React/ReactJS: Set-up Environment

In our previous tutorial, we have set an environment variable called PORT to change the port number as desired while running the app in localhost. Also, there we have made use of one .env file where the environment variable PORT is set.

That was just for the local environment. But there could be several other variables, each assuming different values in different environments. In this tutorial, we will learn how to set different .env files for customized environment variables to take different values when deployed in different environments like development, staging and production.

To start with, we consider the path to the image files in your project. While working on localhost, you most probably would have stored and fetched the images from the local /img directory inside the app. For the sake of simplicity (of accessing), we take a logo image logo.png and place it inside the /img directory of the /public directory. Note again that this is the /public directory and not inside your /src. The JSX might resemble something like

					<img src="./img/logo.png"/>

But for production, this image most probably would be stored in some Content Delivery Network (CDN) and the paths would change. For example, if the image is stored in Amazon Web Services (AWS) S3 bucket, the path could be something like

					<img src=""/>

Now, switching back from production to localhost would mean changing the paths back for each and every image in your app; it would be monotonous. This is where environment variables and .env files come in handy. For our image path example above, we can create one environment variable, say, REACT_APP_LOGO_PATH which would take different path values to the logo image depending on the environment. To achieve this, we maintain two files — .env.local (for localhost) and .env.production (for production) — where its values will be set as required. Note that, custom environment variables start with REACT_APP_, and that is why our variable name above is prepended with REACT_APP_.

The custom environment variables begin with REACT_APP_.

Now this is how our environment variable REACT_APP_LOGO_PATH will be stored in .env files. In .env.local, its value could be


and in .env.production, its value could be


so that we can load respective .env files in respective environments. Now there is the process.env object via which the environment variables are made available throughout the app. In the JSX, we could just use the process.env object to get the LOGO_PATH variable as

					<img src={`${process.env.REACT_APP_LOGO_PATH}logo.png`}/>

NOTE: The environment variables are available throughout the app via the process.env object.

Here is the component for the above example

					import React, { Component } from 'react';

					const Logo = () => {
					        <img alt="" src={`${process.env.REACT_APP_LOGO_PATH}logo.svg`}/>

					export default Logo;

Now that we have assigned two different values to the same environment variable in two different .env files, how exactly do we load a particular .env file for a particular environment? Or to be precise, how do we load .env.local in localhost and .env.production in prodution? This is achieved via the various npm commands, which we will learn in the following section.

.env Files and npm Commands

reactjs environment files

1) npm start

The npm start command is run for development in local server. Below are the list of .env files which it picks in terms of priority from left to right.

.env.development.local, .env.development, .env.local, .env

So, if some values are assigned for the variable REACT_APP_LOGO_PATH in env.local and .env files, it will pick the value assigned in .env.local.

2) npm test

The npm test command is run for the test environment. If you have some specs/test files written in Jest, a node-based test runner, the command will launch Jest in watch mode. The files listed below are picked in the priority from left to right.

.env.test.local, .env.test, .env

3) npm run build

The npm run build command creates the /build directory with the optimized build of your app. The command picks the following files in priority from left to right.

.env.production.local, .env.production, .env.local, .env