# How Do I Test a Single File Using Node Built-in Test Runner?

Node.js itself does not come with a built-in test runner. However, you can use the built-in `assert` module for simple assertion-based tests. If you want a more feature-rich test runner, you might consider using a third-party library like `mocha` or `jest`. Here, I'll show you a basic example using the `assert` module:

## **Write your test file**

Let's say you have a module (`math.js`) with some functions you want to test:

```jsx
// math.js
function add(a, b) {
  return a + b;
}

function subtract(a, b) {
  return a - b;
}

module.exports = { add, subtract };
```

Now, create a test file (`test.js`) for this module:

```jsx
// test.js
const assert = require('assert');
const { add, subtract } = require('./math');

// Test the add function
assert.strictEqual(add(2, 3), 5, 'add function failed');

// Test the subtract function
assert.strictEqual(subtract(5, 2), 3, 'subtract function failed');

console.log('All tests passed!');
```

## **Run the test file**

To run the test file, use the following command in your terminal:

```bash
node test.js
```

If all tests pass, you will see the message "All tests passed!".

This is a very basic example using Node.js's built-in `assert` module. For more advanced features, you might want to explore third-party test runners like `mocha` or `jest`. They offer features such as test organization, asynchronous testing, and test reporting. To use these libraries, you would typically install them via npm and configure your `package.json` to run your test files.