dataset.json

[
    {
       "category": "C",
       "price": 539.0772150437717
    },
    {
       "category": "B",
       "price": 743.6677612263602
    },
    {
       "category": "A",
       "price": 275.2065639795811
    },
    {
       "category": "B",
       "price": 912.7750179381753
    },
    {
       "category": "C",
       "price": 416.7595315030992
    },
    {
       "category": "C",
       "price": 891.8110658837174
    },
    {
       "category": "C",
       "price": 138.9733737849883
    },
    {
       "category": "C",
       "price": 596.8529572177353
    },
    {
       "category": "C",
       "price": 158.57450856771948
    },
    {
       "category": "C",
       "price": 706.449897751821
    },
    {
       "category": "B",
       "price": 382.21079583645735
    },
    {
       "category": "C",
       "price": 650.9998017936671
    },
    {
       "category": "B",
       "price": 809.8615405964318
    },
    {
       "category": "A",
       "price": 957.5352782964179
    },
    {
       "category": "A",
       "price": 773.41642359315
    },
    {
       "category": "B",
       "price": 729.8269145774594
    },
    {
       "category": "C",
       "price": 116.13727979157717
    },
    {
       "category": "B",
       "price": 214.61693437388485
    },
    {
       "category": "A",
       "price": 286.6148036611407
    },
    {
       "category": "C",
       "price": 798.6981083990146
    },
    {
       "category": "B",
       "price": 575.3016729126446
    },
    {
       "category": "C",
       "price": 545.8248639019607
    },
    {
       "category": "B",
       "price": 526.5525069963899
    },
    {
       "category": "B",
       "price": 978.8372701271397
    },
    {
       "category": "A",
       "price": 680.1551976430187
    },
    {
       "category": "B",
       "price": 354.89468096599296
    },
    {
       "category": "A",
       "price": 567.5869633266133
    },
    {
       "category": "C",
       "price": 944.8486361405011
    },
    {
       "category": "B",
       "price": 302.22437625980024
    },
    {
       "category": "B",
       "price": 357.2259674369997
    },
    {
       "category": "B",
       "price": 428.92786312710143
    },
    {
       "category": "C",
       "price": 447.8332985153
    },
    {
       "category": "A",
       "price": 38.64510777757335
    },
    {
       "category": "B",
       "price": 783.6139947695449
    },
    {
       "category": "B",
       "price": 908.5522126582788
    },
    {
       "category": "B",
       "price": 429.51677739937446
    },
    {
       "category": "C",
       "price": 602.5169068896299
    },
    {
       "category": "A",
       "price": 52.22328414623001
    },
    {
       "category": "C",
       "price": 330.15414877554906
    },
    {
       "category": "C",
       "price": 449.21646275320626
    },
    {
       "category": "B",
       "price": 895.6771000924999
    },
    {
       "category": "A",
       "price": 794.3466005297684
    },
    {
       "category": "A",
       "price": 609.0891839643986
    },
    {
       "category": "C",
       "price": 463.32623614025306
    },
    {
       "category": "B",
       "price": 377.3541793886579
    },
    {
       "category": "A",
       "price": 137.92758111392266
    },
    {
       "category": "A",
       "price": 530.1171121336716
    },
    {
       "category": "A",
       "price": 697.1259364020708
    },
    {
       "category": "A",
       "price": 43.06132777960636
    },
    {
       "category": "A",
       "price": 418.045061940556
    },
    {
       "category": "B",
       "price": 30.008411006383405
    },
    {
       "category": "A",
       "price": 573.9415549108637
    },
    {
       "category": "C",
       "price": 738.7188581934994
    },
    {
       "category": "B",
       "price": 309.510529350725
    },
    {
       "category": "A",
       "price": 345.7877707437882
    },
    {
       "category": "A",
       "price": 506.571759259907
    },
    {
       "category": "B",
       "price": 974.2188335208195
    },
    {
       "category": "B",
       "price": 743.348573151069
    },
    {
       "category": "C",
       "price": 979.7731595950072
    },
    {
       "category": "A",
       "price": 746.9749896679157
    },
    {
       "category": "C",
       "price": 624.0546579285733
    },
    {
       "category": "B",
       "price": 780.7058331846932
    },
    {
       "category": "A",
       "price": 983.6347277285857
    },
    {
       "category": "C",
       "price": 297.2411390762917
    },
    {
       "category": "A",
       "price": 296.9615689686642
    },
    {
       "category": "B",
       "price": 633.4689361334154
    },
    {
       "category": "A",
       "price": 859.7382589815377
    },
    {
       "category": "A",
       "price": 791.9271210992722
    },
    {
       "category": "B",
       "price": 264.84403806692455
    },
    {
       "category": "C",
       "price": 899.0029535397846
    },
    {
       "category": "C",
       "price": 283.80731457448974
    },
    {
       "category": "B",
       "price": 260.44876675687533
    },
    {
       "category": "A",
       "price": 779.9125567757485
    },
    {
       "category": "C",
       "price": 358.02406008792076
    },
    {
       "category": "B",
       "price": 854.2759287339275
    },
    {
       "category": "B",
       "price": 301.42057155898794
    },
    {
       "category": "A",
       "price": 102.26667985240034
    },
    {
       "category": "C",
       "price": 259.7004047505851
    },
    {
       "category": "C",
       "price": 816.3981645511957
    },
    {
       "category": "B",
       "price": 149.44393905611176
    },
    {
       "category": "A",
       "price": 589.4355309019919
    },
    {
       "category": "A",
       "price": 496.6274360942371
    },
    {
       "category": "A",
       "price": 4.907893026423249
    },
    {
       "category": "C",
       "price": 714.1830057520871
    },
    {
       "category": "C",
       "price": 797.9285362755708
    },
    {
       "category": "A",
       "price": 102.70547464537749
    },
    {
       "category": "C",
       "price": 339.77234358701213
    },
    {
       "category": "A",
       "price": 39.251131169554476
    },
    {
       "category": "C",
       "price": 304.1441021186415
    },
    {
       "category": "C",
       "price": 811.1514293231301
    },
    {
       "category": "B",
       "price": 203.3173392174339
    },
    {
       "category": "A",
       "price": 520.6830106151288
    },
    {
       "category": "B",
       "price": 475.55166365638837
    },
    {
       "category": "B",
       "price": 556.6855071812453
    },
    {
       "category": "C",
       "price": 973.4023443610911
    },
    {
       "category": "A",
       "price": 610.2753003273295
    },
    {
       "category": "A",
       "price": 891.4954601272102
    },
    {
       "category": "A",
       "price": 258.2465008424231
    },
    {
       "category": "A",
       "price": 864.8811397903327
    },
    {
       "category": "A",
       "price": 489.0261788425223
    }
 ]

