Assert – the basis of testing methodologies

Node.js has a useful built-in testing tool known as the assert module. Its functionality is similar to assert libraries in other languages. Namely, it's a collection of functions for testing conditions, and if the conditions indicate an error, the assert function throws an exception. It's not a complete test framework by any stretch of the imagination, but it can still be used for some amount of testing.

At its simplest, a test suite is a series of assert calls to validate the behavior of the thing being tested. For example, a test suite could instantiate the user authentication service, then make an API call and use assert methods to validate the result, then make another API call to validate its results, and so on.

Consider the following code snippet, which you can save in a file named deleteFile.mjs:

import fs from 'fs';

export function deleteFile(fname, callback) {
    fs.stat(fname, (err, stats) => {
        if...