Javascript advanced sorter

sorter.js

/**
 * Create a compare function for Javascript's Array.sort()
 * Inspired by Triptych's http://stackoverflow.com/a/979325/37020
 *
 * my_array.sort( sorter({key: 'size', reverse: true}) );
 * my_array.sort( sorter({key: function(x) { return parseInt(x.tag); }) );
 */
var sorter = function(spec) {
  if (spec.key === undefined)
    spec.key = function(x) { return x; }; // identity
  if (spec.reverse === undefined)
    spec.reverse = false;

  // either a field name, or a key function
  var key = typeof spec.key === "string" ? function(x) { return x[spec.key]; } : spec.key;

  return function (a,b) {
    var A = key(a), B = key(b);
    var cmp = A < B ? -1 : A > B ? 1 : 0;
    return spec.reverse ? -1 * cmp : cmp;
  };
};