Bump to v4.0.0.

keyBy.js

@@ -0,0 +1,34 @@
+var createAggregator = require('./internal/createAggregator');
+
+/**
+ * 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 last element responsible for generating the key. 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
+ *
+ * var keyData = [
+ *   { 'dir': 'left', 'code': 97 },
+ *   { 'dir': 'right', 'code': 100 }
+ * ];
+ *
+ * _.keyBy(keyData, 'dir');
+ * // => { 'left': { 'dir': 'left', 'code': 97 }, 'right': { 'dir': 'right', 'code': 100 } }
+ *
+ * _.keyBy(keyData, function(o) {
+ *   return String.fromCharCode(o.code);
+ * });
+ * // => { 'a': { 'dir': 'left', 'code': 97 }, 'd': { 'dir': 'right', 'code': 100 } }
+ */
+var keyBy = createAggregator(function(result, value, key) {
+  result[key] = value;
+});
+
+module.exports = keyBy;