如何创建对象数组中数量最多的项目的对象图？

Question

New to JavaScript and this is my first post. So I have this array of objects:

  var allOrders = [
        {
            id: 1,
            order: [
                { itemName: "Shoes", quantity: 1 },
                { itemName: "Socks", quantity: 3 },
                { itemName: "Sandals", quantity: 5 },
                { itemName: "Shirts", quantity: 1 },
            ],
        },
        {
            id: 2,
            order: [
                { itemName: "Hats", quantity: 1 },
                { itemName: "Shoes", quantity: 3 },
                { itemName: "Sandals", quantity: 5 },
                { itemName: "Shirts", quantity: 1 },
            ],
        },
 ]

I have done a nested loop to access all the itemName and quantity.

for (let i = 0; i < allOrders.length; i++) {
  let orders = allOrders[i].order

  for (let j = 0; j < orders.length; j++) {
    let itemName = orders[j].itemName;
    let quantity = orders[j].quantity;
  }
}

What I want to do is return some sort of object mapper that will count the total number of quantity for each unique item in descending order like this

{
   Sandals: 10,
   Shoes: 4,
   Socks: 3,
   Shirts: 3,
   Hats: 1
}

Thank you!

Answer 1

Reduce into an object indexed by itemName , then sort its entries to create a sorted object using Object.fromEntries :

 var allOrders = [ { id: 1, order: [ { itemName: "Shoes", quantity: 1 }, { itemName: "Socks", quantity: 3 }, { itemName: "Sandals", quantity: 5 }, { itemName: "Shirts", quantity: 1 }, ], }, { id: 2, order: [ { itemName: "Hats", quantity: 1 }, { itemName: "Shoes", quantity: 3 }, { itemName: "Sandals", quantity: 5 }, { itemName: "Shirts", quantity: 1 }, ], }, ]; const unordered = allOrders.reduce((a, { order }) => { order.forEach(({ itemName, quantity }) => { a[itemName] = (a[itemName] || 0) + quantity; }); return a; }, {}); const ordered = Object.fromEntries( Object.entries(unordered).sort( (a, b) => b[1] - a[1] ) ); console.log(ordered); 

Do keep in mind that object property iteration order is only guaranteed in ES6+ environments if you use certain iteration methods , like Object.getOwnPropertyNames , or Reflect.ownKeys . (If you use other methods like for..in or Object.keys or Object.entries , the order of the keys is implementation-defined)