[Manyasha]

SpaceAlarm,
может так?

var obj = [{
      mass: "test1"
    }, {
      mass: "test2"
    }, {
      mass: "test3"
    }, {
      mass: "test4"
    }];
    var res = obj.reduce(function(prev, curr) {
      return (prev) ? prev + "," + curr.mass : curr.mass;
    }, "");
    alert(res);

или так:

var obj = [{
      mass: "test1"
    }, {
      mass: "test2"
    }, {
      mass: "test3"
    }, {
      mass: "test4"
    }];
    var res = [];
    for (var i = 0; i < obj.length; i++) {
      res.push(obj[i].mass);
    }
    alert(res.join());