像这样的数组

[{ dep: 'A', qt: 10, price: 5},
 { dep: 'A', qt: 5,  price: 2.30 },
 { dep: 'B', qt: 3,  price: 2.20 },
 { dep: 'C', qt: 1,  price: 4 },
 { dep: 'C', qt: 4,  price: 10 }
 ...etc.. 
]

分组和求和值的优雅方式是什么,结果如下：

[{ dep: 'A', qt: 15, price: 61.5 },
 { dep: 'B', qt: 3, price: 2.20 },
 { dep: 'C', qt: 5: price: 44 }
]

解决方法:

你可以这样做：

function sumInvoices(p, c) {
    return _.extend(p, {qt:p.qt + c.qt, price:p.price + c.qt*c.price});
};

var b = _(a)
  .groupBy('dep')
  .map(function(b) {return b.reduce(sumInvoices, {dep:b[0].dep, qt:0, price:0})})
  .valueOf();