var data = [ {type: "A", val:2},

             {type: "B", val:3},

             {type: "A", val:1},

             {type: "C", val:5} ];

​

console.log(data);

​

var groups = _(data).groupBy('type');

​

var out = _(groups).map(function(g, key) {

  return { 

     type: key, 

     val: _(g).reduce(function(m,x) { 

       return m + x.val;

     }, 0) 

  };

});

​

console.log(out);

​