Hash Maps

Hash Maps (objects in JavaScript) store key-value pairs for O(1) lookup: instant access regardless of size. Use them for counting, grouping, and caching.

// Count character frequency
function charCount(str: string): Record<string, number> {
  const count: Record<string, number> = {}

  for (const char of str) {
    count[char] = (count[char] || 0) + 1
  }

  return count
}

charCount("hello")
// { h: 1, e: 1, l: 2, o: 1 }

// Find first non-repeating character
function firstUnique(str: string): string {
  const count = charCount(str)
  
  for (const char of str) {
    if (count[char] === 1) return char
  }
  
  return ""
}