How to use JavaScript feature flags to deploy a feature safely [React.js example]

By Alex Boswell on October 5, 2021

Feature flags in any language significantly reduce the blast radius of a release if anything goes wrong. Javascript feature flags are no exception. In this post, you will learn how to use javascript feature flags in a React.js application to deploy a small feature safely to your users. Let’s get started!

Feature flag image with JavaScript logo


Prior to diving deeper into the code, below are some of the good to have requisites:

  1. Some knowledge of Javascript and React.js would be essential
  2. Knowing how Node.js and Yarn work are required, knowledge of adding new packages with Yarn is also needed
  3. A Flagsmith account will be used to create the feature flag. Register now – It’s free.

We will jump right into the code part now, brace yourself.

Example: JavaScript feature flags with React.js app

We will build a simple React.js app that will list the latest articles from Dev is a community of software developers getting together to help one another out. It is an amazing community where anyone can publish technical content without any editorial process. Based on Forem it has a handy API and we are going to use the articles API to list the latest articles in a light React.js application.

To create the React.js application using create react app, we will run the following command:

1npx create-react-app devto-react 

It will render output as follows when the setup is complete:

output showing possible yarn commands

Next, we will quickly run the React.js boilerplate application to see if it is running with:

1cd devto-react
3yarn start

It will start the development server and open the default browser at `http://localhost:3000` which looks like:

Screenshot of React.js open screen

Hurray! Our skeleton React app created by Create React App for the JavaScript feature flags tutorial is running. We will change the code to call the API next.

We will change the `src/App.js` file to look like below:

1import React, { useState, useEffect } from 'react';
2import './App.css';
3function App() {
4  const [articles, setArticles] = useState([]);
5  const [message, setMessage] = useState('loading…');
6  useEffect(() => {
7    async function fetchArticles () {
8      try {
9        const data = await (await fetch('')).json();
10        setArticles(data)
11        const message = data.length ? "" : 'No articles found';
12        setMessage(message);
13      } catch (err) {
14        console.log('err: ${err.message}', err);
15        setMessage('could not fetch articles');
16      }
17    }
18    fetchArticles()
19  }, []);
21  return (
22    <div className="App">
23      <header className="App-header">
24        <h2>Latest articles</h2>
25        {message}
26        <div className="articles">
27          <ul>
28          {Array.isArray(articles) &&
29            article => article.url && <li><a href={article.url} target="_blank" rel="noreferrer">{article.title}</a> – by {}</li>
30          )}
31          </ul>
32        </div>
33      </header>
34    </div>
35  );
38export default App;

We are using the React useState hook to define two variables articles and the message. Then using the effect hook we are calling the API in the `fetchArticles` function. The articles fetched are being put into the `articles` variable. In case of any error a message of `No articles found` is put into the message variable.

Below, in the return section, we are looping through all the fetched articles and showing the title of the article with a link to it and the name of the user who has posted the articles.

Consequently, we will change the `src/App.css` to have the following contents

1.App-header {
2  min-height: 100vh;
3  display: flex;
4  flex-direction: column;
5  font-size: calc(10px + 2vmin);
8.App-header h2 {
9  text-align: center;
12.App-link {
13  color: #61dafb;

The CSS has been simplified to show the list of articles, removing the unneeded styles for the logo and the background. Text color and text alignment have been reset too.

The focus of this tutorial is to help you learn how to do JavaScript feature flags in a React.js based application with Flagsmith. When we run it with `yarn start` after making the above changes, the application looks like below at this point:

Screenshot of Latest articles

For the scope of this guide, we will add the reading time in minutes for the articles. As this small feature will be added with a JavaScript feature flag, it will be very easy to turn it on and off simply by flipping a flag within Flagsmith; no re-deployments are necessary. That is the power and convenience of feature flags, as deployment is not a release.

In the next section, we will set up the feature flag for our example within Flagsmith’s UI. Then we will add Flagsmith’s JavaScript SDK, available on NPM, to our project. After that, we will add some code to implement the simple flag to show or hide the reading time for the Dev articles.

Setup Flagsmith

To create a feature flag to show or hide the reading time for articles we will first create a project on Flagsmith. To create the project we will click the “Create Project” button after logging in:

Screenshot of creating a project in Flagsmith

I have named the project Dev-React, and then, we will reach the project page like below:

Screenshot of main page in the Flagsmith application

We will scroll down the page and create a new feature flag called `show_reading_time` as seen below:

Screenshot of creating a new feature in Flagsmith

Now we have created a new feature flag called `show_reading_time` and enabled it. It would be created in both development and production environments in Flagsmith. Next, we will install the `flagsmith` NPM library and use it in the React.js app for our JavaScript feature flags tutorial.

