How can I access this property in my dictionary in js?

I thought I understood how to loop through a dictionary, but my loop is wrong. I try to access the name of each sub item but my code does not work.
Here is what I did:

list = [
      {
        title: 'Groceries',
        items: [
          {
            id: 4,
            title: 'Food',
            cost: 540 ,
          },
          {
            id: 5,
            title: 'Hygiene',
            cost: 235,
          },
          {
            id: 6,
            title: 'Other',
            cost: 20,
          },
        ],
      }];

function calculateCost(){
let total = 0;
Object.keys(list).forEach((k) => { for (i in k.items) { total += i.data; } });
console.log(total);
return total;
}

Your list is an array includes 1 object and this object has two properties title and items the items here is an array of objects each one of these objects has property cost so to calculate the total cost you need to loop through items array, here is how you do it:

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let list = [
      {
        title: 'Groceries',
        items: [
          {
            id: 4,
            title: 'Food',
            cost: 540 ,
          },
          {
            id: 5,
            title: 'Hygiene',
            cost: 235,
          },
          {
            id: 6,
            title: 'Other',
            cost: 20,
          },
        ],
      }];

function calculateCost(){
let total = 0;
  list[0].items.forEach(el => {
    total += el.cost;
  })
  console.log(total)
  return total;
}

calculateCost();

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Your list is an Array, not an Object.
Instead of Object.keys() use Array.prototype.reduce:

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const calculateCost = (arr) => arr.reduce((tot, ob) =>
  ob.items.reduce((sum, item) => sum + item.cost, tot), 0);

const list = [
  {
    title: 'Groceries',
    items: [
      {id: 4, title: 'Food', cost: 10},
      {id: 5, title: 'Hygiene', cost: 20},
      {id: 6, title: 'Other', cost: 30}
    ]
  }, {
    title: 'Other',
    items: [
      {id: 8, title: 'Scuba gear', cost: 39}
    ],
  }
];

console.log(calculateCost(list));  // 99

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Expanding on @Roko's and @mmh4all's answers, the following code adds several verification statements to handle cases where a deeply nested property in your data is not what you expect it to be.

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const calculateCost = (orders) => {
let listOfCosts = [];
// For each 'order' object in the 'orders' array, 
// add the value of the 'cost' property of each item 
// in the order to 'listOfCosts' array.
orders.forEach(order => {
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray
if (!Array.isArray(order.items)) { return; }
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/isNaN
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/parseFloat
const orderCostArr = order.items.map(item =>
isNaN(item.cost) ? 0 : parseFloat(item.cost, 10));
if (orderCostArr.length === 0) { return; }
// Concatenate 'orderCostArr' to the 'listOfCosts' array
//listOfCosts = listOfCosts.concat(orderCostArry);
// Alternate approach is to use the spread syntax (...) to
// push the items in the array returned by 'order.items.map()'
// into the 'listOfCosts' array.
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax
listOfCosts.push(...orderCostArr);
});
// Use the 'reduce' method on the 'listOfCosts' array
// to get the total cost.
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce
const totalCost = listOfCosts.reduce(
(accumulator, currentValue) => accumulator + currentValue, 0);
return totalCost;
};
const list = [
{
title: 'Groceries',
items: [
{ id: 4, title: 'Food', cost: 10 },
{ id: 3, title: 'Baked goods', cost: 20 },
{ id: 5, title: 'Hygiene', cost: 0 },
{ id: 6, title: 'Other' }
]
}, {
title: 'Gear',
items: {},
}, {
title: 'Accessories',
items: [],
}, {
title: 'Bags',
}, {
title: 'Other',
items: [
{ id: 10, title: 'Scuba gear', cost: "5" },
{ id: 8, title: 'Scuba gear', cost: "err" },
{ id: 9, title: 'Scuba gear', cost: 59 }
],
}
];
console.log(calculateCost(list));  // 94

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