From 03f2024b6ecf9bea7941a521779d18ca1d757a9f Mon Sep 17 00:00:00 2001
From: Linus Groh <mail@linusgroh.de>
Date: Sun, 19 Apr 2020 01:12:51 +0100
Subject: [PATCH] LibJS: Improve CallExpression::execute()'s error messages

---
 Libraries/LibJS/AST.cpp | 36 +++++++++++++++++++++++++++---------
 Libraries/LibJS/AST.h   |  2 ++
 2 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/Libraries/LibJS/AST.cpp b/Libraries/LibJS/AST.cpp
index 4fec6571283..f384defa264 100644
--- a/Libraries/LibJS/AST.cpp
+++ b/Libraries/LibJS/AST.cpp
@@ -92,17 +92,24 @@ Value CallExpression::execute(Interpreter& interpreter) const
 
     ASSERT(!callee.is_empty());
 
-    if (is_new_expression()) {
-        if (!callee.is_object()
-            || !callee.as_object().is_function()
-            || (callee.as_object().is_native_function()
-                && !static_cast<NativeFunction&>(callee.as_object()).has_constructor()))
-            return interpreter.throw_exception<TypeError>(String::format("%s is not a constructor", callee.to_string().characters()));
+    if (!callee.is_object()
+        || !callee.as_object().is_function()
+        || (callee.as_object().is_native_function() && !static_cast<NativeFunction&>(callee.as_object()).has_constructor())) {
+        String error_message;
+        auto call_type = is_new_expression() ? "constructor" : "function";
+        if (m_callee->is_identifier() || m_callee->is_member_expression()) {
+            String expression_string;
+            if (m_callee->is_identifier())
+                expression_string = static_cast<const Identifier&>(*m_callee).string();
+            else
+                expression_string = static_cast<const MemberExpression&>(*m_callee).to_string_approximation();
+            error_message = String::format("%s is not a %s (evaluated from '%s')", callee.to_string().characters(), call_type, expression_string.characters());
+        } else {
+            error_message = String::format("%s is not a %s", callee.to_string().characters(), call_type);
+        }
+        return interpreter.throw_exception<TypeError>(error_message);
     }
 
-    if (!callee.is_object() || !callee.as_object().is_function())
-        return interpreter.throw_exception<TypeError>(String::format("%s is not a function", callee.to_string().characters()));
-
     auto& function = static_cast<Function&>(callee.as_object());
 
     Vector<Value> arguments;
@@ -943,6 +950,17 @@ PropertyName MemberExpression::computed_property_name(Interpreter& interpreter)
     return PropertyName(index.to_string());
 }
 
+String MemberExpression::to_string_approximation() const
+{
+    String object_string = "<object>";
+    if (m_object->is_identifier())
+        object_string = static_cast<const Identifier&>(*m_object).string();
+    if (is_computed())
+        return String::format("%s[<computed>]", object_string.characters());
+    ASSERT(m_property->is_identifier());
+    return String::format("%s.%s", object_string.characters(), static_cast<const Identifier&>(*m_property).string().characters());
+}
+
 Value MemberExpression::execute(Interpreter& interpreter) const
 {
     auto object_value = m_object->execute(interpreter);
diff --git a/Libraries/LibJS/AST.h b/Libraries/LibJS/AST.h
index 09835df3ec0..58c1bbe8ca4 100644
--- a/Libraries/LibJS/AST.h
+++ b/Libraries/LibJS/AST.h
@@ -716,6 +716,8 @@ public:
 
     PropertyName computed_property_name(Interpreter&) const;
 
+    String to_string_approximation() const;
+
 private:
     virtual bool is_member_expression() const override { return true; }
     virtual const char* class_name() const override { return "MemberExpression"; }