Why is express.urlencoded not a function while testing with Jest? - node.js

Question

I am testing my app.js using Jest and it says express.urlencoded is not a function.
Im using express 4.16.4 and while I am writing test for app.js that looks somewhat like this
const express = require('express');
const app = express();
app.use(express.json());
app.use(express.urlencoded({ extended: true }));
app.use(require('./routes'));
module.exports = app;
and my test case is:
jest.mock('express');
it('has a parser', () => {
const app = require('../src/app');
const express = require('express');
expect(app.use).toHaveBeenCalledWith(express.urlencoded());
});
This gives me an error saying: express.urlencoded is not a function.
I expect this test to pass because the app works perfectly fine but only inside the test it says that express.urlencoded is not a function.

Answer 1

You have to manually provide factory and mock module methods there, which you want to test. This would look something like this:
const urlencodedMock = jest.fn();
jest.mock('express', () => {
urlencoded: urlencodedMock,
});
it('has a parser', () => {
const app = require('../src/app');
const express = require('express');
expect(app.use).toHaveBeenCalledWith(urlencodedMock);
});
EDIT:
I am not familiar with express, but you can try something like example below. jest.mock() will mock express in a way that it passes express() and then inside the test when that part is passed, it will mock it again, to provide json() and urlencoded() functions to express.
jest.mock('express', () => {
return {
use: () => {
return;
}
}
});
it('has a parser', () => {
const jsonMock = jest.fn();
const urlencodedMock = jest.fn();
const app = require('../src/app');
jest.doMock('express', () => {
json: jsonMock,
urlencoded: urlencodedMock,
});
expect(app.use).toHaveBeenCalledWith(urlencodedMock);
});