Sum the same object of array

Question

var data = [ {id:1, qty:100}, {id:2, qty:200}, {id:1, qty:100}, {id:2, qty:200} ];

How to sum this array become to [ {id:1, qty:200}, {id:2, qty:

Answer 1

Try this:

var sum = [];

data.forEach(function(o) {
    var existing = sum.filter(function(i) { return i.id === o.id })[0];

    if (!existing)
        sum.push(o);
    else
        existing.qty += o.qty;
});

See Fiddle