LibJS: Add the Number.prototype.toFixed method

Userland/Libraries/LibJS/Tests/builtins/Number/Number.prototype.toFixed.js

@@ -0,0 +1,44 @@
+describe("correct behavior", () => {
+    test("length", () => {
+        expect(Number.prototype.toFixed).toHaveLength(1);
+    });
+
+    test("basic functionality", () => {
+        [
+            [0, 5, "0.00000"],
+            [Infinity, 6, "Infinity"],
+            [-Infinity, 7, "-Infinity"],
+            [NaN, 8, "NaN"],
+            [12.81646112, 3, "12.816"],
+            [84.23, 4, "84.2300"],
+            [3.00003, 5, "3.00003"],
+            // Numbers >= 1e+21
+            [1e21, 5, "1e+21"],
+            [1e22, 0, "1e+22"],
+        ].forEach(testCase => {
+            expect(testCase[0].toFixed(testCase[1])).toBe(testCase[2]);
+        });
+    });
+
+    test("decimal fixed digits gets converted to int", () => {
+        expect((30.521).toFixed(1.9)).toBe("30.5");
+        expect((30.521).toFixed(2.2)).toBe("30.52");
+    });
+});
+
+test("errors", () => {
+    test("must be called with numeric |this|", () => {
+        [true, [], {}, Symbol("foo"), "bar", 1n].forEach(value => {
+            expect(() => Number.prototype.toFixed.call(value)).toThrowWithMessage(
+                TypeError,
+                "Not a Number object"
+            );
+        });
+    });
+
+    test("fixed digits RangeError", () => {
+        [-Infinity, -5, 105, Infinity, NaN].forEach(value => {
+            expect(() => (0).toFixed(value)).toThrow(RangeError);
+        });
+    });
+});