将集合转换为具有计数的字符串的映射

function countElements(arr) {
  let map = {};
  for (let i = 0; i < arr.length; i++) {
    if (map[arr[i]]) {
      map[arr[i]]++;
    } else {
      map[arr[i]] = 1;
    }
  }
  
  let result = "";
  for (let key in map) {
    result += key + ":" + map[key] + ",";
  }
  
  return result.slice(0, -1); // 去除最后一个逗号
}

let collection = [1, 2, 3, 2, 1, 3, 3, 4, 5, 4];
let mapping = countElements(collection);
console.log(mapping); // 输出：1:2,2:2,3:3,4:2,5:1