Creating New Project Files

In this unit, you'll explore how Codex can help you create entirely new files and populate them with useful content — just by describing what you want. This includes writing new TypeScript files, generating code from scratch, and setting up files such as unit tests.

This hands-on unit focuses on expanding your workflow to include file generation and scaffolding, making Codex a powerful tool not just for editing — but also for building.

Before we dive into generating test files, let's briefly cover Jest — the testing framework you'll use in this unit. Jest is a popular JavaScript testing framework that makes it easy to write and run tests. When you write tests in Jest, you use three main functions:

• describe(): Groups related tests together
• test() or it(): Defines individual test cases
• expect(): Makes assertions about what your code should do

For example, a basic Jest test looks like this:

describe('math functions', () => {
  test('adds two numbers', () => {
    expect(add(2, 3)).toBe(5);
  });
});

You don't need to memorize the syntax — Codex can generate properly structured Jest tests for you. Just describe what you want tested, and Codex will create the appropriate test structure.

Once you've launched Codex inside your project directory, try a natural prompt like:

Create a new file called math_utils.test.ts with unit tests for math_utils.ts

Codex will:

• Create the new file.
• Read the functions from math_utils.ts.
• Generate appropriate test cases using a testing framework like Jest.
• Show you a preview before applying the change.

You'll have the chance to review the proposed content and approve it — or edit the suggestion before it's applied.

Imagine your file math_utils.ts contains this function:

export function square(x: number): number {
    return x * x;
}

Now you run Codex and type:

Write a test file called math_utils.test.ts to test the square function.

Codex might suggest creating a new file with this content:

import { square } from './math_utils';

describe('square', () => {
    test('squares positive numbers', () => {
        expect(square(2)).toBe(4);
    });

    test('squares negative numbers', () => {
        expect(square(-3)).toBe(9);
    });

    test('squares zero', () => {
        expect(square(0)).toBe(0);
    });
});

You can then review and approve the file creation and content.