LibJS: Throw on assignment of an const variable

Was stubbed out as an assert, should be handled with a runtime exception.

2 files changed, 29 insertions, 2 deletions

diff --git a/Libraries/LibJS/Interpreter.cpp b/Libraries/LibJS/Interpreter.cpp
index c3b74cff9c..756855b499 100644
--- a/Libraries/LibJS/Interpreter.cpp
+++ b/Libraries/LibJS/Interpreter.cpp
@@ -145,8 +145,10 @@ void Interpreter::set_variable(const FlyString& name, Value value, bool first_as
 
         auto possible_match = scope.variables.get(name);
         if (possible_match.has_value()) {
-            if (!first_assignment && possible_match.value().declaration_kind == DeclarationKind::Const)
-                ASSERT_NOT_REACHED();
+            if (!first_assignment && possible_match.value().declaration_kind == DeclarationKind::Const) {
+                throw_exception<TypeError>("Assignment to constant variable");
+                return;
+            }
 
             scope.variables.set(move(name), { move(value), possible_match.value().declaration_kind });
             return;
diff --git a/Libraries/LibJS/Tests/variable-declaration.js b/Libraries/LibJS/Tests/variable-declaration.js
new file mode 100644
index 0000000000..ef634867c9
--- /dev/null
+++ b/Libraries/LibJS/Tests/variable-declaration.js
@@ -0,0 +1,25 @@
+try {
+
+    const ConstantValue = 1;
+    try {
+        ConstantValue = 2;
+    } catch (e) { 
+        assert(e.name === "TypeError");
+        assert(e.message === "Assignment to constant variable");
+        assert(ConstantValue === 1);
+    }
+
+    // Make sure we can define new constants in inner scopes.
+    //
+    const ConstantValue2 = 1;
+
+    do 
+    {
+        const ConstantValue2 = 2;
+    }
+    while (false)
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}