Group an Array of Objects by a Key

📌 Definition:

📖 Detailed Explanation:

Use reduce to build an object whose keys are the group values and whose values are arrays. Modern engines also offer Object.groupBy, but reduce is universally supported.

💻 Solutions:

• JavaScript

function groupBy(arr, key) {
  return arr.reduce((acc, item) => {
    const k = item[key];
    (acc[k] ||= []).push(item);
    return acc;
  }, {});
}
// Usage: groupBy(users, 'role') -> { admin: [...], qa: [...] }

🔑 Key Points:

• reduce builds a keyed object of arrays
• (acc[k] ||= []) initializes the bucket if absent
• Object.groupBy is the newer built-in (check support)
• O(n) single pass

🌍 Real-World Example: