Calling reduce to sum array of objects returns NaN

Question

I have code similar to this:

var temp = [ { "y": 32 }, { "y": 60 }, { "y": 60 } ];
var reduced = temp.reduce(function(a, b) {
  return a.y + b.y;
});

console.log(reduced); // Prints NaN

Why does it print NaN instead of 152?

to explain the issue - once you return the first sum, the typeof a will be a Number - which has no y property — Jaromanda X
– Jaromanda X, Commented Sep 26, 2016 at 8:59
check reduce function's callback parameters , what are they ? — passion
– passion, Commented Sep 26, 2016 at 8:59
Does this answer your question? Javascript reduce on array of objects — Sebastian Simon
– Sebastian Simon, Commented Apr 9, 2021 at 14:31

Nina Scholz · Accepted Answer · 2016-09-26 09:02:30Z

47

You could use a start value and the add only one value from the array.

var temp=[{"name":"Agency","y":32,"drilldown":{"name":"Agency","categories":["APPS & SI","ERS"],"data":[24,8]}},{"name":"ER","y":60,"drilldown":{"name":"ER","categories":["APPS & SI","ERS"],"data":[7,53]}},{"name":"Direct","y":60,"drilldown":{"name":"Direct","categories":["APPS & SI","ERS"],"data":[31,29]}}];

var reduced = temp.reduce(function (r, a) {
        return r + a.y;
        //    ^^^ use the last result without property
    }, 0);
//   ^^^ add a start value
console.log(reduced) // r

edited Sep 26, 2016 at 9:02

answered Sep 26, 2016 at 8:55

Nina Scholz

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2 Comments

webmaster Over a year ago

one liner const reduced = temp.length ? temp.reduce((r, a) => { return r + a.y; }, 0) : 0;

Nina Scholz Over a year ago

@webmaster, even shorter, it takes the start value of reduce for empty arrays: const reduced = temp.reduce((r, { a }) => r + a, 0);

Chen · Accepted Answer · 2016-09-26 09:16:11Z

10

short solution: map the collection to integers collection, and reduce it

var temp=[{"name":"Agency","y":32,"drilldown":{"name":"Agency","categories":["APPS & SI","ERS"],"data":[24,8]}},{"name":"ER","y":60,"drilldown":{"name":"ER","categories":["APPS & SI","ERS"],"data":[7,53]}},{"name":"Direct","y":60,"drilldown":{"name":"Direct","categories":["APPS & SI","ERS"],"data":[31,29]}}];

var reduced = temp
                .map(function(obj) { return obj.y; })
                .reduce(function(a, b) { return a + b; });

console.log(reduced);

edited Sep 26, 2016 at 9:16

answered Sep 26, 2016 at 9:06

Chen

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