how to format JSON object array (Javascript)

Question

I am struggling with a tough requirement where I don't know how to deal with

I received one Array as below:

[{
    "student": "21_A_1",
    "sport": "badminton"
}, {
    "student": "21_C_1",
    "sport": "badminton"
}, {
    "student": "21_A_2",
    "sport": "badminton"
}, {
    "student": "21_B_1",
    "sport": "football"
}, {
    "student": "21_A_2",
    "sport": "football"
}]

Requirement: convert it to an array with format :

[{
    "student": "21_A_1",
    "sport": ["badminton"]
}, {
    "student": "21_C_1",
    "sport": ["badminton"]
}, {
    "student": "21_A_2",
    "sport": ["badminton","football"]
}, {
    "student": "21_B_1",
    "sport": ["football"]
}]

I found that lodash library has a feature "group by" returned quite closed result but not as expectation:

_.groupBy(array, 'student');

Result:

{
    "21_A_1": [{
        "student": "21_A_1",
        "sport": "badminton"
    }],
    "21_C_1": [{
        "student": "21_C_1",
        "sport": "badminton"
    }],
    "21_A_2": [{
        "student": "21_A_2",
        "sport": "badminton"
    }, {
        "student": "21_A_2",
        "sport": "football"
    }],
    "21_B_1": [{
        "student": "21_B_1",
        "sport": "football"
    }]
}

Any suggestion is very appreciated.

Better that you construct your own algo than using lodash for this... — Praveen Kumar Purushothaman
– Praveen Kumar Purushothaman, Commented Jun 14, 2018 at 13:43

Nina Scholz · Accepted Answer · 2018-06-14 13:50:05Z

2

You need to map the grouped items with their sport propery and build new objects with the grouping informations.

var array = [{ student: "21_A_1", sport: "badminton" }, { student: "21_C_1", sport: "badminton" }, { student: "21_A_2", sport: "badminton" }, { student: "21_B_1", sport: "football" }, { student: "21_A_2", sport: "football" }],
    grouped = _(array)
        .groupBy('student')
        .map((group, student) => ({ student, sport: _.map(group, 'sport') }))
        .value();

console.log(grouped);

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

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.15.0/lodash.min.js"></script>

answered Jun 14, 2018 at 13:50

Nina Scholz

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1 Comment

Phong Vu Over a year ago

Thanks a lot for quick solutions, all are working, however, I marked this is as answer because I am using lodash. I don't know which one is the best practices, so if you guys have any best practice please keep sharing it. Because the array can be up to thousands items.

Jagdish Idhate · Accepted Answer · 2018-06-14 13:49:58Z

We can use '.reduce' function of array and aggregate result as below

var test = [{
    "student": "21_A_1",
    "sport": "badminton"
}, {
    "student": "21_C_1",
    "sport": "badminton"
}, {
    "student": "21_A_2",
    "sport": "badminton"
}, {
    "student": "21_B_1",
    "sport": "football"
}, {
    "student": "21_A_2",
    "sport": "football"
}]
const result = test.reduce((res,x)=>{
   res[x.student] = res[x.student] || { student:x.student,sport:[]}
   res[x.student].sport.push(x.sport)
   return res;
},{})

console.log(result)

Yosvel Quintero · Accepted Answer · 2018-06-14 13:57:56Z

Using Array.prototype.reduce() combined with Object.values():

const data = [{"student": "21_A_1","sport": "badminton"}, {"student": "21_C_1","sport": "badminton"}, {"student": "21_A_2","sport": "badminton"}, {"student": "21_B_1","sport": "football"}, {"student": "21_A_2","sport": "football"}];
const result = Object.values(
  data.reduce((a, c) => ((a[c.student] = a[c.student] || {student: c.student, sport: []}).sport.push(c.sport), a), {})
);

console.log(result);

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