Day 31 of 100: Sets and Maps — Deduplication and Lookups

Day 31 of 100 — 100 Days of Playwright with TypeScript Challenge. Phase: JavaScript Fundamentals. JavaScript (Days 1-33) → TypeScript (Days 34-63) → Playwright (Days 64-96) → AI Framework Capstone (Days 97-100).

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Sets store unique values. Maps store key-value pairs with any key type. Both solve real test problems arrays handle badly.

Set — Uniqueness for Free

const ids = [101, 102, 101, 103, 102];
const unique = [...new Set(ids)];        // [101, 102, 103]

// QA check: duplicate detection
const hasDuplicates = new Set(ids).size !== ids.length;  // true

const seen = new Set();
seen.add('test-1');
seen.has('test-1');   // true
seen.size;            // 1

Map — Better Than Object for Dynamic Keys

const durations = new Map();
durations.set('login.spec.ts', 1240);
durations.set('checkout.spec.ts', 3580);

durations.get('login.spec.ts');   // 1240
durations.has('admin.spec.ts');   // false

// Iterate in insertion order
for (const [file, ms] of durations) {
  console.log(`${file}: ${ms}ms`);
}

// Count failures by status — classic aggregation
const byStatus = new Map();
for (const s of [200, 404, 200, 500, 404]) {
  byStatus.set(s, (byStatus.get(s) || 0) + 1);
}
// Map { 200 => 2, 404 => 2, 500 => 1 }

When to Use What

• Set: dedup lists, track visited/seen items
• Map: counters, lookups with non-string keys, ordered key-value data
• Plain object: fixed known keys (configs, payloads)

Tomorrow (Day 32): Optional chaining and nullish coalescing — crash-proof access.