Hm. Linter didn't catch this for some reason, but there is no need for nested ifs here, just use a single one:

      if (S.token.value == "property" &&
          (S.token.type == "name" || S.token.value == "var") &&
          !is_token(peek(), "punc", ":")) {
        next();
        return qmlpropdef();
      }

Upd: ah, we don't have linter enabled in this repo.

Btw, S.token.value == "property" && (S.token.type == "name" || S.token.value == "var") could be reduced to S.token.value == "property" && S.token.type == "name", perhaps this means that there is some other bug somewhere.

Good point, token.value should never be property and var at the same time.