feat: type declarations for new Set methods (#23090)

Closes #22851

cli/tsc/dts/lib.esnext.collection.d.ts

@@ -27,3 +27,34 @@ interface MapConstructor {
         keySelector: (item: T, index: number) => K,
     ): Map<K, T[]>;
 }
+
+interface Set<T> {
+    /**
+     * @returns a new Set containing all the elements in this Set and also all the elements in the argument.
+     */
+    union<U>(other: Set<U>): Set<T | U>;
+    /**
+     * @returns a new Set containing all the elements which are both in this Set and in the argument.
+     */
+    intersection<U>(other: Set<U>): Set<T & U>;
+    /**
+     * @returns a new Set containing all the elements in this Set which are not also in the argument.
+     */
+    difference<U>(other: Set<U>): Set<T>;
+    /**
+     * @returns a new Set containing all the elements which are in either this Set or in the argument, but not in both.
+     */
+    symmetricDifference<U>(other: Set<U>): Set<T | U>;
+    /**
+     * @returns a boolean indicating whether all the elements in this Set are also in the argument.
+     */
+    isSubsetOf(other: Set<unknown>): boolean;
+    /**
+     * @returns a boolean indicating whether all the elements in the argument are also in this Set.
+     */
+    isSupersetOf(other: Set<unknown>): boolean;
+    /**
+     * @returns a boolean indicating whether this Set has no elements in common with the argument.
+     */
+    isDisjointFrom(other: Set<unknown>): boolean;
+}

tests/unit/esnext_test.ts

@@ -41,3 +41,10 @@ Deno.test(function intlListFormat() {
     { type: "element", value: "golang" },
   ]);
 });
+
+Deno.test(function setUnion() {
+  const a = new Set([1, 2, 3]);
+  const b = new Set([3, 4, 5]);
+  const union = a.union(b);
+  assertEquals(union, new Set([1, 2, 3, 4, 5]));
+});