Why isn't my express test passing?
Because first argument of the end callback is err, which in your case is undefined and results in
assert.equal(200, undefined.statusCode)The right way
.end((err, res) => { if (err) return done(err); assert.equal(200, res.statusCode); done();})Even better would be to use directly .expect(200) as answered by @R. Gulbrandsen.
Try to change your test to use the supertest's expect like this:
import assert from 'assert';import request from 'supertest';import app from '../app';describe('The express App', () => { it('should return 200', done => { request(app) .get('/') .expect(200) .end(done); });});