Hashing and maps in JavaScript

Why it matters

Maps and sets help with frequency counting, lookups, and grouping.

Problems to practice

• Valid Anagram
• Top K Frequent Elements
• Longest Consecutive Sequence
• Subarray Sum Equals K
• First Non-Repeating Character

Example: frequency map

const freq = new Map();
for (const ch of str) {
  freq.set(ch, (freq.get(ch) || 0) + 1);
}

Interview guidance

Map preserves insertion order and is often better than plain objects for dynamic keys.