diff --git a/Userland/Libraries/LibJS/AST.cpp b/Userland/Libraries/LibJS/AST.cpp
index 737a248bf4..f3aff618b8 100644
--- a/Userland/Libraries/LibJS/AST.cpp
+++ b/Userland/Libraries/LibJS/AST.cpp
@@ -1313,7 +1313,10 @@ Value Identifier::execute(Interpreter& interpreter, GlobalObject& global_object)
 void Identifier::dump(int indent) const
 {
     print_indent(indent);
-    outln("Identifier \"{}\"", m_string);
+    if (m_argument_index.has_value())
+        outln("Identifier \"{}\" (argument #{})", m_string, m_argument_index.value());
+    else
+        outln("Identifier \"{}\"", m_string);
 }
 
 void SpreadExpression::dump(int indent) const
diff --git a/Userland/Libraries/LibJS/AST.h b/Userland/Libraries/LibJS/AST.h
index 62ae3d13de..d36b639251 100644
--- a/Userland/Libraries/LibJS/AST.h
+++ b/Userland/Libraries/LibJS/AST.h
@@ -768,13 +768,15 @@ private:
 
 class Identifier final : public Expression {
 public:
-    explicit Identifier(SourceRange source_range, FlyString const& string)
-        : Expression(move(source_range))
-        , m_string(string)
+    explicit Identifier(SourceRange source_range, FlyString string, Optional<size_t> argument_index = {})
+        : Expression(source_range)
+        , m_string(move(string))
+        , m_argument_index(move(argument_index))
     {
     }
 
     FlyString const& string() const { return m_string; }
+    Optional<size_t> const& argument_index() const { return m_argument_index; }
 
     virtual Value execute(Interpreter&, GlobalObject&) const override;
     virtual void dump(int indent) const override;
@@ -785,6 +787,7 @@ private:
     virtual bool is_identifier() const override { return true; }
 
     FlyString m_string;
+    Optional<size_t> m_argument_index;
 };
 
 class ClassMethod final : public ASTNode {
diff --git a/Userland/Libraries/LibJS/Parser.cpp b/Userland/Libraries/LibJS/Parser.cpp
index 95ab39c29f..4f9d0de2b6 100644
--- a/Userland/Libraries/LibJS/Parser.cpp
+++ b/Userland/Libraries/LibJS/Parser.cpp
@@ -382,6 +382,8 @@ RefPtr<FunctionExpression> Parser::try_parse_arrow_function_expression(bool expe
     if (function_length == -1)
         function_length = parameters.size();
 
+    m_parser_state.function_parameters.append(parameters);
+
     auto old_labels_in_scope = move(m_parser_state.m_labels_in_scope);
     ScopeGuard guard([&]() {
         m_parser_state.m_labels_in_scope = move(old_labels_in_scope);
@@ -411,6 +413,8 @@ RefPtr<FunctionExpression> Parser::try_parse_arrow_function_expression(bool expe
         return nullptr;
     }();
 
+    m_parser_state.function_parameters.take_last();
+
     if (!function_body_result.is_null()) {
         state_rollback_guard.disarm();
         discard_saved_state();
@@ -651,7 +655,23 @@ NonnullRefPtr<Expression> Parser::parse_primary_expression()
 
             set_try_parse_arrow_function_expression_failed_at_position(position(), true);
         }
-        return create_ast_node<Identifier>({ m_parser_state.m_current_token.filename(), rule_start.position(), position() }, consume().value());
+        auto string = consume().value();
+        Optional<size_t> argument_index;
+        if (!m_parser_state.function_parameters.is_empty()) {
+            size_t i = 0;
+            for (auto& parameter : m_parser_state.function_parameters.last()) {
+                parameter.binding.visit(
+                    [&](FlyString const& name) {
+                        if (name == string) {
+                            argument_index = i;
+                        }
+                    },
+                    [&](BindingPattern const&) {
+                    });
+                ++i;
+            }
+        }
+        return create_ast_node<Identifier>({ m_parser_state.m_current_token.filename(), rule_start.position(), position() }, string, argument_index);
     }
     case TokenType::NumericLiteral:
         return create_ast_node<NumericLiteral>({ m_parser_state.m_current_token.filename(), rule_start.position(), position() }, consume_and_validate_numeric_literal().double_value());
@@ -1386,8 +1406,13 @@ NonnullRefPtr<FunctionNodeType> Parser::parse_function_node(u8 parse_options)
         m_parser_state.m_labels_in_scope = move(old_labels_in_scope);
     });
 
+    m_parser_state.function_parameters.append(parameters);
+
     bool is_strict = false;
     auto body = parse_block_statement(is_strict);
+
+    m_parser_state.function_parameters.take_last();
+
     body->add_variables(m_parser_state.m_var_scopes.last());
     body->add_functions(m_parser_state.m_function_scopes.last());
     return create_ast_node<FunctionNodeType>(
diff --git a/Userland/Libraries/LibJS/Parser.h b/Userland/Libraries/LibJS/Parser.h
index 83a6a01e5e..49e2d9904f 100644
--- a/Userland/Libraries/LibJS/Parser.h
+++ b/Userland/Libraries/LibJS/Parser.h
@@ -196,6 +196,9 @@ private:
         Vector<NonnullRefPtrVector<VariableDeclaration>> m_var_scopes;
         Vector<NonnullRefPtrVector<VariableDeclaration>> m_let_scopes;
         Vector<NonnullRefPtrVector<FunctionDeclaration>> m_function_scopes;
+
+        Vector<Vector<FunctionNode::Parameter>&> function_parameters;
+
         HashTable<StringView> m_labels_in_scope;
         bool m_strict_mode { false };
         bool m_allow_super_property_lookup { false };