var source = [
  {"a": 2, "b": 8, "c": 9},
  {"a": 4, "b": 3, "c": 7},
  {"a": 1, "b": 0, "c": 6}
]

var sortedIndexes;

SomeSortOfSortMethod("a", "asc");

// result of sortedIndexes containing indexes to source array:
// [2, 0, 1]