Refactor double for loop in Javascript

Question

Is it possible to reduce the number of for loops to just one in this case? I need to group an object based on 'grade' property, no matter the data type of the output, even doesn't matter if your solution force me to start over again. I don't know much about algorithms for now so I need your help to improve my code because I am not allowed to use more than one for loop in a single block of code.

var schools = {
    'SCHOOL_1':
    [
        // girls
        {id: '1', school: 'SCHOOL_1', grade: 'A', message: 'Congratulations!'},
        {id: '2', school: 'SCHOOL_1', grade: 'A', message: 'Good work!'},
        {id: '3', school: 'SCHOOL_1', grade: 'A', message: 'Ok'},
        // boys
        {id: '4', school: 'SCHOOL_1', grade: 'A', message: 'Congratulations!'},
        {id: '5', school: 'SCHOOL_1', grade: 'B', message: 'Good work!'},
        {id: '6', school: 'SCHOOL_1', grade: 'B', message: 'Good work!'},
        {id: '7', school: 'SCHOOL_1', grade: 'A', message: 'Congratulations!'},
        {id: '8', school: 'SCHOOL_1', grade: 'B', message: 'Good work!'},

    ],
    'SCHOOL_2':
    [
        // girls
        {id: '9', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'},
        {id: '10', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'},
        {id: '11', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'},
        {id: '12', school: 'SCHOOL_2', grade: 'B', message: 'Good work!'},
        {id: '13', school: 'SCHOOL_2', grade: 'B', message: 'Nice!'},
        // boys
        {id: '14', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'},
        {id: '15', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'},
        {id: '16', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'},
        {id: '17', school: 'SCHOOL_2', grade: 'B', message: 'Congratulations!'},
    ]
}

var result = [];

for (let school in schools) {
    // Group students by grade
    let studentsByGrade = GroupByProperty(schools[school], 'grade');
    // Second loop (I NEED TE REFACTOR THIS)
    for (let grade in studentsByGrade) {
        if (!result[grade]) result[grade] = [];
        result[grade].push(studentsByGrade[grade]);
    }
}

console.log('result',result);

function GroupByProperty(objectArray, property) {
    let result = objectArray.reduce((acc, obj) => {
       var key = obj[property];
       if (!acc[key]) acc[key] = [];
       acc[key].push(obj);
       return acc;
    }, {});

    return result;
}

@ScottSauyet Yeah, but your answer gets the cookie,.. Upvote..! — Keith
– Keith, Commented Nov 20, 2020 at 13:33

Scott Sauyet · Accepted Answer · 2020-11-20 13:32:04Z

By combining Object.values and Array.prototype.flat, you can turn this object into a single list of objects.

This would do it:

function GroupByProperty(objectArray, property) {
    let result = objectArray.reduce((acc, obj) => {
       var key = obj[property];
       if (!acc[key]) acc[key] = [];
       acc[key].push(obj);
       return acc;
    }, {});

    return result;
}

const byGrades = (schools) => 
  GroupByProperty (Object .values (schools) .flat(), 'grade')

var schools = {'SCHOOL_1': [{id: '1', school: 'SCHOOL_1', grade: 'A', message: 'Congratulations!'}, {id: '2', school: 'SCHOOL_1', grade: 'A', message: 'Good work!'}, {id: '3', school: 'SCHOOL_1', grade: 'A', message: 'Ok'}, {id: '4', school: 'SCHOOL_1', grade: 'A', message: 'Congratulations!'}, {id: '5', school: 'SCHOOL_1', grade: 'B', message: 'Good work!'}, {id: '6', school: 'SCHOOL_1', grade: 'B', message: 'Good work!'}, {id: '7', school: 'SCHOOL_1', grade: 'A', message: 'Congratulations!'}, {id: '8', school: 'SCHOOL_1', grade: 'B', message: 'Good work!'}], 'SCHOOL_2': [{id: '9', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'}, {id: '10', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'}, {id: '11', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'}, {id: '12', school: 'SCHOOL_2', grade: 'B', message: 'Good work!'}, {id: '13', school: 'SCHOOL_2', grade: 'B', message: 'Nice!'}, {id: '14', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'}, {id: '15', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'}, {id: '16', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'}, {id: '17', school: 'SCHOOL_2', grade: 'B', message: 'Congratulations!'}]}

console .log (byGrades (schools))

pilchard · Accepted Answer · 2020-11-20 13:37:20Z

Compact solution using Object.values() flat() and reduce() implemented with destructuring and making use of the nullish coalescing operator.

const bygrade = Object.values(schools)
   .flat()
   .reduce((acc, o) => 
      (acc[o.grade] = [...acc[o.grade] ?? [], {...o}], acc), {});

var schools = {
    'SCHOOL_1':
    [
        // girls
        {id: '1', school: 'SCHOOL_1', grade: 'A', message: 'Congratulations!'},
        {id: '2', school: 'SCHOOL_1', grade: 'A', message: 'Good work!'},
        {id: '3', school: 'SCHOOL_1', grade: 'A', message: 'Ok'},
        // boys
        {id: '4', school: 'SCHOOL_1', grade: 'A', message: 'Congratulations!'},
        {id: '5', school: 'SCHOOL_1', grade: 'B', message: 'Good work!'},
        {id: '6', school: 'SCHOOL_1', grade: 'B', message: 'Good work!'},
        {id: '7', school: 'SCHOOL_1', grade: 'A', message: 'Congratulations!'},
        {id: '8', school: 'SCHOOL_1', grade: 'B', message: 'Good work!'},

    ],
    'SCHOOL_2':
    [
        // girls
        {id: '9', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'},
        {id: '10', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'},
        {id: '11', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'},
        {id: '12', school: 'SCHOOL_2', grade: 'B', message: 'Good work!'},
        {id: '13', school: 'SCHOOL_2', grade: 'B', message: 'Nice!'},
        // boys
        {id: '14', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'},
        {id: '15', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'},
        {id: '16', school: 'SCHOOL_2', grade: 'A', message: 'Congratulations!'},
        {id: '17', school: 'SCHOOL_2', grade: 'B', message: 'Congratulations!'},
    ]
}

const bygrade = Object.values(schools).flat().reduce((acc, o) => (acc[o.grade] = [...acc[o.grade] ?? [], {...o}], acc), {});

console.log(bygrade);

Yes, although the OP already had a more generic equivalent of your reduce call in the question (GroupByProperty).

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