reduce-by-key.js

var data = [
      [
        {field: 123, sum: 100},
        {field: 345, sum: 98}
      ],[
        {field: 123, sum: 12},
        {field: 345, sum: 20}
      ]
    ];

console.log(
    data
        .reduce((a, b) => a.concat(b))
        .reduce((a, b) => {
            var idx = a.findIndex(elem => elem.field == b.field);
            if (~idx) {
                a[idx].sum += b.sum;
            }
            else {
                a.push(JSON.parse(JSON.stringify(b)));
            }
            return a;
        }, [])
);

console.log(
    data
        .reduce(function (a, b) { return a.concat(b) })
        .reduce(function (a, b) {
            var idx = -1;
            a.forEach(function (elem, index) {
                if (elem.field == b.field) {
                    idx = index;
                }
            });
            if (~idx) {
                a[idx].sum += b.sum;
            }
            else {
                a.push(JSON.parse(JSON.stringify(b)));
            }
            return a;
        }, [])
);