Unable to reduce an array of objects based on string value

Question

So I have this array of objects which I need to reduce based on each object's prop value as well as count the total.

What I mean to say is, suppose I have this array of objects:

[
     { dateTime: '2021-07-30', count: 4 },
     { dateTime: '2021-07-31', count: 1 },
     { dateTime: '2021-08-01', count: 2 },
     { dateTime: '2021-08-02', count: 1 },
     { dateTime: '2021-08-03', count: 2 },
     { dateTime: '2021-08-04', count: 3 }
]

I want to reduce it to something like this:

[
     { month: '2021-07', count: 5 },
     { month: '2021-08', count: 8 }
]

Here's the snippet:

const data = [{
    dateTime: '2021-07-30',
    count: 4
  },
  {
    dateTime: '2021-07-31',
    count: 1
  },
  {
    dateTime: '2021-08-01',
    count: 2
  },
  {
    dateTime: '2021-08-02',
    count: 1
  },
  {
    dateTime: '2021-08-03',
    count: 2
  },
  {
    dateTime: '2021-08-04',
    count: 3
  }
];

const stackedMonths = data.reduce((acc, curr) => {

  return [{
    month: curr.dateTime.slice(0, 7),
    count: acc + curr.count
  }];

}, []);

console.log(stackedMonths);

After trying the code, I'm getting this which doesn't make any sense:

[ { month: '2021-08', count: '[object Object]2' } ]

What am I doing wrong here? Please note that I'm new to reduce and I'm still learning it.

On this line, count: acc + curr.count, acc is an object. When you concatenate an object with a number, you get a string (unless you have a toValue method on the object that returns a number.) Surely you didn't intend to concatenate an object with a number? — user400654
– user400654, Commented Aug 25, 2021 at 17:53
No I did not. I was trying it out myself from various answers here in SO. But isn't acc an array? — s.khan
– s.khan, Commented Aug 25, 2021 at 17:55
An Array is an Object. You can't add numbers to an Object itself, as an Object isn't a Number (or any form of a Number). — esqew
– esqew, Commented Aug 25, 2021 at 17:57
It should be an array, but if it was, you wouldn't get [object Object]2. Yes, arrays are objects, but arrays have a different outcome when concatenating in this way — user400654
– user400654, Commented Aug 25, 2021 at 17:58

Arvind Kumar Avinash · Accepted Answer · 2021-08-26 12:14:59Z

2

You can use Array#reduce with an object to store the sum for each month.

const arr = [
     { dateTime: '2021-07-30', count: 4 },
     { dateTime: '2021-07-31', count: 1 },
     { dateTime: '2021-08-01', count: 2 },
     { dateTime: '2021-08-02', count: 1 },
     { dateTime: '2021-08-03', count: 2 },
     { dateTime: '2021-08-04', count: 3 }
];
const res = Object.values(arr.reduce((acc, curr)=>{
  const month = curr.dateTime.slice(0, -3);
  (acc[month] ??= {month, count: 0}).count += curr.count;
  return acc;
}, {}));
console.log(res);

If the value of month does not yet exist as a key on the accumulator, it is set to {month: month, count: 0}. Then, the count is incremented by the current object's count.

edited Aug 26, 2021 at 12:14

Arvind Kumar Avinash

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answered Aug 25, 2021 at 17:51

Unmitigated

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6 Comments

s.khan Over a year ago

Hi. Can you please explain this part (acc[month] ??= {month, count: 0}).count += curr.count;?

Unmitigated Over a year ago

@s.khan If the value of month does not yet exist as a key on the accumulator, it is set to {month: month, count: 0}. Then, the count is incremented by the current object's count.

esqew Over a year ago

Very clever use of Object.values and ??=, love the succinctness.

s.khan Over a year ago

@Unmitigated hey man, I'm getting an error in NodeJS because of ??= operator.

Unmitigated Over a year ago

@s.khan You can replace that with acc[month] = acc[month] || {month, count: 0}.

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