如何根据id和数量统计数组中的项目总数？

function countProjectsByIdAndQuantity(arr) {
  var idQuantityDict = {};
  var totalProjects = 0;
  
  for (var i = 0; i < arr.length; i++) {
    var id = arr[i].id;
    var quantity = arr[i].quantity;
    
    if (id in idQuantityDict) {
      idQuantityDict[id] += quantity;
    } else {
      idQuantityDict[id] = quantity;
    }
    
    totalProjects += quantity;
  }
  
  console.log("每个id对应的总数量：", idQuantityDict);
  console.log("所有项目的总数量：", totalProjects);
}

// 示例数据
var projects = [
  { id: 1, quantity: 5 },
  { id: 2, quantity: 3 },
  { id: 1, quantity: 2 },
  { id: 3, quantity: 7 },
  { id: 2, quantity: 4 }
];

countProjectsByIdAndQuantity(projects);