During server software development (i.e. node) I always strive to work offline, in that my development workflow doesn't depend on any online services.

However, sometimes that's not possible as there's some 3rd party API dependancy - as there was in my latest client work. The problem I found, was that the latency between my location and the 3rd party API was so high, that it was impacting on the workflow cycle of: debug, change, test, repeat. The load time was taking anywhere from 15-30 seconds each time.

So this was my quick work around to easily mock out the API so my development process could be faster.

UK EVENTAttend ffconf.org 2024

The conference for people who are passionate about the web. 8 amazing speakers with real human interaction and content you can't just read in a blog post or watch on a tiktok!

The aims

The aim was to, without much work at all, replicate specific API calls without modifying my application code. i.e. I could switch my mock API on and off as I needed.

It was also important that the time required to put a mock API in place, did not outweigh the time lost on the latency. Specifically, if the custom mock API turned into days of work, it would have been a total waste of time.

The simple solution

This code uses Express 4. It's entirely possible that there was a library that already existed, but the small amount of code that I had to write wins over finding, researching, checking and learning a new library.

Capturing mock data

First I had to run my way through each API call and save the raw output as a .json file in a local directory (not tracked in git) called mock-data.

Since all my internal API calls used a config value as the root of the API URL, it meant I could change my local config to point to my local mock API instead of the staging or production endpoint:

var config = require('./config');
var root = config.api;

function getOrder(id) {
  return request({ // a promise
    url: root + '/order',
    type: 'json',
    body {
      id: id

I would change my config.api value to point to `http://localhost/mock-api' and the server I was developing would actually reply to the API requests using the code in the next section.

Mock API Code

The following code would be in (something like) routes/mock-api.js:

var express = require('express');
var fs = require('fs');
var router = express.Router();
// directory path to the mock json files
var mockPath = __dirname + '/mock-data/';

var design = {
  'POST /order': 'getOrder',
  'POST /order/tickets': 'addTickets',
  'DELETE /order/tickets': 'removeTicket',

module.exports = router;

Object.keys(design).forEach(function (req) {
  var method = req.split(' ').shift();
  var path = req.split(' ').pop();
  var mock = '{}';
  try {
    mock = require(mockPath + design[req]);
  } catch (e) {}

  router.route(path)[method.toLowerCase()](function (req, res) {

In my main routes.js file I can then conditionally load up my mock router:

// snip...
if (config.mock) {
  app.use('/mock-api', require('./routes/mock-api'));

Now requests to my server (the same server I'm doing my main development against), can respond to API requests with pre-baked data. Now my damn latency is a thing of the past!

Taking it further

I struggle to look at the above code without seeing ways to improve it - but as I pointed out in the aims: keep it simple, and keep the solution quick.

Here's a few ideas that spring to mind that might make the code more useful in different situations:

  • Respond with different file types (rather than only .json)
  • Respond with inline JSON data (that would be defined in the design)
  • Support variables in the URL to pick different static responses (like GET /order/:id)

I have done some similar work in my static server which also has limited support for interpolation in the response.

For now though, this simple version suits my requirements!