Mocha, how to test async code without UnhandledPromiseRejectionWarning - node.js

Question

I'm testing my server-side api endpoints with mochajs and I can't figure out how to do it properly.
I started with code that had the following logic:
it('test', (doneFn) => {
// Add request handler
express.get('/test', (req, res, next) => {
// Send response
res.status(200).end();
// Run some more tests (which will fail and throw an Error)
true.should.be.false;
// And that's the problem, normally my framework would catch the
// error and return it in the response, but that logic can't work
// for code executed after the response is sent.
});
// Launch request
requests.get(url('/test'), (err, resp, body) => { // Handle response
// I need to run some more tests here
true.should.be.true;
// Tell mocha test is finished
doneFn();
});
});
But the test doesn't fail because it throws in the request handling callback.
So I googled around and found that my problem could be solved using promises, and it does, now the test fails. This is the resulting code:
it('test', (doneFn) => {
let handlerPromise;
// Add request handler
express.get('/test', (req, res, next) => {
// Store it in a promise
handlerPromise = new Promise(fulfill => {
res.status(200).end();
true.should.be.false; // Fail
fulfill();
});
});
// Send request
requests.get(url('/test'), (err, resp, body) => {
// Run the other tests
true.should.be.true;
handlerPromise
.then(() => doneFn()) // If no error, pass
.catch(doneFn); // Else, call doneFn(error);
});
});
But now I end up with a deprecation warning because the error is handled in a different process tick than the one it was thrown.
The errors are: UnhandledPromiseRejectionWarning and PromiseRejectionHandledWarning
How can I make my test fail after the response is sent, and avoid having an
unhandledPromiseRejectionWarning?

Answer 1

This works
it('test', (doneFn) => {
let bindRequestHandler = new Promise((reslove, reject) => {
// Add request handler
app.testRouter.get('/test', (req, res, next) => {
// Send response
res.status(200).end();
try { // Here, we need a try/catch/reject logic because we're in a callback (not in the promise scope)
// Run some more tests (which will fail and throw an Error)
true.should.be.false;
} catch (error) { // Catch the failing test errors and reject them
reject(error);
}
resolve();
});
});
let sendRequest = new Promise((reslove, reject) => {
// Launch request
requests.get(url('/test'), (err, resp, body) => { // Handle response
try {
// I need to run some more tests here
true.should.be.true;
} catch (error) { // Catch the failing test errors and reject them
reject(error);
}
reslove();
});
});
Promise.all([bindRequestHandler, sendRequest])
.then(res => doneFn())
.catch(doneFn);
});