LibJS: Disallow duplicated variable declarations

Author: https://github.com/davidot
Commit: 40b8689f9b
Pull-request: https://github.com/SerenityOS/serenity/pull/8667
Reviewed-by: https://github.com/IdanHo
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibJS/Tests/duplicated-variable-declarations.js

@@ -0,0 +1,30 @@
+describe("duplicated variable declarations should throw", () => {
+    test("given two declarations in the same statement", () => {
+        expect("let a, a;").not.toEval();
+        expect("const a, a;").not.toEval();
+
+        expect("var a, a;").toEval();
+    });
+
+    test("fail to parse if repeated over multiple statements", () => {
+        expect("let a; var a;").not.toEval();
+        expect("let b, a; var c, a;").not.toEval();
+        expect("const a; var a;").not.toEval();
+        expect("const b, a; var c, a;").not.toEval();
+    });
+
+    test.skip("should fail to parse even if variable first declared with var", () => {
+        expect.skip("var a; let a;").toEval();
+        expect.skip("var a; let b, a;").toEval();
+        expect.skip("var a; const a;").toEval();
+        expect.skip("var a; const b, a;").toEval();
+    });
+
+    test.skip("special cases with for loops", () => {
+        expect("for (var a;;) { let a; }").toEval();
+        expect("for (let a;;) { let a; }").toEval();
+        expect("for (var a;;) { var a; }").toEval();
+
+        expect("for (let a;;) { var a; }").not.toEval();
+    });
+});