LibJS: Implement a naive String.prototype.localeCompare

Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.localeCompare.js

@@ -0,0 +1,39 @@
+test("basic functionality", () => {
+    expect(String.prototype.localeCompare).toHaveLength(1);
+
+    expect("".localeCompare("")).toBe(0);
+    expect("a".localeCompare("a")).toBe(0);
+    expect("6".localeCompare("6")).toBe(0);
+
+    function compareBoth(a, b) {
+        const aTob = a.localeCompare(b);
+        const bToa = b.localeCompare(a);
+
+        expect(aTob).toBe(1);
+        expect(aTob).toBe(-bToa);
+    }
+
+    compareBoth("a", "");
+    compareBoth("1", "");
+    compareBoth("a", "A");
+    compareBoth("7", "3");
+    compareBoth("0000", "0");
+
+    expect("undefined".localeCompare()).toBe(0);
+    expect("undefined".localeCompare(undefined)).toBe(0);
+
+    expect("null".localeCompare(null)).toBe(0);
+    expect("null".localeCompare(undefined)).not.toBe(0);
+    expect("null".localeCompare()).toBe(-1);
+
+    expect(() => {
+        String.prototype.localeCompare.call(undefined, undefined);
+    }).toThrowWithMessage(TypeError, "undefined cannot be converted to an object");
+});
+
+test("UTF-16", () => {
+    var s = "😀😀";
+    expect(s.localeCompare("😀😀")).toBe(0);
+    expect(s.localeCompare("\ud83d")).toBe(1);
+    expect(s.localeCompare("😀😀s")).toBe(-1);
+});