Implement groupBy(items, key): given an array of objects and a property name, return an object whose keys are the distinct values of that property and whose values are arrays of the matching objects.
- Groups must appear in the order their key value is first seen.
- Inside each group, objects keep their original order and are printed unchanged.
Constraints
- At most 1000 objects; the input is at most 50000 characters.
- Every object has the given property, and its value is a string of letters (never a number-like string).
Input and output
- Input
- Line 1: the property name Line 2: a JSON array of objects
- Output
- One line: the grouped object as compact JSON.
Examples
Input 1
branch
[{"name":"Asha","branch":"cse"},{"name":"Ravi","branch":"ece"},{"name":"Meena","branch":"cse"}]Output
{"cse":[{"name":"Asha","branch":"cse"},{"name":"Meena","branch":"cse"}],"ece":[{"name":"Ravi","branch":"ece"}]}Groups appear in order of first appearance; items keep their original order.
Input 2
city
[{"id":1,"city":"Pune"}]Output
{"Pune":[{"id":1,"city":"Pune"}]}One item, one group.
Show hints
- reduce with an accumulator object is the classic answer.
- Create the group array the first time you see a value.
Show the explanation
One pass with reduce: for each item, read item[key]; if the accumulator has no group for it yet, create an empty array; push the item.
const grouped = items.reduce((acc, item) => {
(acc[item[key]] ??= []).push(item);
return acc;
}, {});String keys keep insertion order in JavaScript objects, which gives "first seen" order for free. (Integer-like keys such as "2" would be moved to the front, which is why the constraints exclude them; a Map avoids the issue.) Modern runtimes also provide Object.groupBy(items, fn), which does the same thing.
Original UniCircle question. Last reviewed 22 Oct 2026.