Jest / NodeJS, how to build a test for "unhandledRejection" - node.js

Question

I'm trying to have 100% coverage.
I'm stuck at creating a test for this part of my code:
// Manually throwing the exception will let winston handle the logging
process.on("unhandledRejection", (ex) => {
throw ex;
});
I tried to use the following:
const logger = require("../../../startup/logging");
describe("startup / logging", () => {
it("should log the error in case of unhandledRejection", () => {
process.emit("unhandledRejection");
const loggerError = jest.spyOn(logger, "error");
expect(loggerError).toHaveBeenCalled();
});
});
But the test fails with: thrown: undefined
This is the full code of logging.js:
const { createLogger, format, transports } = require("winston");
require("winston-mongodb");
// This will forward the error in the pipeline to our error handler
require("express-async-errors");
// Manually throwing the exception will let winston handle the logging
process.on("unhandledRejection", (ex) => {
throw ex;
});
// Log to files
const logger = createLogger({
format: format.combine(
format.timestamp({
format: 'YYYY-MM-DD HH:mm:ss'
}),
format.errors({ stack: true }),
format.splat(),
format.json()
),
transports: [
new transports.File({filename: "./logs/combined.log", level: "verbose"}),
],
transports: [
new transports.File({filename: "./logs/error.log", level: "error"}),
new transports.File({filename: "./logs/combined.log"}),
],
exceptionHandlers: [
new transports.File({ filename: "./logs/exceptions.log" }),
new transports.File({ filename: "./logs/combined.log" }),
],
handleExceptions: true,
});
// Log to database
logger.add(new transports.MongoDB({
level: "error",
db: "mongodb://localhost:27017/rest-api-mongodb",
options: {
useUnifiedTopology: true,
useNewUrlParser: true,
},
metaKey: "stack",
handleExceptions: true,
}));
module.exports = logger;
This is the error middleware that is triggered in case of unhandledRejection:
// Winston is a logger, it allows to store errors in a log and mongoDB
const logger = require("../startup/logging");
// This function will handle all errors in the router
// It works thanks to require("express-async-errors"); that forwards the error in the pipeline
// It does not work outside of the context of express
module.exports = function (err, req, res, next) {
logger.error(err.message, err);
// error, warn, info, berbose, debug, silly
res.status(500).send("Something on the server failed.");
}
Any help is appreciated!

Answer 1

Try triggering unhandledRejection by using a real unhandled rejection. For example, call Promise.reject() without attaching a handler.

Answer 2

After playing around with the code, I made it work as follows:
require("../../../startup/logging");
describe("startup / logging", () => {
it("should throw an error if there is an unhandledRejection", () => {
const emitUnhandledRejection = () => process.emit("unhandledRejection");
expect(emitUnhandledRejection).toThrow();
});
});