1 d3.extent = function(array, f) {

     2   var i = -1,

     3       n = array.length,

     4       a,

     5       b,

     6       c;

     7   if (arguments.length === 1) {

     8     while (++i < n && ((a = c = array[i]) == null || a != a)) a = c = undefined;

     9     while (++i < n) if ((b = array[i]) != null) {

    10       if (a > b) a = b;

    11       if (c < b) c = b;

    12     }

    13   } else {

    14     while (++i < n && ((a = c = f.call(array, array[i], i)) == null || a != a)) a = undefined;

    15     while (++i < n) if ((b = f.call(array, array[i], i)) != null) {

    16       if (a > b) a = b;

    17       if (c < b) c = b;

    18     }

    19   }

    20   return [a, c];

    21 };