jQuery attr vs prop?

Question

Now this isn't just another What's the difference question, I have done some tests(http://jsfiddle.net/ZC3Lf/) modifying the prop and attr of <form action="/test/"></form>​ with the output being:

1) prop Modification test
Prop: http://fiddle.jshell.net/test/1
Attr: http://fiddle.jshell.net/test/1

2) Attr Modification test
Prop: http://fiddle.jshell.net/test/1
Attr: /test/1

3) Attr then Prop Modification test
Prop: http://fiddle.jshell.net/test/11
Attr: http://fiddle.jshell.net/test/11

4) Prop then Attr Modification test
Prop: http://fiddle.jshell.net/test/11
Attr: http://fiddle.jshell.net/test/11

Now I am confused about a couple of things, as far as my knowledge goes:
Prop: The value in its current state after any modifications via JavaScript
Attr: The value as it was defined in the html on page load.

Now if this is correct,

• Why does modifying the prop seem to make the action fully qualified, and conversely why does modifying the attribute not?
• Why does modifying the prop in 1) modify the attribute, that one makes no sense to me?
• Why does modifying the attr in 2) modify the property, are they meant to be linked that way?

---

Test Code

HTML

JavaScript

var element = $('form');
var property = 'action';

/*You should not need to modify below this line */

var body = $('body');
var original = element.attr(property);

body.append('<h1>Prop Modification test</h1>');
element.prop(property, element.prop(property) + 1);

body.append('Prop: '+element.prop(property)+'<br />');
body.append('Attr: '+element.attr(property)+'<hr />');

//reset
element.prop(property, original);
element.attr(property, original);

body.append('<h1>Attr Modification test</h1>');
element.attr(property, element.attr(property) + 1);

body.append('Prop: '+element.prop(property)+'<br />');
body.append('Attr: '+element.attr(property)+'<hr />');

//reset
element.prop(property, original);
element.attr(property, original);

body.append('<h1>Attr then Prop Modification test</h1>');
element.attr(property, element.attr(property) + 1);
element.prop(property, element.prop(property) + 1);

body.append('Prop: '+element.prop(property)+'<br />');
body.append('Attr: '+element.attr(property)+'<hr />');

//reset
element.prop(property, original);
element.attr(property, original);

body.append('<h1>Prop then Attr Modification test</h1>');
element.prop(property, element.prop(property) + 1);
element.attr(property, element.attr(property) + 1);

body.append('Prop: '+element.prop(property)+'<br />');
body.append('Attr: '+element.attr(property)+'<hr />');

Answer 1

Unfortunately none of your links work :(

Some insight though, attr is for all attributes. prop is for properties.

In older jQuery versions (<1.6), we just had attr. To get to DOM properties such as nodeName, selectedIndex, or defaultValue you had to do something like:

var elem = $("#foo")[0];
if ( elem ) {
  index = elem.selectedIndex;
}

That sucked, so prop was added:

index = $("#foo").prop("selectedIndex");

This was great, but annoyingly this wasn't backward compatible, as:

<input type="checkbox" checked>

has no attribute of checked, but it does have a property called checked.

So, in the final build of 1.6, attr does also do prop so that things didn't break. Some people wanted this to be a clean break, but I think that the right decision was made as things didn't break all over the place!

Regarding:

Prop: The value in it's current state after any modifications via JavaScript

Attr: The value as it was defined in the html on page load.

This isn't always true, as many times the attribute is actually changed, but for properties such as checked, there isn't an attribute to change, so you need to use prop.

References:

http://blog.jquery.com/2011/05/03/jquery-16-released/

http://ejohn.org/blog/jquery-16-and-attr