Add support for _.sum to take iteratee.

lodash.src.js

@@ -11304,6 +11304,8 @@
      * @memberOf _
      * @category Math
      * @param {Array|Object|string} collection The collection to iterate over.
+     * @param {Function|Object|string} [iteratee] The function invoked per iteration.
+     * @param {*} [thisArg] The `this` binding of `iteratee`.
      * @returns {number} Returns the sum.
      * @example
      *
@@ -11312,16 +11314,41 @@
      *
      * _.sum({ 'a': 4, 'b': 6, 'c': 2 });
      * // => 12
+     *
+     * _.sum([]);
+     * // => 0
+     *
+     * var users = [
+     *   { 'user': 'barney', 'age': 36 },
+     *   { 'user': 'fred',   'age': 40 }
+     * ];
+     *
+     * _.sum(users, function(chr) {
+     *   return chr.age;
+     * });
+     * // => 76
+     *
+     * // using the `_.property` callback shorthand
+     * _.sum(users, 'age');
+     * // => 76
      */
-    function sum(collection) {
+    function sum(collection, iteratee, thisArg) {
+      if (thisArg && isIterateeCall(collection, iteratee, thisArg)) {
+        iteratee = null;
+      }
       if (!isArray(collection)) {
         collection = toIterable(collection);
       }
+      var func = getCallback();
+      if (!(func === baseCallback && iteratee == null)) {
+        iteratee = func(iteratee, thisArg, 3);
+      }
+
       var length = collection.length,
           result = 0;
 
       while (length--) {
-        result += +collection[length] || 0;
+        result += +(iteratee == null ? collection[length] : iteratee(collection[length])) || 0;
       }
       return result;
     }

test/test.js

@@ -2984,6 +2984,17 @@
       }
     });
 
+    test('`_.sum` should use `_.callback` internally', 1, function() {
+      if (!isModularize) {
+        _.callback = getPropB;
+        strictEqual(_.sum(objects), 1);
+        _.callback = callback;
+      }
+      else {
+        skipTest();
+      }
+    });
+
     test('`_.takeRightWhile` should use `_.callback` internally', 1, function() {
       if (!isModularize) {
         _.callback = getPropB;
@@ -13464,6 +13475,8 @@
   QUnit.module('lodash.sum');
 
   (function() {
+    var objects = [{ 'a': 2 }, { 'a': 3 }, { 'a': 1 }];
+
     test('should return the sum of an array of numbers', 1, function() {
       strictEqual(_.sum([6, 4, 2]), 12);
     });
@@ -13491,6 +13504,40 @@
         strictEqual(_.sum(value), 6);
       });
     });
+
+    test('should work with an `iteratee` argument', 1, function() {
+      var actual = _.sum(objects, function(object) {
+        return object.a;
+      });
+
+      deepEqual(actual, 6);
+    });
+
+    test('should support the `thisArg` argument', 1, function() {
+      var actual = _.sum([1, 2.5, 1.5], function(num) {
+        return this.floor(num);
+      }, Math);
+
+      strictEqual(actual, 4);
+    });
+
+    test('should work with a "_.property" style `iteratee`', 2, function() {
+      var actual = _.sum(objects, 'a');
+
+      strictEqual(actual, 6);
+
+      var arrays = [[2], [3], [1]];
+      actual = _.sum(arrays, 0);
+
+      strictEqual(actual, 6);
+    });
+
+    test('should perform basic sum when used as an iteratee for methods like `_.map`', 1, function() {
+      var array = [{ 'a': 1, 'b': 2, 'c': 3 }, [1, 2, 1]],
+          actual = _.map(array, _.sum);
+
+      deepEqual(actual, [6, 4]);
+    });
   }());
 
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