<p>I'm trying to achieve good coverage with end-to-end testing of my code base, so I want to test the <code>.catch()</code> code as well. </p><p> My API uses node.js and mongoose. </p><p> I use chai and mocha for testing.</p> <p>I tried something like: </p> <p><em>File src/controllers/user.controller.js:</em></p> <pre class="brush:php;toolbar:false;">const User = require("../models/user.model"); const getUser = async(req, res) => { try { const user = await User.findOne({name: req.name}); return res.status(200).json(user); } catch (err) { //This is the code I want to test console.error(`Error finding user ${req.name}:`, err); return res.status(err.code).json({ message: err }); } }</pre> <p><em>File src/models/user.model.js:</em></p> <pre class="brush:php;toolbar:false;">const mongoose = require("mongoose"); const UserSchema = mongoose.Schema({ name: { type: String, required: "Name is required", }, }); module.exports = mongoose.model("User", UserSchema);</pre> <p><em>File test/user.test.js:</em></p> <pre class="brush:php;toolbar:false;">const chai = require("chai"); const chaiHttp = require("chai-http"); const spies = require("chai-spies"); const User = require("../src/models/user.model"); chai.use(chaiHttp); chai.use(spies); chai.should(); describe("mongoose errors should be handled", function() { describe("Problematic User.findOne method", function() { const _User_findOne_Backup = User.findOne; beforeEach(function() { // This function should override the real findOne function, but it didn't succeed!User.findOne = function() { return Promise.reject("Forced error"); }; }); afterEach(function() { //Restore the real function after each test User.findOne = _User_findOne_Backup; }); it("Registration should return a server error", function() { const spy = chai.spy(); return chai .request(server) .post("/api/getUser") .send({name: "Alice"}) .then(spy) .catch((err) => { const res = err.response; res.should.have.status(500); }) .then(() => { spy.should.not.have.been.called(); }) ; }); }); });</pre> <p>The problem is that in my tests, the fake <code>User.findOne()</code> method is never called: the original mongoose <code>findOne</code> method is called successfully , so the <code>getUser</code> method never throws an exception, causing my test to fail...</p> <p>Maybe I'm missing something obvious, but I really can't find it... :-(</p><p> If more code or context is needed, please let me know...</p> <p><strong>Update: </strong> Following @Bergi's suggestion, I added complete information about my (simplified) model and required modules...</p>

const mongoose = require("mongoose");

const UserSchema = mongoose.Schema({
  name: {
    type: String,
    required: "Name is required",
  },
});
module.exports = mongoose.model("User", UserSchema);

const User = require("./user.model");

const getUser = async (req, res) => {
  try {
    const user = await User.findOne({ name: req.body.name });
    return res.status(200).json(user);
  } catch (err) {
    console.error(`Error finding user ${req.body.name}:`, err);
    return res.status(500).json({ message: err });
  }
}

module.exports = {
  getUser
}

const express = require('express');
const userController = require('./user.controller');

const app = express();

app.use(express.json())
app.post('/api/getUser', userController.getUser)

module.exports = app;

const chai = require("chai");
const chaiHttp = require("chai-http");
const User = require("./user.model");
const server = require('./server');
chai.use(chaiHttp);
chai.should();

describe("should handle mongoose errors", function () {
  describe("faulty User.findOne method", function () {
    const _User_findOne_Backup = User.findOne;
    beforeEach(function () {
      User.findOne = function () {
        return Promise.reject("forced error");
      };
    });
    afterEach(function () {
      User.findOne = _User_findOne_Backup;
    });

    it("signup should respond with a server error", function () {
      return chai
        .request(server)
        .post("/api/getUser")
        .send({ name: "Alice" })
        .catch((err) => {
          const res = err.response;
          res.should.have.status(500);
        })
        ;
    });
  });
});

  should handle mongoose errors
    faulty User.findOne method
Error finding user Alice: forced error
      ✓ signup should respond with a server error


  1 passing (21ms)

--------------------|----------|----------|----------|----------|-------------------|
File                |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
--------------------|----------|----------|----------|----------|-------------------|
All files           |    89.19 |      100 |    88.89 |    88.57 |                   |
 server.js          |      100 |      100 |      100 |      100 |                   |
 user.controller.js |       80 |      100 |      100 |       75 |               6,7 |
 user.model.js      |      100 |      100 |      100 |      100 |                   |
 user.test.js       |    88.89 |      100 |    85.71 |    88.89 |             26,27 |
--------------------|----------|----------|----------|----------|-------------------|

"chai": "^4.2.0",
"chai-http": "^4.3.0",
"mongodb": "^3.6.3",
"mongoose": "^5.11.9",
"express": "^4.17.1"