Encapsulate Conditionals

Complex boolean expressions in if statements hide intent.

// dirty — what does this check?
interface User { age: number; isActive: boolean; isBanned: boolean; emailVerified: boolean; }

function solve(user: User): string {
  if (user.age >= 18 && user.isActive && !user.isBanned && user.emailVerified) {
    return 'eligible';
  }
  return 'not-eligible';
}

Refactor by extracting the condition into a named predicate isEligible(user).
solve(user, 'isEligible') → true/false · solve(user, 'getStatus') → 'eligible'/'not-eligible'