我需要获取一个字符串，并输出该字符串中出现次数最多的单词

function getMostFrequentWord(str) {
  // 将字符串转换为小写
  str = str.toLowerCase();
  
  // 去除标点符号和特殊字符，只保留字母和空格
  str = str.replace(/[^a-zA-Z ]/g, "");
  
  // 将字符串分割成单词数组
  var words = str.split(" ");
  
  // 统计每个单词出现的次数
  var wordCount = {};
  for (var i = 0; i < words.length; i++) {
    var word = words[i];
    if (wordCount[word]) {
      wordCount[word]++;
    } else {
      wordCount[word] = 1;
    }
  }
  
  // 找出出现次数最多的单词
  var maxCount = 0;
  var mostFrequentWord = "";
  for (var word in wordCount) {
    if (wordCount[word] > maxCount) {
      maxCount = wordCount[word];
      mostFrequentWord = word;
    }
  }
  
  return mostFrequentWord;
}

var inputString = "This is a test string. This string is just a test.";
var mostFrequent = getMostFrequentWord(inputString);
console.log("出现次数最多的单词是：" + mostFrequent);