Install and use Flagsmith JavaScript client

We have created the feature flag in Flagsmith on the UI, now it is time to use it in the Dev React app. To do this for our JavaScript feature flags tutorial, we will first get the Flagsmith JavaScript client from NPM running the following:

1yarn add flagsmith

After the `flagsmith` client is installed we will change the code in `src/App.js`. First, we will add the following line on line 2 to import Flagsmith library into the app:

1import flagsmith from 'flagsmith';

Subsequently, we will add the following at line 8 to initialize the `showReadingTime` variable which will be false by default:

1const [showReadingTime, setShowReadingTime] = useState(false);

Then we will add the code below in `useEffect` function below `fetchArticles` call at line 22 as follows:

2    environmentID:"VKE5KnX8xXZuztTDGbcvBU",
3    cacheFlags: true,
4    enableAnalytics: true,
5    onChange: (oldFlags, params) => {
6    setShowReadingTime(flagsmith.hasFeature('show_reading_time'));
7    }
8  });

You can get the environment ID from the “Initialize your code” section of the feature flag page as seen below:

Screenshot of Flagsmith application showing the environmentID

Consequently add the following code where you see the looping through articles towards line 40:

2          {Array.isArray(articles) &&
3            article => article.url && <li><a href={article.url} target="_blank" rel="noreferrer">{article.title}</a> – by {} {showReadingTime ? '(Around ${article.reading_time_minutes} minute${article.reading_time_minutes > 1 ? "s": ""} read)' : ""}</li>
4          )}

We have first imported Flagsmith’s JavaScript client, then set the `showReadingTime` on useEffect so that it is only called once on page load. Then when we render the articles we check if the reading time should be shown and then we print the reading time else we print the empty string. Next up we will see the changes in action.

Test JavaScript feature flags changes

At this point if we again start the app it will show something like the below:

Screenshot of latest articles with reading times noted next to each one

Great! We can see the reading time of the articles. Now if we would like to turn off showing the reading time, it is very easy. We can just go to the Flagsmith interface and turn the feature off like below:

Screenshot of turning reading time off in the Flagsmith application

After we hit “Confirm Changes” and the flag is off, if we come back and refresh the Dev React app page used for our example we will not see the reading times anymore as follows:

Screenshot of latest articles without reading times

Wasn’t that easy? You can deploy the app and change the environment ID to be production for your production environment. In the next step, we will deploy the react application to Surge.

Deploy React App to

We have run the application locally, so to deploy it to a more production-like environment we will use It is a very easy platform to deploy our front-end application. To deploy our React application to surge first we will replace the key from development to production one. To do this we will go to the “Production” environment and then click “Initialising your project” under the features section as seen below:

Screenshot of Flagsmith application showing Production environment ID

We had done a similar thing for the development environment, as we want to deploy to a production-like environment we are replacing the environment ID to be of production. We will copy the environment id and replace it on line 23 of `src/App.js` which will look like below after the change:

2    environmentID:"BLPF23hJZ4ekaYV48epC55",
3    cacheFlags: true,
4    enableAnalytics: true,
5    onChange: (oldFlags, params) => {
6     setShowReadingTime(flagsmith.hasFeature('show_reading_time'));
7    }
8  });

To deploy our React app to surge we will run the following commands:

1yarn build

It will give the following output:

Screenshot of output from yarn build setup

It will also create a `build` folder with the output of the build process which has the index.html file and other static assets.

To install the Surge command line we will run the following:

1npm i -g surge

Consequently, we will go into the build folder and deploy the built app with:

1cd build

After that type in your email and a password. Then it will find your current working directory as the project. Then if you want, change the subdomain, I have used `` else use the random subdomain it provides. The hit enter and in a couple of seconds your app will be deployed to Surge over a CDN as seen below:

Screenshot of output from yarn build success

Pretty neat! Below is how the app looks on with the toggle for reading time “on” for Javascript feature flag,  taken from the “production” environment:

Screenshot of latest articles running on

You can try it out too at You will get an email to verify your email address, if you are interested in Surge do check out their help section.  In the following part, we will look at the next steps.

Next Steps

Implementing JavaScript feature flags is a pretty easy task with amazing benefits. You can use the same Flagsmith JavaScript client in any frontend application ranging from vanilla JavaScript to Jquery to even Angular or Vue.js applications. It is also possible to use the SDK from a CDN with the following code:

1<script src=""></script>

The main concept here is to deploy the feature safely, if there is an issue we should be able to turn it off in seconds, not minutes or hours. That is easily possible with feature flags on an open-source platform like Flagsmith or with the free plan. The feature can be turned on and off with a couple of clicks, not with a deployment process. That adds unprecedented power and flexibility to the software engineering teams. The safety of feature release is another desirable by-product of the whole process.


We just saw how to implement JavaScript feature flags on a simple React.js app that calls an API to show the latest Dev articles. This idea can be easily implemented in any frontend JavaScript application. Feature release can be done safely with high confidence and low time to turn on or off the released feature without the need to deploy any new code.