Grouping a javascript array over a loop

Question

I have the following array:

[
    {'id': 1, 'rating': 5, 'rating_category': "a"},
    {'id': 1, 'rating': 1, 'rating_category': "a"},
    {'id': 1, 'rating': 4, 'rating_category': "b"},
    {'id': 1, 'rating': 5, 'rating_category': "b"},
]

And expect the following array:

[
   { category: "a", rating_average: 3, rating_count: 2},
   { category: "b", rating_average: 4.5, rating_count: 2},
]

My approach works only partially. I just don't get the array format I want. I get a [key : {}, key : {}] but I want to get [{},{}]. My problem is checking if the key already exists in the array.

const data = [
    {'id': 1, 'rating': 5, 'rating_category': "a"},
    {'id': 1, 'rating': 1, 'rating_category': "a"},
    {'id': 1, 'rating': 4, 'rating_category': "b"},
    {'id': 1, 'rating': 5, 'rating_category': "b"},
]

r = []
data.forEach(e => {
  if (! r[e.rating_category]) {
    r[e.rating_category] = {
      rating: e.rating, 
      count: 1      
    };
  } else {    
    r[e.rating_category]['rating'] += e.rating
    r[e.rating_category]['count']++
  }
  
})
console.log(r.a, r.b)

Barmar · Accepted Answer · 2022-01-12 20:17:03Z

1

Add the category as a property in the object. Then at the end you can use Object.values() to get all the properties as an array.

r should be an object, not an array.

const data = [
    {'id': 1, 'rating': 5, 'rating_category': "a"},
    {'id': 1, 'rating': 1, 'rating_category': "a"},
    {'id': 1, 'rating': 4, 'rating_category': "b"},
    {'id': 1, 'rating': 5, 'rating_category': "b"},
]

const r = {};
data.forEach(e => {
  if (! r[e.rating_category]) {
    r[e.rating_category] = {
      category: e.rating_category,
      rating: e.rating, 
      count: 1      
    };
  } else {    
    r[e.rating_category]['rating'] += e.rating
    r[e.rating_category]['count']++
  }
  
})
console.log(Object.values(r))

answered Jan 12, 2022 at 20:17

Barmar

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3 Comments

Maik Lowrey Over a year ago

Thank you! that is what i looking for!

Kinglish Over a year ago

appears to be missing rating_average

Barmar Over a year ago

His code just creates the rating total. I assume he'll calculate the averages later once he solves the basic problem of grouping the results the way he wants.

mhodges · Accepted Answer · 2022-01-12 20:23:57Z

1

Here's a way you can do it fairly efficiently. Basically the idea is to group the ratings by category first, then calculate the length and average of the ratings for each category and then format your return value, like so:

let data = [
    {'id': 1, 'rating': 5, 'rating_category': "a"},
    {'id': 1, 'rating': 1, 'rating_category': "a"},
    {'id': 1, 'rating': 4, 'rating_category': "b"},
    {'id': 1, 'rating': 5, 'rating_category': "b"},
]
// group the ratings by category
let grouped = data.reduce((res, curr) => {
  if (!res[curr.rating_category]) {
    res[curr.rating_category] = [];
  }
  res[curr.rating_category].push(curr.rating);
  return res;
}, {});
// calculate length & average of each category and format return value
let formatted = Object.entries(grouped).map(([key, value]) => {
  let average = value.reduce((res, curr) => res + curr, 0) / value.length;
  return { category: key, rating_count: value.length, rating_average: average }
});

console.log(formatted)

answered Jan 12, 2022 at 20:23

mhodges

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1 Comment

Maik Lowrey Over a year ago

Thanks. also a good approach!

Nina Scholz · Accepted Answer · 2022-01-12 21:15:42Z

You could take a function for grouping, like the (hopefully) upcoming new prototype Array#groupBy and take the average by using a sum function and return a new object for each group.

Array.prototype.groupBy ??= function (callbackfn, thisArg) {
    const O = Object(this);
    const len = O.length >>> 0;
    if (typeof callbackfn !== 'function') throw new TypeError(callbackfn + ' is not a function');

    let k = 0;
    const groups = {};

    while (k < len) {
        const Pk = Number(k).toString();
        const kValue = O[Pk];
        const propertyKey = callbackfn.call(thisArg, kValue, Number(k), O);
        (groups[propertyKey] ??= []).push(kValue);
        ++k;
    }
    return groups;
};

const
    sum = (array, key) => array.reduce((s, v) => s + (key ? v[key] : v), 0),
    data = [{ id: 1, rating: 5, rating_category: "a" }, { id: 1, rating: 1, rating_category: "a" }, { id: 1, rating: 4, rating_category: "b" }, { id: 1, rating: 5, rating_category: "b" }],
    result = Object
        .entries(data.groupBy(({ rating_category }) => rating_category))
        .map(([category, a]) => ({ category, rating_average: sum(a, 'rating') / a.length, rating_count: a.length }));

console.log(result);

.as-console-wrapper { max-height: 100% !important; top: 0; }

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