Recognize properties introduced by jQuery extend

[amcasey]

The old VS JS language service used to make runtime types available at design-time, allowing it to support jQuery's extend function for adding properties dynamically. The new one uses static types and so does not offer these symbols during completion, navigate-to, etc. We've had some customer inquiries about the possibility of supporting this functionality in the TS language service.

https://developercommunity.visualstudio.com/content/problem/529989/aspnet-在html-文件-js-智能提示失效.html
https://developercommunity.visualstudio.com/content/problem/560277/javascript-go-to-members-doesnt-work-with-property.html

The latter provided this example:

(function ($) {

    mynamespace.myClass = Class.extend({
      init: function (options) {
        // unless you specifically type this.testProperty = "abc" in JS, the 'Go to Members' will not find the below property even though it's explicitely defined.
      },
       __properties: {

       'testProperty': {
                /// <summary>
                /// Gets or sets the test property
                /// </summary>
                /// <value type="String"></value>

                get: function () {
                    return this._testProperty;
                },

                set: function (value) {
                    this._testProperty = value;
                }
            }
       }
   });

})(jQuery);

[sandersn]

Notably, the example is based on John Resig's post from 2008, not on jquery.extend: https://johnresig.com/blog/simple-javascript-inheritance/

Jquery.extend is basically Object.assign, but is sometimes used for inheritance. Here's an example from a decade-old Stack Overflow question.

var A=function(){
};

$.extend(A.prototype, {
    init:function(){
        alert('A init');
    }
});
var B=function(){

};

$.extend(B.prototype,A.prototype,{
    init:function(){
        alert('B init');
    }
});
var p=new A();
p.init();
var x=new B();
x.init();

This is almost the same as the A.prototype = { init: ... } pattern and would be relatively easy to support. It just doesn't work right now because it $.extend works by side-effect.