[jquery] Add self-extending overloads of jQuery.extend(). (#29924)

types/jquery/JQueryStatic.d.ts

@@ -940,6 +940,14 @@ $( "#log" ).append( JSON.stringify( object1 ) );
 ```
      */
     extend<T, U>(deep: true, target: T, object1: U): T & U;
+    /**
+     * Merge the contents of two or more objects together into the first object.
+     * @param deep If true, the merge becomes recursive (aka. deep copy). Passing false for this argument is not supported.
+     * @param target The object to extend. It will receive the new properties.
+     * @see \`{@link https://api.jquery.com/jQuery.extend/ }\`
+     * @since 1.1.4
+     */
+    extend<T>(deep: true, target: T): this & T;
     /**
      * Merge the contents of two or more objects together into the first object.
      * @param deep If true, the merge becomes recursive (aka. deep copy). Passing false for this argument is not supported.
@@ -1431,6 +1439,14 @@ $( "#log" ).append( "<div><b>settings -- </b>" + JSON.stringify( settings ) + "<
 ```
      */
     extend<T, U>(target: T, object1: U): T & U;
+    /**
+     * Merge the contents of two or more objects together into the first object.
+     * @param target An object that will receive the new properties if additional objects are passed in or that will
+     *               extend the jQuery namespace if it is the sole argument.
+     * @see \`{@link https://api.jquery.com/jQuery.extend/ }\`
+     * @since 1.0
+     */
+    extend<T>(target: T): this & T;
     /**
      * Merge the contents of two or more objects together into the first object.
      * @param target An object that will receive the new properties if additional objects are passed in or that will

types/jquery/jquery-tests.ts

@@ -463,6 +463,9 @@ function JQueryStatic() {
         // $ExpectType { name: string; } & EventTarget
         $.extend(true, t, u);
 
+        // $ExpectType JQueryStatic & { name: string; }
+        $.extend(true, t);
+
         // $ExpectType any
         $.extend(true, t, u, v, w, x, y, z, a);
 
@@ -484,6 +487,9 @@ function JQueryStatic() {
         // $ExpectType { name: string; } & EventTarget
         $.extend(t, u);
 
+        // $ExpectType JQueryStatic & { name: string; }
+        $.extend(t);
+
         // $ExpectType any
         $.extend(t, u, v, w, x, y, z, a);
     }