TS

// This is fully type-checked, not just stripped. Break a type on
// purpose and the error appears on the right before it runs.

type Level = 'basic' | 'intermediate' | 'advanced' | 'expert';

interface Learner {
  name: string;
  score: number;
  level: Level;
}

const learners: Learner[] = [
  { name: 'Aisha', score: 92, level: 'advanced' },
  { name: 'Kenji', score: 78, level: 'intermediate' },
  { name: 'Lena',  score: 85, level: 'expert' },
];

// The return type is inferred as Learner[] - hover-free, but real.
function passing(people: Learner[], mark: number) {
  return people.filter((p) => p.score >= mark);
}

console.log(passing(learners, 80).map((l) => l.name));

// Try any of these and watch the type error appear:
//   const n: number = 'not a number';
//   passing(learners, 'eighty');
//   learners[0].level = 'beginner';

const byLevel: Record<Level, number> = {
  basic: 0, intermediate: 0, advanced: 0, expert: 0,
};
for (const l of learners) byLevel[l.level] += 1;
console.log(byLevel);

Output

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