group-map-reduce.js

function groupMapReduce2(groupFunction){
    return function mapFunction(mapFn) {
        return function reduceFunction(reduceFn) {
            return function run(collection) {

            }
        }
    }
}


const groupMapReduce1 = groupFn => mapFn => reduceFn => collection => {

}

function groupBy(collection, keyFunction) {
    let acc = {};
    collection.forEach(currentElement => {
        const group = keyFunction(currentElement);
        const currentListForGroup = acc[group] || [];
        const listUpdate = [...currentListForGroup, currentElement];
        acc[group] = listUpdate
    });

    return acc;
}

function groupByUsingReduce(collection, keyFunction) {
    const initialValue = {};
    return collection.reduce((acc, currentElement) => {
      const group = keyFunction(currentElement);
      const currentListForGroup = acc[group] || [];
      const listUpdate = [...currentListForGroup, currentElement];
  
      return Object.assign({}, acc, {
        [group]: listUpdate
      });
    }, initialValue);
  }

function mapValues(object, transformCollection) {
    const entries = Object.entries(object); 
    /**
     * [
     *  [A, [products...]],
     *  [B, [products...]]
     * ]
     */

    return entries.map(entry => {
        const [key, values] = entry;
        const mappedValues = transformCollection(values); // [prices ...]
        return {
            key,
            values: mappedValues
        }
    })
    .reduce((acc, currentElement) => {
        const {key, values} = currentElement;
        return Object.assign({}, acc, {
            [key]: values
        });
    }, {});
}

const groupMapReduce = (collection, groupFn, mapFn, reduceFn) => {
    /**
     * [products...]
     * group
     * {A: [product, p...], B: [...]}
     * map
     * {A: [prices...], B: [prices...]}
     * reduce
     * {A: averageOfA, B: averageOfB, ...}
     */
    const groups = groupByUsingReduce(collection, groupFn); 
    const mapValuesTransform = elements => elements.map(mapFn);
    const mappedGroups = mapValues(groups, mapValuesTransform);
    const reduceGroups = mapValues(mappedGroups, reduceFn);

    return reduceGroups;
}



const products = [
    {
        category: "A",
        price: 10.2
    },
    {
        category: "B",
        price: 10.2
    },
    {
        category: "C",
        price: 10.2
    }
];


const getProductCategory = product => product.category;
const getProductPrice = product => product.price;
const calculateAverage = products => { // [10.2, 3.1, 4.5]
    const initialValue = 0;   
    const length =  products.length;

    /**
     * += 
     * 1. assignacion => acc =
     * 2. retorno => suma acc + current
     */

    const sum = products.reduce((acc, element) => {
        return acc + element
    }, initialValue)

    /**
     * [10.2, 3.1, 4.5]
     * 
     * 1. acc: 0, element: 10.2
     * 2. acc: 10.2 element: 3.1
     * 3. acc: 13.3 element: 4.5
     * result = 17.8
     */

    const average = sum / length;
    return average;

}

const categoryAverages = groupMapReduce(
    dataset,
    getProductCategory, // group
    getProductPrice,  // map
    calculateAverage // reduce
);

const categoryCounts = groupMapReduce(
    dataset,
    getProductCategory,
    () => 1,
    products => products.reduce((acc, current) => acc + current, 0)
);

/**
 * {
 *  CategoryName: averageOfCategory[Number],
 *  
 * }
 */

console.log(JSON.stringify({categoryAverages, categoryCounts}, null, 3));