QBS JavaScript lastIndexOf always returns -1

import qbs
 
Product {
    type: ["dummy"]
    Rule {
        multiplex: true
        alwaysRun: true
        Artifact {
            filePath: "dummy.txt"
            fileTags: ["dummy"]
        }
        prepare: {
            var cmd = new JavaScriptCommand();
            cmd.silent = false;
            cmd.description = 'Testing JavaScript';
            cmd.sourceCode = function() {
                var str1 = "C:/dir1/dir2/dir3";
                var index1 = str1.indexOf('/');
                var index2 = str1.lastIndexOf("/");
                console.error('indexes: ' + index1 + ' ' + index2);
 
                var str2 = "bbbbaaaaa";
                console.error('indexes: ' + str2.indexOf("a") + ' ' + str2.lastIndexOf("a"));
 
                var str3 = "112211221";
                console.error('indexes: ' + str3.indexOf("2") + ' ' + str3.lastIndexOf("2"));
            };
            return [cmd];
        }
    }
}