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Learn JavaScript

Build the Functionality

Explore how to build essential JavaScript functions to create the functionality of a budget tracker app. Learn to manage income and expenses with reusable code, structure your program logically, and implement a simple command-line interface for interactive use. This lesson guides you step-by-step in coding functions, organizing your application, and testing your code for a working budget tracking solution.

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. ...