LibJS: Add a traceback to Error

Since we have the traceback we now also generate the stack string on
demand instead of immediately on construction.

Userland/Libraries/LibJS/Runtime/Error.cpp

@@ -54,29 +54,46 @@ ThrowCompletionOr<void> Error::install_error_cause(Value options)
 
 void Error::populate_stack()
 {
-    AK::StringBuilder stack_string_builder {};
+    auto& vm = this->vm();
+    m_traceback.ensure_capacity(vm.execution_context_stack().size());
+    for (ssize_t i = vm.execution_context_stack().size() - 1; i >= 0; i--) {
+        auto* context = vm.execution_context_stack()[i];
+        auto function_name = context->function_name;
+        if (function_name.is_empty())
+            function_name = "<unknown>"sv;
+        m_traceback.empend(
+            move(function_name),
+            // We might not have an AST node associated with the execution context, e.g. in promise
+            // reaction jobs (which aren't called anywhere from the source code).
+            // They're not going to generate any _unhandled_ exceptions though, so a meaningless
+            // source range is fine.
+            context->current_node ? context->current_node->source_range() : SourceRange {});
+    }
+}
 
+String Error::stack_string() const
+{
+    StringBuilder stack_string_builder;
     // Note: We roughly follow V8's formatting
     // Note: The error's name and message get prepended by ErrorPrototype::stack
     // Note: We don't want to capture the global exectution context, so we omit the last frame
     // FIXME: We generate a stack-frame for the Errors constructor, other engines do not
-    for (size_t i = vm().execution_context_stack().size() - 1; i > 0; --i) {
-        auto const* frame = vm().execution_context_stack()[i];
+    for (size_t i = 0; i < m_traceback.size() - 1; ++i) {
+        auto const& frame = m_traceback[i];
+        auto function_name = frame.function_name;
+        // Note: Since we don't know whether we have a valid SourceRange here we just check for some default values.
+        if (!frame.source_range.filename.is_null() || frame.source_range.start.offset != 0 || frame.source_range.end.offset != 0) {
 
-        auto function_name = frame->function_name;
-        if (auto const* current_node = frame->current_node) {
-            auto const& source_range = current_node->source_range();
-
-            if (function_name.is_empty())
-                stack_string_builder.appendff("    at {}:{}:{}\n", source_range.filename, source_range.start.line, source_range.start.column);
+            if (function_name == "<unknown>"sv)
+                stack_string_builder.appendff("    at {}:{}:{}\n", frame.source_range.filename, frame.source_range.start.line, frame.source_range.start.column);
             else
-                stack_string_builder.appendff("    at {} ({}:{}:{})\n", function_name, source_range.filename, source_range.start.line, source_range.start.column);
+                stack_string_builder.appendff("    at {} ({}:{}:{})\n", function_name, frame.source_range.filename, frame.source_range.start.line, frame.source_range.start.column);
         } else {
             stack_string_builder.appendff("    at {}\n", function_name.is_empty() ? "<unknown>"sv : function_name.view());
         }
     }
 
-    m_stack_string = stack_string_builder.build();
+    return stack_string_builder.build();
 }
 
 #define __JS_ENUMERATE(ClassName, snake_name, PrototypeName, ConstructorName, ArrayType)                         \

Userland/Libraries/LibJS/Runtime/Error.h

@@ -10,9 +10,15 @@
 #include <AK/FlyString.h>
 #include <LibJS/Runtime/Completion.h>
 #include <LibJS/Runtime/Object.h>
+#include <LibJS/SourceRange.h>
 
 namespace JS {
 
+struct TracebackFrame {
+    FlyString function_name;
+    SourceRange source_range;
+};
+
 class Error : public Object {
     JS_OBJECT(Error, Object);
 
@@ -23,13 +29,15 @@ public:
     explicit Error(Object& prototype);
     virtual ~Error() override = default;
 
-    String const& stack_string() const { return m_stack_string; }
+    [[nodiscard]] String stack_string() const;
 
     ThrowCompletionOr<void> install_error_cause(Value options);
 
+    Vector<TracebackFrame, 32> const& traceback() const { return m_traceback; }
+
 private:
     void populate_stack();
-    String m_stack_string {};
+    Vector<TracebackFrame, 32> m_traceback;
 };
 
 // NOTE: Making these inherit from Error is not required by the spec but

Userland/Libraries/LibJS/Runtime/Exception.h

@@ -10,16 +10,12 @@
 #include <AK/FlyString.h>
 #include <AK/Vector.h>
 #include <LibJS/Heap/Cell.h>
+#include <LibJS/Runtime/Error.h>
 #include <LibJS/Runtime/Value.h>
 #include <LibJS/SourceRange.h>
 
 namespace JS {
 
-struct TracebackFrame {
-    FlyString function_name;
-    SourceRange source_range;
-};
-
 class Exception : public Cell {
 public:
     explicit Exception(Value);