Count how many objects in an array has a certain value (JavaScript)

Question

I have an array of objects, like so:

[
{
"_id": "5b09cc3495cb6c0487f1166b",
"name": "ccc",
"email": "[email protected]",
"phone": "790467522",
"kidsNo": "1",
"adultsNo": "1",
"fullDate": "2018/5/1",
"year": "2018",
"month": "5",
"day": "1",
"chosenHour": "11:00",
"chosenRoom": "x",
"__v": 0
},
{
"_id": "5b09cc6095cb6c0487f1166c",
"name": "asd",
"email": "[email protected]",
"phone": "790467522",
"kidsNo": "2",
"adultsNo": "3",
"fullDate": "2018/5/1",
"year": "2018",
"month": "5",
"day": "1",
"chosenHour": "12:00",
"chosenRoom": "x",
"__v": 0
},
{
"_id": "5b0b1560c7b4fd0c33b2d52e",
"name": "dddd",
"email": "[email protected]",
"phone": "123123112",
"kidsNo": "2",
"adultsNo": "1",
"fullDate": "2018/5/17",
"year": "2018",
"month": "5",
"day": "17",
"chosenHour": "11:00",
"chosenRoom": "x",
"__v": 0
}
]

In the future this array will contain much more objects. I'm trying to solve this using map and for it seems to be quite complicated. That's the challenge: how I can count how many objects have certain value? How can I get to know how many times someone booked something to day===1? The best result would be an array like this:

[{dayOne: 2}, {dayTwo: 5}, {dayThree:1}.......and so on],

where value is the value of how many times a day was booked(key), hence how many times certain object(with certain value) has appeared in the array?

Thank you in advance!

I'm curious about your output format. What is the advantage of that over {day1: 2, day2: 5, day3: 1, /* ... */ } or {1: 2, 2: 5, 3: 1, /* ... */} or even [2, 5, 1, /* ... */ ]? Any of those looks easier to use and easier to create. — Scott Sauyet
– Scott Sauyet, Commented May 27, 2018 at 23:36

score 6 · Accepted Answer · 2018-05-27 23:39:53Z

6

To count objects by a condition, you can use .filter --

let firstDayCount = arr.filter(x => x.day === "1").length;

To group the result by days, you can use .reduce --

let countByDays = 
  arr.reduce((res, { day }) => {
     res[day] = res[day] || 0;
     res[day] += 1;
     return res; 
   }, {});

If you want to format your output, you can then use a dictionary of names --

let dayNames = { 1: "dayOne", 2: "dayTwo" /* and so on */}

let formattedResult = 
  Object.keys(countByDays)
    .map(n => { [dayNames[n]]: countByDays[n] });

Note that using a .filter for counting creates an intermediate throw-away array. We're not storing a reference anywhere, so it has to be GCed soon, but if it really affects your performance measurably in a real-life scenario, you can use a .reduce instead -- something that is called "deforestation":)

let count = arr.reduce((cnt, el) => el.day === "1" ? cnt += 1 : cnt, 0);

It'll still create an intermediate anonymous object though -- a reducer function -- so if your profiler shows this place as a bottleneck, you might be best off using a for loop. As always in such cases, it's up to you to find the right spot between performance and readability in your own real-world scenarios.

edited May 27, 2018 at 23:39

answered May 27, 2018 at 22:01

user9539019

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5 Comments

Ele Over a year ago

function filter for counting is the worst approach!

user9539019 Over a year ago

Worst compared to what?

Ele Over a year ago

To the fact of creating a new array.

user9539019 Over a year ago

We're not storing a reference anywhere, so I assume it's going ot be GCed soon. Though if it really affects the performance significantly in a real-life scenario, one can always go with a for loop :)

Ele Over a year ago

For counting the best is a for-loop as you said or a reduce.

Matus Dubrava · Accepted Answer · 2018-05-27 22:26:06Z

To get result exactly in that format you can do this.

const objs = [
{
"_id": "5b09cc3495cb6c0487f1166b",
"name": "ccc",
"email": "[email protected]",
"phone": "790467522",
"kidsNo": "1",
"adultsNo": "1",
"fullDate": "2018/5/1",
"year": "2018",
"month": "5",
"day": "1",
"chosenHour": "11:00",
"chosenRoom": "x",
"__v": 0
},
{
"_id": "5b09cc6095cb6c0487f1166c",
"name": "asd",
"email": "[email protected]",
"phone": "790467522",
"kidsNo": "2",
"adultsNo": "3",
"fullDate": "2018/5/1",
"year": "2018",
"month": "5",
"day": "1",
"chosenHour": "12:00",
"chosenRoom": "x",
"__v": 0
},
{
"_id": "5b0b1560c7b4fd0c33b2d52e",
"name": "dddd",
"email": "[email protected]",
"phone": "123123112",
"kidsNo": "2",
"adultsNo": "1",
"fullDate": "2018/5/17",
"year": "2018",
"month": "5",
"day": "17",
"chosenHour": "11:00",
"chosenRoom": "x",
"__v": 0
}
]

const days = [['dayOne', 1], ['dayTwo', 2], ['dayThree', 3]];

const res = days.reduce((acc, v) => {
  const obj = {};
  obj[v[0]] = objs.filter(x => x.day == v[1]).length;
  return acc.concat(obj);
}, []);

console.log(res);

