访问属性不存在-了解JavaScript中的对象？

Question

最近我开始学习javascript，直到遇到无法理解的问题..

说的那部分

arr[lastItem].id

（.id）的含义是什么？有人可以帮助我了解这一点吗。.谢谢您。

// ---- BUDGET CONTROLLER MODEL
var budgetController = (function() {

    // Expenses Constructor
    var Expense = function(id, description, value) {
        this.id = id;
        this.description = description;
        this.value = value;
    };

    // Income Constructor
    var Income = function(id, description, value) {
        this.id = id;
        this.description = description;
        this.value = value;
    };

    // Data Structure
    var data = {
        allItems: {
            expense: [],
            income: []
        },
        totals: {
            expense: 0,
            income: 0
        }
    };

    return {
        addItem: function(type, des, val) {
            var newItem;
            var ID;
            var arr = data.allItems[type];
            var len = arr.length;
            var lastItem = len - 1; // last item index (position)

            // Create & check ID 
            ID = len > 0 ? arr[lastItem].id + 1 : 0; // <--- what's (.id) mean ???

            // Create new item based on expense or income
            if (type === 'expense') {
                newItem = new Expense(ID, des, val);
            } else if (type === 'income') {
                newItem = new Income(ID, des, val);
            }

            // Store the new item
            data.allItems[type].push(newItem);

            // Return the new item
            return newItem;
        },
    };

})();

完整的代码可以在这里找到 https://codepen.io/safi2266/pen/JQeBQp

app.js https://codepen.io/safi2266/pen/JQeBQp.js