Bump to v4.0.0.

countBy.js

@@ -0,0 +1,33 @@
+import createAggregator from './internal/createAggregator';
+
+/** Used for built-in method references. */
+var objectProto = Object.prototype;
+
+/** Used to check objects for own properties. */
+var hasOwnProperty = objectProto.hasOwnProperty;
+
+/**
+ * Creates an object composed of keys generated from the results of running
+ * each element of `collection` through `iteratee`. The corresponding value
+ * of each key is the number of times the key was returned by `iteratee`.
+ * The iteratee is invoked with one argument: (value).
+ *
+ * @static
+ * @memberOf _
+ * @category Collection
+ * @param {Array|Object} collection The collection to iterate over.
+ * @param {Function|Object|string} [iteratee=_.identity] The iteratee invoked per element.
+ * @returns {Object} Returns the composed aggregate object.
+ * @example
+ *
+ * _.countBy([6.1, 4.2, 6.3], Math.floor);
+ * // => { '4': 1, '6': 2 }
+ *
+ * _.countBy(['one', 'two', 'three'], 'length');
+ * // => { '3': 2, '5': 1 }
+ */
+var countBy = createAggregator(function(result, value, key) {
+  hasOwnProperty.call(result, key) ? ++result[key] : (result[key] = 1);
+});
+
+export default countBy;