How to automate tests in Replit

Open the Shell tab from the Tools dock and install a testing framework. For JavaScript or TypeScript projects, Jest is the most widely used framework and requires minimal configuration. For Python projects, pytest is the standard choice. Both frameworks auto-discover test files based on naming conventions, so you do not need to manually register each test.

1# For JavaScript/TypeScript projects:
2npm install --save-dev jest
3
4# For Python projects:
5pip install pytest

Create a tests directory in your project and add a test file. Start with a simple test that verifies basic functionality — a function that returns the right value, or an API endpoint that responds with the expected status code. This confirms your testing setup works before adding more complex tests. Place test files in a tests/ directory to keep them separate from source code.

1// tests/app.test.js
2const { add, multiply } = require('../utils/math');
3
4describe('Math utilities', () => {
5  test('adds two numbers correctly', () => {
6    expect(add(2, 3)).toBe(5);
7    expect(add(-1, 1)).toBe(0);
8  });
9
10  test('multiplies two numbers correctly', () => {
11    expect(multiply(3, 4)).toBe(12);
12    expect(multiply(0, 100)).toBe(0);
13  });
14});

Open package.json and add scripts for running tests. The test script is the standard entry point that npm expects. Add a test:watch script for continuous testing during development — it re-runs tests whenever files change. The test:coverage script generates a coverage report showing which lines of code are tested.

1{
2  "scripts": {
3    "test": "jest",
4    "test:watch": "jest --watch",
5    "test:coverage": "jest --coverage",
6    "start": "node index.js"
7  }
8}

Open the .replit file (enable 'Show hidden files' if needed) and modify the run command to execute tests before starting the application. The double-ampersand (&&) ensures the app only starts if all tests pass. If any test fails, the run stops and you see the failure output in the Console. This is the simplest way to enforce testing discipline in Replit — every press of the Run button verifies your code first.

1# .replit file
2run = "npm test && node index.js"

To prevent broken code from reaching production, add the test command to the deployment build configuration in your .replit file. The build command runs before every deployment. If tests fail during the build step, the deployment is aborted and the failing code never goes live. This is the Replit equivalent of a CI/CD gate.

1[deployment]
2build = ["sh", "-c", "npm install && npm test && npm run build"]
3run = ["sh", "-c", "node index.js"]
4deploymentTarget = "cloudrun"

If your project uses Python instead of JavaScript, the workflow is similar but with different tools. Create a tests directory with test files following pytest's naming convention (test_*.py). Configure the .replit run command to run pytest before starting the application. Pytest auto-discovers test files and provides clear pass/fail output with detailed error messages.

1# tests/test_app.py
2from utils.math import add, multiply
3
4def test_add():
5    assert add(2, 3) == 5
6    assert add(-1, 1) == 0
7
8def test_multiply():
9    assert multiply(3, 4) == 12
10    assert multiply(0, 100) == 0
11
12# .replit run command for Python:
13# run = "pytest && python main.py"