Underscore.js: Sum of items in a collection

Question

I made a small plnkr here to show what I am trying to achieve. I have a big dataset, where I like to sum the individual type to get a total.

I could think of iterating and adding the results to an object hash, but wonder more elegant way to solve it with underscore. I am using underscore.js, but never tried map reduce or other functional paradigm. Please update the plnkr to learn how to do this.

http://plnkr.co/edit/B5HGxhwvWsfvOR97z7TL?p=preview

var data = [ {'type': "A", 'val':2},
  {'type': "B", 'val':3},
  {'type': "A", 'val':1},
  {'type': "C", 'val':5} ];


 _.each(data, function (elm, index) {
   console.log(elm);  
 });

 /*
 Desired output

 out = [ {'type': "A", 'total':3},
  {'type': "B", 'total':3},
  {'type': "C", 'total':5} ];

 */

Answer 1

var data = [ { type: "A", val: 2 },
             { type: "B", val: 3 },
             { type: "A", val: 1 },
             { type: "C", val: 5 } ];

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) };
});

DEMO