Single Responsibility

This function does three distinct jobs: fetches, filters, and formats — a maintenance nightmare.

// dirty — one function, three responsibilities
interface User { name: string; age: number; isActive: boolean; }

function solve(rawUsers: User[]) {
  // fetch (simulated here as the input)
  const active = rawUsers.filter(u => u.isActive);
  const seniors = active.filter(u => u.age >= 60);
  return seniors.map(u => u.name.toUpperCase() + ' (' + u.age + ')');
}

Refactor into three functions, each with one job:

• filterActive(users) — keep only active users
• filterSeniors(users) — keep only users aged 60+
• format(users) — format as "NAME (age)" strings

solve(users, 'filterActive') dispatches to the right function.