Exercise 2: Group By (category)

Problem Statement

Given products: `{id, category}`, function `groupByCategory(items)` banao jo category wise grouped object return kare.

Sample Output:

groupByCategory([{id:1,category:"laptop"},{id:2,category:"mobile"},{id:3,category:"laptop"}])
=> {laptop: [{id:1,category:"laptop"},{id:3,category:"laptop"}], mobile: [{id:2,category:"mobile"}]}

Solution

const groupByCategory = (items) =>
  items.reduce((acc, it) => {
    (acc[it.category] ??= []).push(it);
    return acc;
  }, {});

Explanation

Overall goal:

Input: items (array of objects) jisme har item ke andar category field hai.
Output: ek object { [category]: [items...] } jaha har key ek category hai.

Line 1: function header

const groupByCategory = (items) => → function ka naam clearly batata hai ki hum category ke basis par group karenge.
items parameter: ye array of products / objects hai.

Line 2: reduce start

items.reduce((acc, it) => { ... }, {});
reduce ka kaam: pure array ko traverse karke ek single result banana.
acc (accumulator) → yaha final object build ho raha hai.
it (item) → current element from items.
Last argument {} → initial value, yani start me acc = {} (empty object).

Line 3: acc[it.category] ??= []

it.category → current item ki category, jaise "laptop" ya "mobile".
acc[it.category] → us category ke liye abhi tak jo array bana hai.
??= operator:
Agar acc[it.category] null/undefined hai → usko [] (empty array) bana do.
Agar already array hai → usko as‑is chhod do.
Is line ka matlab: “agar is category ke liye array nahi bana, to ab bana do.”

Line 4: .push(it)

.push(it) → ab current item it ko us category wale array me add kar rahe hain.
Ab acc kuch aisa dikh sakta hai:
{ laptop: [/* laptops */], mobile: [/* mobiles */] }

Line 5: return acc

return acc; → reduce ke har step ke baad updated accumulator wapas bhejna zaroori hai.
Next item ke liye ye hi acc starting point banega.

Final output:

Loop khatam hone par reduce complete object return karega, jo hum function se return kar dete hain.

Real world:

Product listing pages (category wise cards).
Blog posts ko topic ya author ke group me dikhana.
Analytics me data ko group karna (status, role, type).

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Exercise 3