javascript sum of json objects

After trying to find many ways to do it here, I’m stuck with this.

I have a Json object and would like to sum all the results.
This code is showing all amounts, but not showing their sum in the end.

var str = e.data.substring(e.data.indexOf("["));
    if (IsJsonString(str)) {
        var body = JSON.parse(str);
        var data = body[1];

        if(data && data.id == "ID" && data.some.string == "string"){

            var fields= [
                data.some
              ];
              var sum = fields.reduce(function(acc, val){
                return (+acc.amount) + (+val.amount);
              }, {amount: 0});
              console.log(sum);
        }
    }