for (k in ['a', 'b']) {
      console.log(k);
    }

    var a = {0:'a', 1:'b'};
    for (k in a) {
      console.log(k);
    }

Edit: turns out, you can even have doubles, too:

    var weird = {3.14:'hello', 6.28:'world'};
    // for loop above emits: 3.14, 6.28
    console.log(weird[3.14]); // emits 'hello'