Array.prototype.sortBy.js

Array.prototype.sortBy = (function() {
  var sorters = {
    string: function(a, b) {
      if (a < b) {
        return -1;
      } else if (a > b) {
        return 1;
      } else {
        return 0;
      }
    },

    number: function(a, b) {
      return a - b;
    }
  };

  return function(prop) {
    var type = typeof this[0][prop] || 'string';
    return this.sort(function(a, b) {
      return sorters[type](a[prop], b[prop]);
    });
  };
})();

sortArrayByKey.js

var sortArrayByKey = (function() {
  var sorters = {
    string: function(a, b) {
      if (a < b) {
        return -1;
      } else if (a > b) {
        return 1;
      } else {
        return 0;
      }
    },

    number: function(a, b) {
      return a - b;
    }
  };

  return function(ary, prop) {
    var type = typeof ary[0][prop] || 'string';
    return ary.sort(function(a, b) {
      return sorters[type](a[prop], b[prop]);
    });
  };
})();

Also note, this only supports strings and numbers, but you could add more sorters inside the closure for other types.