflects the
class attribute), many that reflect their attribute but with restrictions/modifications (
multiple), and so on. The spec covers the various kinds of reflection.