reduce is not giving right sum

Question

My Object groupBy.Food looks like

[
Object
amount: "15.0"
category: Object
debit: true
__proto__: Object
, 
Object
amount: "10.0"
category: Object
debit: true
__proto__: Object
, 
Object
amount: "11.1"
category: Object
debit: true
__proto__: Object
]

All I want is sum of amount in each object. I am using Lodash reduce as

var s = _.reduce(groupBy.Food, function(s, entry){
   return s + parseFloat(entry.amount);
});

When I see value of s I get

s
"[object Object]1011.1"

What is that I am not doing right here?

Answer 1

By default, reduce starts out with the first two items in the list, so s will be the first item in the array and entry will be the second item the first time your function is called. Give it a value to start with:

var s = _.reduce(groupBy.Food, function(s, entry) {
    return s + parseFloat(entry.amount);
}, 0);

(Array’s reduce behaves the same way.)

Answer 2

You can also use _.sum(array) or _.sumBy(array, [iteratee=_.identity]) for summing up for both array and object. See below examples.

_.sum([4, 2, 8, 6]); // ➜ 20

var objects = [{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }];
_.sumBy(objects, function(o) { return o.n; }); // ➜ 20
_.sumBy(objects, 'n');// ➜ 20