Note that you will need to extend the days array for every day you want to include like that. So If you want to include all 31 possible days, you need this.

const objs = [
{
"_id": "5b09cc3495cb6c0487f1166b",
"name": "ccc",
"email": "[email protected]",
"phone": "790467522",
"kidsNo": "1",
"adultsNo": "1",
"fullDate": "2018/5/1",
"year": "2018",
"month": "5",
"day": "1",
"chosenHour": "11:00",
"chosenRoom": "x",
"__v": 0
},
{
"_id": "5b09cc6095cb6c0487f1166c",
"name": "asd",
"email": "[email protected]",
"phone": "790467522",
"kidsNo": "2",
"adultsNo": "3",
"fullDate": "2018/5/1",
"year": "2018",
"month": "5",
"day": "1",
"chosenHour": "12:00",
"chosenRoom": "x",
"__v": 0
},
{
"_id": "5b0b1560c7b4fd0c33b2d52e",
"name": "dddd",
"email": "[email protected]",
"phone": "123123112",
"kidsNo": "2",
"adultsNo": "1",
"fullDate": "2018/5/17",
"year": "2018",
"month": "5",
"day": "17",
"chosenHour": "11:00",
"chosenRoom": "x",
"__v": 0
}
]

const days = [['dayOne', 1], ['dayTwo', 2], ['dayThree', 3], ['dayFour', 4], ['dayFive', 5],
              ['daySix', 6], ['daySeven', 7], ['dayEight', 8], ['dayNine', 9], ['dayTen', 10],
              ['dayEleven', 11], ['dayTwelve', 12], ['dayThirten', 13], ['dayFourteen', 14],
              ['dayFifteen', 15], ['daySixteen', 16], ['daySeventeen', 17], ['dayEighteen', 18],
              ['dayNineteen', 19], ['dayTwenty', 20], ['dayTwentyone', 21], ['dayTwentytwo', 22],
              ['dayTwentythree', 23], ['dayTwentyfour', 24], ['dayTwentyfive', 25], ['dayTwentysix', 26],
              ['dayTwentyseven', 27], ['dayTwentyeight', 28], ['dayTwentynine', 29], ['dayThirty', 30],
              ['dayThirtyone', 31]];

const res = days.reduce((acc, v) => {
  const obj = {};
  obj[v[0]] = objs.filter(x => x.day == v[1]).length;
  return acc.concat(obj);
}, []);

console.log(res);

Which in this example returns [{dayOne: 2}, {dayTwo: 0}, ..., {daySeventeen}: 1, {dayNineteen: 0}, ...]

Ele · Accepted Answer · 2018-05-27 22:47:33Z

0

An alternative is using the function reduce to group and count.

const array = [{"_id": "5b09cc3495cb6c0487f1166b","name": "ccc","email": "[email protected]","phone": "790467522","kidsNo": "1","adultsNo": "1","fullDate": "2018/5/1","year": "2018","month": "5","day": "1","chosenHour": "11:00","chosenRoom": "x","__v": 0},{"_id": "5b09cc6095cb6c0487f1166c","name": "asd","email": "[email protected]","phone": "790467522","kidsNo": "2","adultsNo": "3","fullDate": "2018/5/1","year": "2018","month": "5","day": "1","chosenHour": "12:00","chosenRoom": "x","__v": 0},{"_id": "5b0b1560c7b4fd0c33b2d52e","name": "dddd","email": "[email protected]","phone": "123123112","kidsNo": "2","adultsNo": "1","fullDate": "2018/5/17","year": "2018","month": "5","day": "17","chosenHour": "11:00","chosenRoom": "x","__v": 0}],
      result = Object.values(array.reduce((a, {day}) => {
        let key = `day${day}`;
        (a[key] || (a[key] = {[key]: 0}))[key]++;
        return a;
      }, {}));

console.log(result);

answered May 27, 2018 at 22:47

Ele

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3 Comments

user9539019 Over a year ago

Note though that Object.values won't work in IE without a polyfill.

Ele Over a year ago

@LyoshaKorogoda only in old versions.

user9539019 Over a year ago

Hmm, that's really unexpected :) I've tried it in IE11 just now and it worked, though I always avoided it because MDN said otherwise --developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

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