Learn JavaScript in the Age of AI/

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Build the Functionality

Build the Functionality

Create the core features of the “Budget Tracker” project in this lesson.

We'll cover the following...

In the previous lesson, you:

  • Chatted with a simulated client and gathered needs.

  • Converted notes into functional requirements.

  • Created a clean skeleton of eight function stubs.

Next, we’ll code the two core functions, scaffold the rest with AI, and add tests plus a reference solution.

We’ll keep a tiny in-memory data model:

// Monthly income (one number we can update)
let income = 0.0;
// A growing list of expenses
// Each entry: { category: string, amount: number }
let expenses = [];
// Allowed categories for expense logging
const allowedCategories = [
  "Food", "Rent", "Transport", "Entertainment",
  "Utilities", "Health", "Education", "Other"
];

We’ll use the following conventions:

  • Use positive amounts only.

  • Use arrays and objects (no external libraries or file I/O).

  • Keep functions small and reusable.

Build the basics

We’ll implement addIncome and addExpense first. These are the heartbeat of the app; everything else plugs into them.

addIncome(amount)

Before we start coding our first function, let’s take a look at how ...