get-ssl-certificate | dependency utility that returns a website 's SSL certificate | TLS library

This would happen when you are working with asynchronous code (and promises). From your test code it appears that the method sslCertificate.get() returns a Promise. This promise would get resolved (successfully executed) or rejected (throw an error) asynchronously. In JS the asynchronous execution starts only after the current synchronous execution pauses/completes.

In the context of your test, you pass a callback method when the promise resolves (using .then()). This callback gets executed only after the promise is resolved and since your test code would not pause for this execution, it completes successfully - meaning sslCertificate.get('instagram.com').then(callback) never throws any error or exception. After the test is executed, the promise gets a chance to resolve and now asynchronously executes your callback. Therefore, you get the UnhandledPromiseRejectionWarning: AssertionError.

This can be handled by using mocha async tests in two ways:

Method 1: Using async/await (my personal recommendation for readability):

1. Make your test function async.
2. await for the promise to resolve.
3. Perform your assertions.

Here's some code: