The goal of this assignment is to learn to run two programs concurrently. One program is an Express application called server. The second is a create-react-app program called client.

NOTE: I believe that most of this assignment can be automated by running the get-gist script and picking the Concurrently option from the menu. Run that command in the directory that contains both your client and server projects. For instance, in the case below in the Getting Started section, run it in the weekXX-concurrently directory.

Getting Started

In most cases, you will already have a directory that contains a server and client project. In case you do not, here is how to set one up starting from the root of your repository:

mkdir weekXX-concurrently
cd weekXX-concurrently/
CreateExpressProject server
create-react-app client

Install concurrently in the root of your project:

npm install --save concurrently

Setting the Port

We don’t want both programs to try to run on the same port. To solve this potential conflict, set the port in server/bin/www to 30026. One way to do that is like this:

var port = normalizePort('30026');

A second, more flexible way, looks like this:

var port = normalizePort(process.env.SERVER_PORT || '30026');    

Then in .bashrc, or some other script, set SERVER_PORT:

export SERVER_PORT=30026

In the Week10-Concurrently directory set up package.json and the starter script as described in the next few sections of the project.

Also, in server/bin/www you may need to modify the call to server.listen like this:

server.listen(port, () => console.log("Listening on", port));

This, or something similar, may have already been done for you when you created the project.

NPM Package

Here is an example package.json that shows how to launch our existing client and server programs with concurrently.

Place this package.json file in the root folder that hosts both the client and the server programs. It is expecting both client and server to be sub-directories.

    "name": "lookup-server",
    "version": "0.0.1",
    "private": true,
    "dependencies": {
        "express": "4.16.4"
    "scripts": {
        "start": "concurrently 'npm run server' 'npm run client'",
        "server": "babel-node server/bin/www",
        "client": "babel-node start-client.js",
        "lint": "eslint ."
    "devDependencies": {
        "@babel/cli": "^7.4.3",
        "@babel/core": "^7.4.3",
        "@babel/node": "^7.2.2",
        "babel-eslint": "^9.0.0",
        "concurrently": "4.1.0",
        "elven-code": "^4.6.3",
        "eslint": "5.12.0",
        "eslint-config-google": "^0.12.0",
        "eslint-plugin-import": "2.17.1",
        "eslint-plugin-jsx-a11y": "6.2.1",
        "eslint-plugin-react": "7.12.4"

If you look at the above file, you will also see a lint option.

Start the Client

We need to preform a little dance to start the create-react-app program found in the client directory. Here is our little waltz, which we will save as start-client.js in the root of your project. (Don’t put it in client in server; put it in the root of the project.):

#!/usr/bin/env node

const args = [ 'start' ];
const opts = { stdio: 'inherit', cwd: 'client', shell: true };
const cp = require('child_process');
cp.spawn('npm', args, opts);

Use Concurrently

Now run npm install && npm start

Turn it in

Provide the following:

  • branch
  • folder

For instance:

  • Branch: Week02
  • Folder: Week02-Concurrently

Dependency Error

You might get an error like this: There might be a problem with the project dependency tree. If this happens, open up the copy of package.json in the root of your directory and set the versions of babel-eslint and eslint as follows:

"babel-eslint": "^9.0.0",
"eslint": "5.12.0",

Perhaps the versions that create-react-app wants will differ from this, but they are probably looking for something along these lines.