LibJS: Disallow '\8' and '\9' in strict mode due to being octal escapes

Author: https://github.com/davidot
Commit: https://github.com/SerenityOS/serenity/commit/7624c3de0e2
Pull-request: https://github.com/SerenityOS/serenity/pull/11088
Issue: https://github.com/SerenityOS/serenity/issues/11078
Reviewed-by: https://github.com/IdanHo
Reviewed-by: https://github.com/alimpfard
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibJS/Tests/string-escapes.js

@@ -62,4 +62,11 @@ describe("octal escapes", () => {
         // Because of the non string statement in the middle strict mode is not enabled.
         expect("'\\123'; somethingElse; 'use strict'").toEval();
     });
+
+    test("invalid octal escapes fail in strict mode", () => {
+        expect("'use strict'; '\\8'").not.toEval();
+        expect("'use strict'; '\\800'").not.toEval();
+        expect("'use strict'; '\\9'").not.toEval();
+        expect("'use strict'; '\\912'").not.toEval();
+    });
 });

Userland/Libraries/LibJS/Token.cpp

@@ -198,6 +198,12 @@ String Token::string_value(StringValueStatus& status) const
             continue;
         }
 
+        if (lexer.next_is('8') || lexer.next_is('9')) {
+            status = StringValueStatus::LegacyOctalEscapeSequence;
+            builder.append(lexer.consume());
+            continue;
+        }
+
         lexer.retreat();
         builder.append(lexer.consume_escaped_character('\\', "b\bf\fn\nr\rt\tv\v"));
     }