使用ES5计算数组中每个唯一元素的出现次数

function countOccurrences(arr) {
  var counts = {};
  
  for (var i = 0; i < arr.length; i++) {
    var element = arr[i];
    
    if (counts[element]) {
      counts[element]++;
    } else {
      counts[element] = 1;
    }
  }
  
  var result = [];
  
  for (var key in counts) {
    result.push({ element: key, count: counts[key] });
  }
  
  return result;
}

var arr = [1, 2, 3, 2, 1, 3, 4, 5, 4, 4];
var occurrences = countOccurrences(arr);
console.log(occurrences);

[
  { element: '1', count: 2 },
  { element: '2', count: 2 },
  { element: '3', count: 2 },
  { element: '4', count: 3 },
  { element: '5', count: 1 }
]