如何对 express-http-proxy 进行单元测试

Question

I'm running a proxy service that adds a header to a request and forwards it to another API using express-http-proxy :

app.use(
  proxy(proxyUrl, {
    proxyReqOptDecorator(proxyReqOpts, srcReq) {
      proxyReqOpts.headers.customHeader = 'custom header'
      return proxyReqOpts
    },
  })
)

Question:

How to unit test such a service?

This is how I would test a normal express server:

describe('proxy service', function() {
  let server
  beforeAll(() => {
    server = app
  })
  afterAll(() => {
    server.close()
  })

  describe('successful execution', () => {
    test('responds to /', done => {
      request(server)
        .get('/')
        .expect(200, done)
    })
  })
})

I want to validate what would be sent the external API without making the actual request.

Answer 1

I have the same problem, my solution for now (until I find better one) is like this. (Using Jest)

First split the code on single responsability functions. You should get something like this.

const injectHeaders = (req) => { // This is a pure function easy to UT
  ...
  const headerA = ...
  return (opts) => { // This is a pure function easy to UT
    
    opts.headers[HEADERS.A] = headerA;
   
    return opts;
  };
};

Then your proxy code should look like this more or less

app.use(
  proxy(proxyUrl, {
    proxyReqOptDecorator: injectHeaders,
  })
)

And finale you can test it like this:

const expressProxy = require('express-http-proxy');

jest.mock('express-http-proxy')

test('proxy called with correct params', () => {
 ...execute app.use code ...

expect(expressProxy).toHaveBeenCalledWith(proxyUrl, {
        proxyReqOptDecorator: injectHeaders,
      })
})