Lodash groupBy method
语法：
_.groupBy(collection, [iteratee=_.identity])
Creates an object composed of keys generated from the results of running each element of collection thru iteratee. The order of grouped values is determined by the order they occur in collection. The corresponding value of each key is an array of elements responsible for generating the key. The iteratee is invoked with one argument: (value).
参数：
collection (Array|Object) − The collection to iterate over.
[iteratee=_.identity] (Function) − The iteratee to transform keys.
输出：
(Object) − Returns the composed aggregate object.
例子：
var _ = require('lodash');
var list = [1.1 ,1.4, 2.1, 2.3];
var result = _.groupBy(list, Math.floor);
console.log(result);
result = _.groupBy(['one', 'two', 'three'], 'length');
console.log(result);
Save the above program in
tester.js. Run the following command to execute this program.
命令：
输出：
{ '1': [ 1.1, 1.4 ], '2': [ 2.1, 2.3 ] }
{ '3': [ 'one', 'two' ], '5': [ 'three' ] }
