code: function singleIndexOf(arr, v) {
    var pos = -1;
    var i, l;
    if (!arr) {
      return -1;
    }
    for (i = 0, l = arr.length; i < l; i++) {
      if (arr[i] === v) {
        if (pos >= 0) {
          return -2;
        }
        pos = i;
      }
    }
    return pos;
  }
docs: Find index of value in array.
  
  @param arr {Array} Can also be a false-ish value.
  @param v {Object} Value to find.
  
  Returns -1 if not found, -2 if found multiple times.