A scriptable HTTP proxy, designed for use in UI tests for web applications.

Usage no npm install needed!

<script type="module">
  import middleMan from 'https://cdn.skypack.dev/middle-man';



A scriptable HTTP proxy, designed for use in UI tests for web applications.

Build Status

Rich web applications often interact with external services by making HTTP requests. Testing applications like this can be difficult for a number of reasons:

  • Servers may be unavailable due to service outages, maintenance, or local unavailability of network connection
  • Responses may be slow, further increasing the time required to execute UI tests
  • Requests may be throttled. This introduces artificial limits on the application's test schedule
  • Requests may have side effects, so automated tests may exhibit different behavior during repeated execution
  • Secure operations may require sensitive credentials, necessitating the creation of shared "dummy" accounts and management of shared, semi-private credentials

MiddleMan is a tool intended to be used as an intermediary between UI tests for web applications and the external services with which they interact. Through a simple, Express-inspired API, test writers can control exactly how web requests are handled in the context of the current-running UI tests.


Initialization This tool is intended for use in Selenium-powered UI tests, but it is completely agnostic of test framework and Selenium binding.

var createSeleniumBinding = require('your-favorite-selenium-binding');
var startTests = require('your-favorite-testing-framework');

var MiddleMan = require('middle-man');
var middleMan = new MiddleMan();
var mmPort = 8003;

  .then(function() {
    return createSeleniumBinding({
      server: 'http://localhost:4444/wd/hub',
      capabilities = {
        browserName: 'firefox',
        proxy: {
          proxyType: 'manual',
          httpProxy: 'localhost:' + mmPort
  }).then(function() {

Generic request handlers In some cases, you may want to unilaterally react to certain types of requests. The on method allows you to register code to modify every request/response pairs that match some criteria:

 * Set the CORS flag to ensure that requests are not subject to any same-origin
 * policy for the duration of the tests.
function handleCors(req, res, next) {
  res.setHeader('Access-Control-Allow-Origin', '*');

  // If your handler specifies a third parameter, it will be provided with a
  // function that *must* be invoked before the request is passed through.

// Bind to all HTTP verbs and all URL paths
middleMan.on('*', /.*/, handleCors);

In some cases, you may be able to handle the HTTP request yourself (instead of allowing it to "pass through"). The second parameter is a Node.js response object, and when it is closed, the proxy will not issue it to the web (nor invoke any additional request handlers).

function handleCorsPreflight(req, res) {
    'authorization, authorization, content-type'
    'Access-Control-Allow-Methods', 'GET,HEAD,PUT,PATCH,POST,DELETE'
  res.setHeader('Access-Control-Allow-Origin', '*');
  res.setHeader('Vary', 'Origin');

  // No need for the `next` parameter here--we don't need the request to pass
  // through.

// Bind to OPTIONS requests for all URL paths
middleMan.on('OPTIONS', /.*/, handleCorsPreflight);

Scripting specific responses In the course of a UI test, your script probably causes the application to issue web requests. This tool allows you to programatically control the response behavior:

test('awesome test', function() {
  var hasDeleted = false;
  function handleDelete(req, res) {
    hasDeleted = true;
  function handlePost(req, res) {
    assert(hasDeleted, 'Deletes old utilization before creating new one.');
    res.end(JSON.stringify({ utilizations: { id: 99 } }));

  // This pattern allows you to verify that the requests are actually made--
  // the test will time out if the Promises returned by `MiddleMan#once` are
  // not resolved.
      middleMan.once('DELETE', '/v1/utilizations/99', handleDelete),
      middleMan.once('POST', '/v1/utilizations', handlePost),
        name: 'Jerry Seinfeld',
        day: 'thursday',
        type: 'Vacation'
    ]).then(function() {

API Documentation

The API is defined within the source code itself using JSDoc formatting.


Copyright (c) 2015 Mike Pennisi
Licensed under the MIT Expat license.