LibJS: Allow invalid string in tagged template literals

Since tagged template literals can inspect the raw string it is not a
syntax error to have invalid escapes. However the cooked value should be
`undefined`.
We accomplish this by tracking whether parse_string_literal
fails and then using a NullLiteral (since UndefinedLiteral is not a
thing) and finally converting null in tagged template execution to
undefined.

1 files changed, 5 insertions, 0 deletions

diff --git a/Userland/Libraries/LibJS/AST.cpp b/Userland/Libraries/LibJS/AST.cpp
index c6f1fdd106..94e272af9e 100644
--- a/Userland/Libraries/LibJS/AST.cpp
+++ b/Userland/Libraries/LibJS/AST.cpp
@@ -3545,6 +3545,11 @@ Completion TaggedTemplateLiteral::execute(Interpreter& interpreter, GlobalObject
         // tag`${foo}`             -> "", foo, ""                -> tag(["", ""], foo)
         // tag`foo${bar}baz${qux}` -> "foo", bar, "baz", qux, "" -> tag(["foo", "baz", ""], bar, qux)
         if (i % 2 == 0) {
+            // If the string contains invalid escapes we get a null expression here, which we then convert
+            // to the expected `undefined` TV.
+            if (value.is_nullish())
+                value = js_undefined();
+
             strings->indexed_properties().append(value);
         } else {
             arguments.append(value);