字符串中字母的出现次数

Question

function countLetters(str) {
  const counts = {};
  for (let i = 0; i < str.length; i++) {
    const char = str.charAt(i).toLowerCase();
    if (/[a-z]/.test(char)) {
      counts[char] = (counts[char] || 0) + 1;
    }
  }
  return counts;
}

const str = 'Hello, world!';
const counts = countLetters(str);