JSFiddle - React, Tailwind, and code Playground

JavaScript

function mergeDates(listDates, listItems) {

    for (var i = 0; i < listDates.length; i++) {
        var index = findListIndex(listItems, listDates[i].id);
        if (index != null) listItems[index].savedDate = listDates[i].savedDate;
    }

    return listItems;

}

function findListIndex(listItems, id) {
    for (var i = 0; i < listItems.length; i++) {
        if (listItems[i].id == id) return i;
    }
    return null;
}



var savedItems = [{
    id: "1",
    savedDate: "1111-11-11"
}, {
    id: "2",
    savedDate: "2222-22-22"
}];

var items = [{
    id: "2",
    itemName: "Cookies",
    price: "5"
}, {
    id: "1",
    itemName: "Milk",
    price: "4"
}];

var listWithDate = mergeDates(savedItems, items);

for (var i = 0; i < listWithDate.length; i++) {
    alert(listWithDate[i].savedDate);
}