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How to set up a location-based site banner

Jun 15, 2023

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Many sites want to set up banners to display information for different users, such as regional announcements or country-based alerts. Doing this with feature flags is simple and this tutorial shows you how to set it up for a Next.js app (but it can be done with any framework).

Create a Next.js app

The first step is creating a basic app, which we Next.js to do. Run the script below, choose no to using TypeScript, yes to using the app router, and the defaults for everything else.

Terminal
npx create-next-app@latest banner

Next, go into your banner folder and install posthog-js.

Terminal
cd banner
npm i posthog-js

After installing PostHog, create a providers.js file in the app folder which contains the PostHog initialization with your project API key and instance address (you can get both in your project settings)

JavaScript
// app/providers.js
'use client'
import posthog from 'posthog-js'
import { PostHogProvider } from 'posthog-js/react'


if (typeof window !== 'undefined') {
  posthog.init(
    '<ph_project_api_key>',{
      api_host:https://us.i.posthog.com
    }
  )
}


export default function PHProvider({ children }) {
  return <PostHogProvider client={posthog}>{children}</PostHogProvider>
}

Once we created the provider, we can add it to our layout.js file.

JavaScript
// app/layout.js
import './globals.css'
import Providers from './providers'


export default function RootLayout({ children }) {
  return (
    <html lang="en">
      <Providers>
        <body>{children}</body>
      </Providers>
    </html>
  )
}

Finally, we can run the app with npm run dev. When we go to the page running locally, we see events captured into our PostHog instance.

Events

Set up the banner feature flag

First, make sure that the GeoIP app is enabled (it should be by default).

Once you have confirmed that, we can create our feature flag controlling the site banner. Go to feature flags, click "Create new flag," and name the key as "site-banner." For release conditions, match users where the continent code equals NA (or whichever geography you want), roll out to 100% of users, and press save.

Adding a flag payload

Optionally, we can edit the flag to add a payload with content and a link. We can then access this payload when we implement the banner.

JSON
{
   "title": "Greetings North American",
   "link": "http://posthog.com/"
}

This enables us to change the banner content without rewriting any code, just editing the flag payload in PostHog.

Implementing the banner in our app

With our flag created, we can go back into our app and implement our banner. To do this, create a banner.js file in the app folder. In this file, import useFeatureFlagEnabled from posthog-js/react, use it to check the site-banner flag, and set up your banner to conditionally show if the flag is true. With some banner styling, this looks like this:

JavaScript
// app/banner.js
'use client'
import { useFeatureFlagEnabled } from 'posthog-js/react'


export default function Banner() {


  const flagEnabled = useFeatureFlagEnabled('site-banner')


  return (
    <>
      {flagEnabled && (
        <div style={{ 
          backgroundColor: '#0071ff33',
          color: '#ffffff',
          textAlign: 'center',
          padding: '10px',
        }}>
          <p style={{
            fontSize: '18px',
            fontWeight: 'bold',
          }}>
            Welcome North American!
          </p>
        </div>
      )}
    </>
  )
}

With the banner set up, we import and add it to our layout.js file.

JavaScript
// app/layout.js
import './globals.css'
import Providers from './providers'
import Banner from './banner'


export default function RootLayout({ children }) {
  return (
    <html lang="en">
      <Providers>
        <Banner />
        <body>{children}</body>
      </Providers>
    </html>
  )
}

When we go to our locally running site now, we should see our new banner at the top of our page.

Banner

Handling the flag payload

If you added a payload to your flag, you can set up your banner to handle it. We must change useFeatureFlagEnabled for useFeatureFlagPayload, and then add the variables you get from the payload to your component.

JavaScript
// app/banner.js
'use client'
import Link from 'next/link'
import { useFeatureFlagPayload } from 'posthog-js/react'


export default function Banner() {


  const flagPayload = useFeatureFlagPayload('site-banner')


  return (
    <>
      {flagPayload && (
        <div style={{ 
          backgroundColor: '#0071ff33',
          color: '#ffffff',
          textAlign: 'center',
          padding: '10px',
        }}>
          <p style={{
            fontSize: '18px',
            fontWeight: 'bold',
          }}>
            <Link href={flagPayload.link}>
              {flagPayload.title}
            </Link>
          </p>
        </div>
      )}
    </>
  )
}

With this, we have full control of the visibility and content of our site banner from the feature flag in PostHog.

Further reading