javascript - Combining objects in an array with same key (date) -


i have array of objects , want combine objects have same date values have array like. looping through them .map(i => ) , chekcing on previous index i'm getting wrong values.

 [{ id: '8',    name: 'hfh',    consumed_date: '2017-03-14',    calories: 8952 },  { id: '7',    name: 'onion',    consumed_date: '2017-03-14',    calories: 224},  { id: '6',    name: 'onion',    consumed_date: '2017-03-14',    calories: 279},  { id: '2',    name: 'ready-to-serve chocolate drink',    consumed_date: '2017-01-01',    calories: 3036} ]

and want end array like

[{date: 2017-03-14, calories: 9455}, date: 2017-01-01, calories: 3036}]

reduce perfect situations this: need "loop" through items don't want straight-forward "a => modify(a)"-like behavior of map, rather need reduce or collapse several values else (in case both input , output array of values reduced one).

we need variable hold state (the progress far) while loop through items. in more imperative style solved variable declared outside of for-loop, in reduce "progress far"-variable (acc) passed along each iteration, along next item.

const reducedarray = arr.reduce((acc, next) => { // acc stands accumulator   const lastitemindex = acc.length -1;   const acchascontent = acc.length >= 1;    if(acchascontent && acc[lastitemindex].consumed_date == next.consumed_date) {     acc[lastitemindex].calories += next.calories;   } else {     // first time seeing entry. add it!     acc[lastitemindex +1] = next;   }   return acc; }, []);  console.log(reducedarray)

(this solution assumes data sorted on date.)


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