How to set up feature flags in Nuxt

Feature flags help you release features and conditionally show content. This tutorial shows you how integrate them in your Nuxt.js app using PostHog.

We'll create a basic Nuxt app, add PostHog, create a feature flag, and then implement the flag to control content in our app.

Create your Nuxt app

For this tutorial, we create a basic Nuxt 3 app. First, ensure Node.js is installed (version 18.0.0 or newer). Then run the following command:

npx nuxi@latest init <project-name>

Name it whatever you like (we call ours nuxt-feature-flags), select npm as the package manager, and use the defaults for the remaining options.

Replace the code in app.vue with a simple heading:

<template>
  <main>
    <h1>Nuxt.js 3 feature flags</h1>
  </main>
</template>

Run npm run dev to start your app.

Adding PostHog on the client side

This tutorial shows how to integrate PostHog with Nuxt 3. If you're using Nuxt 2, see our Nuxt docs for how to integrate PostHog.

Since PostHog handles the management and evaluation of feature flags, we must set it up in our app. If you don't have a PostHog instance, you can sign up for free here.

Start by installing the posthog-js library to get access to the JavaScript Web SDK.

npm install posthog-js

Then, add your PostHog API key and host to your nuxt.config.ts file. You can find your project API key in your PostHog project settings

export default defineNuxtConfig({
 devtools: { enabled: true },
  runtimeConfig: {
    public: {
      posthogPublicKey: '<ph_project_api_key>',
      posthogHost: 'https://us.i.posthog.com'
    }
  }
})

Create a new plugin by creating a new folder called plugins in your base directory and then a new file posthog.client.js:

mkdir plugins
cd plugins 
touch posthog.client.js

Add the following code to your posthog.client.js file:

import { defineNuxtPlugin } from '#app'
import posthog from 'posthog-js'

export default defineNuxtPlugin(nuxtApp => {
  const runtimeConfig = useRuntimeConfig();
  const posthogClient = posthog.init(runtimeConfig.public.posthogPublicKey, {
    api_host: runtimeConfig.public.posthogHost,
  })
  
  return {
    provide: {
      posthog: () => posthogClient
    }
  }
})

Once you’ve done this, reload your app. You should begin seeing events in the PostHog events explorer.

Creating a feature flag

With PostHog set up, your app is ready for feature flags. To create one, go to the feature flags tab in PostHog and click "New feature flag." Enter a flag key (like my-cool-flag), set the release condition to roll out to 100% of users, and press "Save."

You can customize your release conditions with rollout percentages, and user or group properties to fit your needs.

Implementing flag code on the client side

Once created, we can add our feature flag to our app. We do this using the posthog.onFeatureFlags callback.

In our example, we fetch our feature flag and update the text on the page based on its value:

<template>
  <main>
    <h1>{{ title }}</h1>
  </main>
</template>

<script setup>

const title = ref('No variant');
onMounted(() => {
  const { $posthog } = useNuxtApp()
  if ($posthog) {
    const posthog = $posthog()
    if (posthog.isFeatureEnabled('my-cool-flag')) {
      title.value = 'Our flag is enabled!';
    } else {
      title.value = 'Our flag is disabled!';
    }
  }
});

</script>

When you run your app now, you should see the updated text.

Implementing flag code on the server side

You may notice the text flickers when you load your app. This is because PostHog is making a request on the client side to fetch the flag value. There are two ways to prevent this:

1. Fetch the flag on the server side.
2. Bootstrap flags on the client side (not covered in this tutorial).

We'll show you how to fetch the flag on the server side. First, install the posthog-node SDK:

npm install posthog-node

Then, we use the PostHog Node library to fetch the feature flag using useAsyncData. Replace the code in app.vue with the following:

<template>
  <main>
    <h1>{{ title }}</h1>
  </main>
</template>

<script setup>
import { useAsyncData, useRuntimeConfig } from 'nuxt/app';
import { PostHog } from 'posthog-node';

const { data: titleData, error } = await useAsyncData('titleData', async () => {
  const runtimeConfig = useRuntimeConfig();
  const posthog = new PostHog(
    runtimeConfig.public.posthogPublicKey,
    { host: runtimeConfig.public.posthogHost }
  );

  let returnedValue = '';
  const distinctId = 'placeholder-user-id'
  const isFlagEnabled = await posthog.isFeatureEnabled('my-cool-flag', distinctId);
  if (isFlagEnabled) {
    returnedValue = 'Our flag is enabled!';
  } else {
    returnedValue = 'Our flag is disabled!'
  }
  return returnedValue;
});
const title = computed(() => titleData.value);

</script>

Now when you reload your app, the flicker is gone.

Setting the correct distinctId

You may notice that we set distinctId = 'placeholder-user-id' in our flag call above. In production apps, to ensure you fetch the correct flag value for your user, distinctId should be set to their unique ID.

For logged-in users, you typically use their email as their distinctId. However, for logged-out users, you can use the distinct_id property from their PostHog cookie:

<!-- rest of your code -->

<script setup>
import { useAsyncData, useRuntimeConfig, useCookie } from 'nuxt/app';
import { PostHog } from 'posthog-node';

const { data: titleData, error } = await useAsyncData('titleData', async () => {
  const runtimeConfig = useRuntimeConfig();
  const posthog = new PostHog(
    runtimeConfig.public.posthogPublicKey,
    { host: runtimeConfig.public.posthogHost }
  );
  let returnedValue = 'No cookie';

  const cookies = useCookie(`ph_${runtimeConfig.public.posthogPublicKey}_posthog`);
  if (cookies && cookies.value) {
    const distinctId = cookies.value.distinct_id;
    const isFlagEnabled = await posthog.isFeatureEnabled('my-cool-flag', distinctId);
    if (isFlagEnabled) {
      returnedValue = 'Our flag is enabled!';
    } else {
      returnedValue = 'Our flag is disabled!';
    }
  }
  return returnedValue;
});
const title = computed(() => titleData.value);

</script>

Further reading

• How to set up analytics in Nuxt
• How to set up A/B tests in Nuxt
• How to set up surveys in Nuxt

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