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, Sep 26 '16 at 08:59
check reduce function's callback parameters , what are they ? — passion, Sep 26 '16 at 08:59
Does this answer your question? [Javascript reduce on array of objects](https://stackoverflow.com/questions/5732043/javascript-reduce-on-array-of-objects) — Sebastian Simon, Apr 09 '21 at 14:31

Nina Scholz · Accepted Answer · 2016-09-26T09:02:30.827

37

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 '16 at 09:02

answered Sep 26 '16 at 08:55

Nina Scholz

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one liner const reduced = temp.length ? temp.reduce((r, a) => { return r + a.y; }, 0) : 0; – webmaster Feb 28 '18 at 13:43
@webmaster, even shorter, it takes the start value of reduce for empty arrays: `const reduced = temp.reduce((r, { a }) => r + a, 0);` – Nina Scholz Feb 28 '18 at 13:53

Chen · Answer 2 · 2016-09-26T09:16:11.093

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);

Calling reduce to sum array of objects returns NaN

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