Add _.meanBy.

fp/_mapping.js

@@ -67,7 +67,7 @@ exports.aryMethod = {
     'get', 'groupBy', 'gt', 'gte', 'has', 'hasIn', 'includes', 'indexOf',
     'intersection', 'invertBy', 'invoke', 'invokeMap', 'isEqual', 'isMatch',
     'join', 'keyBy', 'lastIndexOf', 'lt', 'lte', 'map', 'mapKeys', 'mapValues',
-    'matchesProperty', 'maxBy', 'merge', 'minBy', 'multiply', 'omit', 'omitBy',
+    'matchesProperty', 'maxBy', 'meanBy', 'merge', 'minBy', 'multiply', 'omit', 'omitBy',
     'overArgs', 'pad', 'padEnd', 'padStart', 'parseInt', 'partial', 'partialRight',
     'partition', 'pick', 'pickBy', 'pull', 'pullAll', 'pullAt', 'random', 'range',
     'rangeRight', 'rearg', 'reject', 'remove', 'repeat', 'restFrom', 'result',

lodash.js

@@ -1513,7 +1513,7 @@
      * `isPlainObject`, `isRegExp`, `isSafeInteger`, `isSet`, `isString`,
      * `isUndefined`, `isTypedArray`, `isWeakMap`, `isWeakSet`, `join`, `kebabCase`,
      * `last`, `lastIndexOf`, `lowerCase`, `lowerFirst`, `lt`, `lte`, `max`,
-     * `maxBy`, `mean`, `min`, `minBy`, `noConflict`, `noop`, `now`, `pad`,
+     * `maxBy`, `mean`, `meanBy`, `min`, `minBy`, `noConflict`, `noop`, `now`, `pad`,
      * `padEnd`, `padStart`, `parseInt`, `pop`, `random`, `reduce`, `reduceRight`,
      * `repeat`, `result`, `round`, `runInContext`, `sample`, `shift`, `size`,
      * `snakeCase`, `some`, `sortedIndex`, `sortedIndexBy`, `sortedLastIndex`,
@@ -14820,7 +14820,34 @@
      * // => 5
      */
     function mean(array) {
-      return sum(array) / (array ? array.length : 0);
+      return meanBy(array, identity);
+    }
+
+    /**
+     * 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|Object|string} [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
+     */
+    function meanBy(array, iteratee) {
+      return sumBy(array, iteratee) / (array ? array.length : 0);
     }
 
     /**
@@ -15240,6 +15267,7 @@
     lodash.max = max;
     lodash.maxBy = maxBy;
     lodash.mean = mean;
+    lodash.meanBy = meanBy;
     lodash.min = min;
     lodash.minBy = minBy;
     lodash.multiply = multiply;

test/test.js

@@ -12010,6 +12010,19 @@
       }
     });
 
+    QUnit.test('`_.meanBy` should use `_.iteratee` internally', function(assert) {
+      assert.expect(1);
+
+      if (!isModularize) {
+        _.iteratee = getPropA;
+        assert.strictEqual(_.meanBy(objects), 2 / 3);
+        _.iteratee = iteratee;
+      }
+      else {
+        skipAssert(assert);
+      }
+    });
+
     QUnit.test('`_.minBy` should use `_.iteratee` internally', function(assert) {
       assert.expect(1);
 
@@ -13958,6 +13971,44 @@
 
   /*--------------------------------------------------------------------------*/
 
+  QUnit.module('lodash.meanBy');
+
+  (function() {
+    var objects = [{ 'a': 2 }, { 'a': 3 }, { 'a': 1 }];
+
+    QUnit.test('should work with an `iteratee` argument', function(assert) {
+      assert.expect(1);
+
+      var actual = _.meanBy(objects, function(object) {
+        return object.a;
+      });
+
+      assert.deepEqual(actual, 2);
+    });
+
+    QUnit.test('should provide the correct `iteratee` arguments', function(assert) {
+      assert.expect(1);
+
+      var args;
+
+      _.meanBy(objects, function() {
+        args || (args = slice.call(arguments));
+      });
+
+      assert.deepEqual(args, [{ 'a': 2 }]);
+    });
+
+    QUnit.test('should work with "_.property" shorthands', function(assert) {
+      assert.expect(2);
+
+      var arrays = [[2], [3], [1]];
+      assert.strictEqual(_.meanBy(arrays, 0), 2);
+      assert.strictEqual(_.meanBy(objects, 'a'), 2);
+    });
+  }());
+
+  /*--------------------------------------------------------------------------*/
+
   QUnit.module('lodash.memoize');
 
   (function() {
@@ -25146,7 +25197,7 @@
     var acceptFalsey = lodashStable.difference(allMethods, rejectFalsey);
 
     QUnit.test('should accept falsey arguments', function(assert) {
-      assert.expect(307);
+      assert.expect(308);
 
       var emptyArrays = lodashStable.map(falsey, alwaysEmptyArray);