How to serve an API locally with webpack

June 22, 2020

While developing a data driven web-app, the frontend may need to fetch data from various APIs provided by backend which may not be ready at the given time. In this case, we need to mock the response and serve it via dummy server, either local or remote. With Webpack this can be achieved without any dummy server. Webpack devserver allows custom handlers which can be used to serve the API response.

We need to add a devServer entry to the webpack config, which would look something like this:

const data = require("./data.json");

module.exports = {
  mode: "development",
  entry: "./src/index",
  output: {
    filename: "bundle.js",
    path: path.resolve(__dirname, "build")
  plugins: [],
  devServer: {
    before: function(app) {
      app.get("/getData", function(req, res) {
    open: true,
    port: 3000
  resolve: {
    extensions: [".js"]
  module: {}

There are various amazing things that we can do with devserver (check here), we consider three of them here

  1. open - launch the browser once the app is compiled
  2. port - set the port to serve the app
  3. before - We can define custom paths with endpoints in this function, from the docs -

Provides the ability to execute custom middleware prior to all other middleware internally within the server. This could be used to define custom handlers.

So for given webpack config, if the content of data.json is - [{a: 1}, {b: 2}, {c: 3}] then response for http://localhost:3000/getData will be

  { a: 1 },
  { b: 2 },
  { c: 3 }

The app param of before function can be considered as express object and be used in the same context, app.get has req and res params, which are request and response object for the API call made.

To see this in action please checkout this project on github.

Bonus: I use this technique to get around with CORS on my local setup, the following code does the trick for me:

  devServer: {
    before: function(app) {
      app.get('/api', async function(req, res) {
        try {
          const queryURL = req.query.q;
          const resp = await fetch(queryURL);
          const body = await resp.text();
        } catch (e) {

Here I fetch the remote URL on the server rather than the browser, so there is no CORS restriction and then serve on the response on the same server hence, eliminating CORS.

I highly recommend to look into devserver options, this can save a lot of your efforts 😊

Trishul Goel

Trishul Goel is a frontend developer, loves to talk about modern Javascript, frontend architecture, opensource, promotes PWAs (#teamWeb) and is expert in developing browser extensions..