Overall Goal:

• Users array se role-wise count banana.
• Output: {admin: 2, user: 5} format.

Solution:

• const countByRole = (users) => users.reduce((acc, u) => { acc[u.role] = (acc[u.role] ?? 0) + 1; return acc; }, {});

Part 1: Reduce setup

• users.reduce((acc, u) => {...}, {})
• reduce → array ko traverse karke single object banana.
• {} → initial value (empty object).
• acc → accumulator (final object build ho raha hai).
• u → current user from array.

Part 2: Count logic

• acc[u.role] = (acc[u.role] ?? 0) + 1;
• u.role → current user ki role (e.g., "admin", "user").
• acc[u.role] ?? 0 → nullish coalescing:
• Agar role pehli baar aayi → undefined → 0 use karo.
• Agar role pehle se hai → existing count use karo.
• + 1 → count increment karo.
• acc[u.role] = ... → updated count store karo.

Part 3: Return accumulator

• return acc; → updated accumulator return (next iteration ke liye).

Example:

• Input: [{role: "admin"}, {role: "user"}, {role: "admin"}]
• Step 1: {admin: 1}
• Step 2: {admin: 1, user: 1}
• Step 3: {admin: 2, user: 1}

Real world:

• Dashboards: user statistics display.
• Analytics: category-wise counts.
• Reports: group by operations.