function meanBy(array, iteratee) {
      return baseMean(array, getIteratee(iteratee, 2));
    }
This method is like `_.mean` except that it accepts `iteratee` which is
invoked for each element in `array` to generate the value to be averaged.
The iteratee is invoked with one argument: (value).

@static
@memberOf _
@since 4.7.0
@category Math
@param {Array} array The array to iterate over.
@param {Function} [iteratee=_.identity] The iteratee invoked per element.
@returns {number} Returns the mean.
@example

var objects = [{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }];

_.meanBy(objects, function(o) { return o.n; });
// => 5

// The `_.property` iteratee shorthand.
_.meanBy(objects, 'n');
// => 5