Search⌘ K
AI Features
Home
Courses
Learn JavaScript

Build the Functionality

Explore how to build the core functionality of a JavaScript Budget Tracker by coding key functions like addIncome and addExpense. Learn to organize your code with variables, reusable functions, and an execution flow. Use AI assistance to scaffold and test additional app features step-by-step.

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. ...