示例数据：

let userUsages = 
[ { _id: 5cad99fe0506582a4430491c,user: 5c9d8cc3161d6c257c021340,key: '1234',created: 2019-04-10T07:20:20.600Z },{ _id: 5cad99af06ebd54558e88716,user: 5c9d8cc3161d6c257c021340,key: '1234',created: 2019-04-10T07:22:23.932Z },{ _id: 5cad9cd46a97d8470c09b817,user: 5c9d8cc3161d6c257c021340,key: '1234',created: 2019-04-10T07:35:48.273Z },{ _id: 5cad9d066a97d8470c09b819,user: 5c9d8cc3161d6c257c021340,key: '2345',created: 2019-04-10T07:36:38.448Z },{ _id: 5cad9d096a97d8470c09b81a,user: 5c9d8cc3161d6c257c021340,key: '2345',created: 2019-04-10T07:36:41.580Z },{ _id: 5cad9d0e6a97d8470c09b81b,user: 5c9d8cc3161d6c257c021340,key: '2345',created: 2019-04-10T07:36:46.250Z },]

按照key进行分组：

let keyContainer = {};
userUsages.forEach(element => {keyContainer[element.key] = keyContainer[element.key] || [];keyContainer[element.key].push(element);
});
console.info(keyContainer);

分组结果：

{ '1234':[ { _id: 5cad99fe0506582a4430491c,user: 5c9d8cc3161d6c257c021340,key: '1234',created: 2019-04-10T07:20:20.600Z },{ _id: 5cad99af06ebd54558e88716,user: 5c9d8cc3161d6c257c021340,key: '1234',created: 2019-04-10T07:22:23.932Z },{ _id: 5cad9cd46a97d8470c09b817,user: 5c9d8cc3161d6c257c021340,key: '1234',created: 2019-04-10T07:35:48.273Z } ],'2345':[ { _id: 5cad9d066a97d8470c09b819,user: 5c9d8cc3161d6c257c021340,key: '2345',created: 2019-04-10T07:36:38.448Z },{ _id: 5cad9d096a97d8470c09b81a,user: 5c9d8cc3161d6c257c021340,key: '2345',created: 2019-04-10T07:36:41.580Z },{ _id: 5cad9d0e6a97d8470c09b81b,user: 5c9d8cc3161d6c257c021340,key: '2345',created: 2019-04-10T07:36:46.250Z } ],}

JS - 数组分组