LibJS: Fix that constant declaration in for loop was mutable in body

3 files changed, 34 insertions, 4 deletions

diff --git a/Userland/Libraries/LibJS/AST.cpp b/Userland/Libraries/LibJS/AST.cpp
index 313a5d6071..51d37625be 100644
--- a/Userland/Libraries/LibJS/AST.cpp
+++ b/Userland/Libraries/LibJS/AST.cpp
@@ -921,12 +921,22 @@ struct ForInOfHeadState {
         else {
             VERIFY(expression_lhs && is<VariableDeclaration>(*expression_lhs));
             iteration_environment = new_declarative_environment(*interpreter.lexical_environment());
+
             auto& for_declaration = static_cast<VariableDeclaration const&>(*expression_lhs);
+
+            // 14.7.5.4 Runtime Semantics: ForDeclarationBindingInstantiation, https://tc39.es/ecma262/#sec-runtime-semantics-fordeclarationbindinginstantiation
+            // 1. For each element name of the BoundNames of ForBinding, do
             for_declaration.for_each_bound_name([&](auto const& name) {
-                if (for_declaration.declaration_kind() == DeclarationKind::Const)
-                    MUST(iteration_environment->create_immutable_binding(vm, name, false));
-                else
-                    MUST(iteration_environment->create_mutable_binding(vm, name, true));
+                // a. If IsConstantDeclaration of LetOrConst is true, then
+                if (for_declaration.is_constant_declaration()) {
+                    // i. Perform ! environment.CreateImmutableBinding(name, true).
+                    MUST(iteration_environment->create_immutable_binding(vm, name, true));
+                }
+                // b. Else,
+                else {
+                    // i. Perform ! environment.CreateMutableBinding(name, false).
+                    MUST(iteration_environment->create_mutable_binding(vm, name, false));
+                }
             });
             interpreter.vm().running_execution_context().lexical_environment = iteration_environment;
 
diff --git a/Userland/Libraries/LibJS/Tests/loops/for-in-basic.js b/Userland/Libraries/LibJS/Tests/loops/for-in-basic.js
index f7b018ca38..f25c43725d 100644
--- a/Userland/Libraries/LibJS/Tests/loops/for-in-basic.js
+++ b/Userland/Libraries/LibJS/Tests/loops/for-in-basic.js
@@ -98,6 +98,16 @@ describe("special left hand sides", () => {
             eval("for (f() in [0]) { expect().fail() }");
         }).toThrowWithMessage(ReferenceError, "Invalid left-hand side in assignment");
     });
+
+    test("Cannot change constant declaration in body", () => {
+        const vals = [];
+        for (const v in [1, 2]) {
+            expect(() => v++).toThrowWithMessage(TypeError, "Invalid assignment to const variable");
+            vals.push(v);
+        }
+
+        expect(vals).toEqual(["0", "1"]);
+    });
 });
 
 test("remove properties while iterating", () => {
diff --git a/Userland/Libraries/LibJS/Tests/loops/for-of-basic.js b/Userland/Libraries/LibJS/Tests/loops/for-of-basic.js
index 29fcb5d2b9..b5753d76dc 100644
--- a/Userland/Libraries/LibJS/Tests/loops/for-of-basic.js
+++ b/Userland/Libraries/LibJS/Tests/loops/for-of-basic.js
@@ -142,4 +142,14 @@ describe("special left hand sides", () => {
             eval("for (f() of [0]) { expect().fail() }");
         }).toThrowWithMessage(ReferenceError, "Invalid left-hand side in assignment");
     });
+
+    test("Cannot change constant declaration in body", () => {
+        const vals = [];
+        for (const v of [1, 2]) {
+            expect(() => v++).toThrowWithMessage(TypeError, "Invalid assignment to const variable");
+            vals.push(v);
+        }
+
+        expect(vals).toEqual([1, 2]);
+    });
 });