javascript - Why does change() called only for receiving radio?

I have the following handler setup

form.find("input[name=url_source]").change(function() {
            updateUrlSource(jQuery(this))
            });

I was thinking it will be attached to all radio buttons with the name. But actually I see in debugger, that only one call occurs and only first condition satisfies below:

function updateUrlSource(source) {
   if( source.is(':checked') ) {
      // ...
   }
   else {
      // ...
   }
}

Is this true? Why?

Answer

But actually I see in debugger, that only one call occurs and only first condition satisfies below:

Correct, change is only fired on the input that received the check, not the others that (may have) lost it. Details buried somewhere in this section of the spec, but I believe this is the critical bit:

If the element is mutable, then: The pre-click activation steps consist of setting the element's checkedness to true. The canceled activation steps consist of setting the element's checkedness to false. The activation behavior is to fire a simple event that bubbles named change at the element.

(My emphasis, and many thanks to Barmar for the assist!)

Here's an example: Live Copy

  


  
(Look in the console for the output as you click...
  


   Group 1 one
   Group 1 two
   Group 1 three
  

   Group 2 one
   Group 2 two
   Group 2 three