google/closure-compiler — pull request #631 by phistuck (2014-09-22 07:32:42+00:00)
Title: Added a typedef for the jQuery AJAX setting object

The jQuery AJAX setting dictionary is a defined dictionary with known fields. When the various methods accept such a dictionary, they expect very specific properties (the developer can indeed add more properties, of course). Adding a typedef for this type of dictionary reduces the need to quote properties.
Currently, $.ajax({url: "/", beforeSend: function (){} ,cache: true,username: "d",xhrFields: {}}) compiles to $.ajax({url:"/",a:function(){},b:!0,c:"d",d:{}}), causing errors.

With the use of the typedef in the various functions, $.ajax({url: "/", type: "post"}) and friends will be minified without renaming the relevant properties, causing the code not to fail anymore.