使用JavaScript，如何从一个句子中获得X次Y字母计数？

function countLetter(sentence, letter) {
  // 将句子转换为小写，以便不区分大小写
  const lowerCaseSentence = sentence.toLowerCase();
  
  // 初始化计数器
  let count = 0;
  
  // 遍历句子中的每个字符
  for (let i = 0; i < lowerCaseSentence.length; i++) {
    // 如果当前字符与目标字母匹配，则增加计数器
    if (lowerCaseSentence[i] === letter.toLowerCase()) {
      count++;
    }
  }
  
  return count;
}

// 示例用法
const sentence = "This is a sample sentence.";
const letter = "s";
const count = countLetter(sentence, letter);
console.log(`句子中字母"${letter}"出现了${count}次。`);