Reduce array of object to totals by property object

Question

I'm looking for a smart ES6 way to reduce array of objects into totals-by-property-object.

for a sample data:

const src = [{mon:1,tue:0,wed:3,thu:5,fri:7,sat:0,sun:4}, {mon:5,tue:3,wed:2,thu:0,fri:1,sat:0,sun:6}];

following code:

const res = src.reduce((totals,item) => Object.keys(item).forEach(weekday => totals[weekday] += item[weekday]),{})

throws an error:

Uncaught TypeError: Cannot read property 'mon' of undefined

even if reduce is initialized with {mon:0, tue:0 ...} instead of {}.

Is there a non-for-loop solution?

p.s. expected output is an object where each property is a sum of array objects by that property, e.g. {mon:6, tue:3, wed:5, thu:5, fri:8, sat:0, sun:10} in my case

Answer 1

You need to return totals as accumulator for reduce.

If you have allways all days in the objects and you don't mind to mutate the first object, you could work without a start object.

const
    src = [{ mon: 1, tue: 0, wed: 3, thu: 5, fri: 7, sat: 0, sun: 4 }, { mon: 5, tue: 3, wed: 2, thu: 0, fri: 1, sat: 0, sun: 6 }],
    res = src.reduce((totals, item) => 
        (Object.keys(item).forEach(d => totals[d] += item[d]), totals));

console.log(res);

Answer 2

You need to return totals after you modify it:

const src = [{mon:1,tue:0,wed:3,thu:5,fri:7,sat:0,sun:4}, {mon:5,tue:3,wed:2,thu:0,fri:1,sat:0,sun:6}];

const res = src.reduce((totals, item) => {
  Object.keys(item).forEach(weekday => totals[weekday] = (totals[weekday] || 0) + item[weekday]);
  
  return totals;
}, {});

console.log(res);