LibJS: Add the Set built-in object

Userland/Libraries/LibJS/Tests/builtins/Set/Set.js

@@ -0,0 +1,31 @@
+test("constructor properties", () => {
+    expect(Set).toHaveLength(0);
+    expect(Set.name).toBe("Set");
+});
+
+describe("errors", () => {
+    test("invalid array iterators", () => {
+        [-100, Infinity, NaN, {}, 152n].forEach(value => {
+            expect(() => {
+                new Set(value);
+            }).toThrowWithMessage(TypeError, "is not iterable");
+        });
+    });
+    test("called without new", () => {
+        expect(() => {
+            Set();
+        }).toThrowWithMessage(TypeError, "Set constructor must be called with 'new'");
+    });
+});
+
+describe("normal behavior", () => {
+    test("typeof", () => {
+        expect(typeof new Set()).toBe("object");
+    });
+
+    test("constructor with single array argument", () => {
+        var a = new Set([0, 1, 2]);
+        expect(a instanceof Set).toBeTrue();
+        expect(a).toHaveSize(3);
+    });
+});

Userland/Libraries/LibJS/Tests/test-common.js

@@ -97,6 +97,17 @@ class ExpectationError extends Error {
             });
         }
 
+        toHaveSize(size) {
+            this.__expect(
+                typeof this.target.size === "number",
+                () => "toHaveSize: target.size not of type number"
+            );
+
+            this.__doMatcher(() => {
+                this.__expect(Object.is(this.target.size, size));
+            });
+        }
+
         toHaveProperty(property, value) {
             this.__doMatcher(() => {
                 let object = this.target;