From cae39fb62f9519ce5cea0ee484393ef8df48dd43 Mon Sep 17 00:00:00 2001
From: Ramb Memburg <46289413+memburg@users.noreply.github.com>
Date: Thu, 11 Jun 2026 13:43:39 -0400
Subject: [PATCH] reject unsupported let/const declarations explicitly

---
 reference/Language.md          |  2 ++
 reference/REFERENCE.md         |  2 ++
 spec/giavascript_spec.cr       | 11 ++++++++---
 src/giavascript/interpreter.cr | 22 ++++++++++++++++++++++
 4 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/reference/Language.md b/reference/Language.md
index cb9ec44..5188533 100644
--- a/reference/Language.md
+++ b/reference/Language.md
@@ -83,3 +83,5 @@ Status of core JavaScript language features in GiavaScript.
 ## Notes
 
 - This reflects current behavior in the interpreter and specs.
+- `let` and `const` declarations return explicit errors: `Error: unsupported declaration 'let'` and `Error: unsupported declaration 'const'`.
+- Use `var` for variable declarations.
diff --git a/reference/REFERENCE.md b/reference/REFERENCE.md
index 0abfa73..4f5dbee 100644
--- a/reference/REFERENCE.md
+++ b/reference/REFERENCE.md
@@ -107,6 +107,8 @@ Status of core JavaScript language features in GiavaScript.
 ### Notes
 
 - This reflects current behavior in the interpreter and specs.
+- `let` and `const` declarations return explicit errors: `Error: unsupported declaration 'let'` and `Error: unsupported declaration 'const'`.
+- Use `var` for variable declarations.
 
 ## Type Methods and Properties
 
diff --git a/spec/giavascript_spec.cr b/spec/giavascript_spec.cr
index b041ecc..eeb5715 100644
--- a/spec/giavascript_spec.cr
+++ b/spec/giavascript_spec.cr
@@ -1562,11 +1562,16 @@ describe GiavaScript do
     interpreter.eval("continue;").should eq(["Error: continue can only be used inside loops"])
   end
 
-  it "rejects let and const declarations" do
+  it "rejects let declarations with an explicit unsupported-declaration error" do
     interpreter = GiavaScript::Interpreter.new
 
-    interpreter.eval("let value = 1;").should eq(["Error: invalid assignment target 'let value'"])
-    interpreter.eval("const total = 2;").should eq(["Error: invalid assignment target 'const total'"])
+    interpreter.eval("let value = 1;").should eq(["Error: unsupported declaration 'let'"])
+  end
+
+  it "rejects const declarations with an explicit unsupported-declaration error" do
+    interpreter = GiavaScript::Interpreter.new
+
+    interpreter.eval("const total = 2;").should eq(["Error: unsupported declaration 'const'"])
   end
 
   it "rejects class declarations" do
diff --git a/src/giavascript/interpreter.cr b/src/giavascript/interpreter.cr
index 73ef6c9..a95e84d 100644
--- a/src/giavascript/interpreter.cr
+++ b/src/giavascript/interpreter.cr
@@ -85,6 +85,13 @@ module GiavaScript
       end
     end
 
+    class UnsupportedDeclarationRawStatement < CompiledRawStatement
+      getter keyword : String
+
+      def initialize(@keyword : String)
+      end
+    end
+
     def initialize(@console_output : IO = STDOUT)
       @env = build_global_env
       @function_runtime = FunctionRuntime.new
@@ -185,6 +192,10 @@ module GiavaScript
         return "Error: return can only be used inside functions"
       end
 
+      if keyword = unsupported_declaration_keyword(stmt)
+        return "Error: unsupported declaration '#{keyword}'"
+      end
+
       if match = stmt.match(/^(.+?)\s*(\+\+|--)$/)
         target_source = match[1].strip
         operator = match[2]
@@ -811,6 +822,8 @@ module GiavaScript
         rescue ex : ExpressionError
           ex.message || "Error: invalid right-hand side '#{compiled.source}'"
         end
+      when UnsupportedDeclarationRawStatement
+        "Error: unsupported declaration '#{compiled.keyword}'"
       end
     end
 
@@ -843,6 +856,8 @@ module GiavaScript
                    rescue
                      FallbackRawStatement.new(source)
                    end
+                 elsif keyword = unsupported_declaration_keyword(key)
+                   UnsupportedDeclarationRawStatement.new(keyword)
                  elsif assignment = split_assignment_statement(key)
                    begin
                      target = parse_assignment_target(assignment[:lhs])
@@ -948,6 +963,13 @@ module GiavaScript
       next_char.nil? || !(next_char.ascii_letter? || next_char.ascii_number? || next_char == '_')
     end
 
+    private def unsupported_declaration_keyword(source : String) : String?
+      return "let" if starts_with_keyword?(source, "let")
+      return "const" if starts_with_keyword?(source, "const")
+
+      nil
+    end
+
     private def call_function(name : String, args : Array(Value), env : Environment) : Value
       @function_runtime.invoke_function(name, args, env) do |stmt, local_env, inside_function, inside_loop|
         eval_statement(stmt, local_env, inside_function, inside_loop)