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;
}

• Keith: Object.values(schools).flat()
• Scott Sauyet: @Keith: beat me to it by seconds.
• Keith: @ScottSauyet Yeah, but your answer gets the cookie,.. Upvote..!

Answer 1

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))

Answer 2

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);

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