Every modern web application needs a database, and setting one up usually means wrestling with server configurations, connection pooling, and security policies. If you're building a new product and want to ship fast, you've likely looked at BaaS platforms. Supabase provides an open source alternative to Firebase, giving you a full Postgres database, auto-generated APIs, and authentication out of the box.
To connect your frontend application to this backend infrastructure, you need two critical pieces of configuration data: the project URL and the public anon key. Without these credentials, your client-side code can't talk to your database or fetch user sessions. But misusing them leads to leaky data and compromised applications.
You'll find your project URL and anon key inside your Supabase dashboard under the project settings API tab. The project URL acts as the unique network endpoint for your specific database instance. It tells your client application exactly where to send HTTP and WebSocket requests. The anon key, short for anonymous key, is a public JSON Web Token that grants safe, unauthenticated access to your database endpoints from the browser.
Understanding how these two values function together keeps your app secure while letting users interact with public data. Let's break down what they do, how to configure them safely, and why treating them like passwords will break your application.
What is a Supabase Project URL?
Your project URL is the unique address assigned to your backend instance. It looks like a standard web domain, usually structured with your unique project reference ID followed by the Supabase domain suffix.
When you initialize the client library in JavaScript, TypeScript, or any other supported language, you pass this URL as the primary argument. Every query you write—whether fetching rows from a table or signing up a new user—routes through this endpoint.
The URL itself isn't a secret. Anyone inspecting network traffic in browser developer tools can see it. That's by design. Because your frontend code runs on user devices, it has to know where to send network requests.
Structuring Your Environment Variables
Never hardcode your project URL directly into source files. If you migrate from a staging database to a production instance, hardcoding forces you to manually update every file in your repository.
Instead, store the value in an environment configuration file. For Next.js projects, use the NEXT_PUBLIC_ prefix so the value is accessible on the client side. Vite applications use VITE_.
import { createClient } from '@supabase/supabase-js'
const supabaseUrl = process.env.NEXT_PUBLIC_SUPABASE_URL
const supabaseAnonKey = process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY
export const supabase = createClient(supabaseUrl, supabaseAnonKey)
If you miss the environment prefix, your client initialization fails because the variables evaluate to undefined in the browser.
Decoding the Supabase Anon Key
The anon key causes the most confusion for developers coming from traditional backend architectures. People see a long string of characters starting with eyJ... and automatically assume it's a master password that needs to be hidden at all costs.
That's incorrect.
The anon key is a public token designed to be sent to client browsers, mobile devices, and public-facing apps. It tells the Supabase API gateway that the request is coming from an unauthenticated or anonymous visitor.
When a user opens your app for the first time, they haven't logged in yet. The app uses the anon key to make requests to the database. Crucially, this doesn't mean the user has full access to everything.
How Row Level Security Protects the Anon Key
Security in Supabase doesn't rely on hiding the anon key. It relies on Row Level Security, known as RLS.
Postgres enforces RLS policies on every table. When a request arrives using the anon key, Postgres checks who is making the request and evaluates your defined security rules. If a table has RLS enabled and no policy allows public reading, the database returns an empty array or an error.
For example, if you have a table storing private user profiles, an anon key request will fail to read those rows unless a specific policy grants access.
- Enable RLS on every new table you create in the dashboard.
- Write explicit policies for SELECT, INSERT, UPDATE, and DELETE operations.
- Test your policies by querying data while signed out.
If you forget to enable RLS, anyone with your project URL and anon key can read, modify, or delete data in that table. The key itself is safe to expose, but your database schema must be locked down.
Setting Up Your Client Connection
Getting your application connected takes just a few lines of code, but getting the configuration right prevents subtle runtime bugs. You should initialize your client once in a dedicated utility file and import that instance across your components.
Let's look at a clean setup for a modern web project. First, install the official package using your terminal:
npm install @supabase/supabase-js
Next, create your environment file in the root directory. Name it .env.local for local development.
NEXT_PUBLIC_SUPABASE_URL=https://xyzprojectref.supabase.co
NEXT_PUBLIC_SUPABASE_ANON_KEY=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
Then, initialize the client module as shown earlier. Keeping this logic centralized ensures you don't accidentally create multiple client instances, which exhausts connection pools and causes memory leaks.
You can read more about client configuration options in the Supabase JavaScript Client Documentation.
Common Mistakes and Security Pitfalls
Developers often make avoidable mistakes when handling project credentials. Recognizing these traps early saves hours of debugging and prevents security incidents.
Confusing the Anon Key with the Service Role Key
Supabase generates two primary keys for every project: the anon key and the service_role key.
The service role key bypasses all RLS policies entirely. It has root access to your database. Never expose the service role key in frontend code, mobile apps, or public repositories. If you put the service role key in a React component, any user can open inspect element, grab the key, and wipe your entire database.
Only use the service role key in secure server environments, such as API routes, serverless functions, or backend Node.js servers.
Exposing Secrets in Public Repositories
Pushing your .env file to GitHub is a classic mistake. Even though the anon key is meant to be public, keeping your local configuration files private is still best practice because they often contain other sensitive variables, like third-party API tokens.
Add your environment files to your .gitignore file immediately when starting a project.
# .gitignore
.env
.env.local
.env.*.local
Disabling Row Level Security for Convenience
When developers run into errors where queries return no data, they sometimes disable RLS on the table to make it work. This opens a massive security hole.
Instead of turning off security, write a proper policy. If you want public data to be readable by everyone, create a policy that explicitly permits read access for the anon role.
create policy "Allow public read access"
on public.articles
for select
to public
using (true);
This approach keeps your database secure while giving you explicit control over what is visible to the public.
Frequently Asked Questions
Can I change my project URL or anon key if they get leaked?
Your project URL is tied to your project reference ID and cannot be changed easily without migrating to a new project. However, you can rotate your API keys in your Supabase dashboard settings if your service role key was accidentally exposed.
Why is my query returning empty data even with the anon key?
This usually happens because Row Level Security is enabled on the table, but no policy exists to allow the anon role to read the data. Check your database policies in the dashboard and ensure you have a SELECT policy configured for public or authenticated roles.
Should I use the anon key in mobile apps?
Yes. Mobile apps built with Flutter, React Native, or native iOS and Android run on user devices, meaning the code is technically accessible to the user. Use the anon key combined with RLS and Supabase Auth to handle user logins safely.
What is the difference between the anon key and authenticated users?
The anon key identifies requests coming from visitors who haven't logged in. Once a user signs in through Supabase Auth, the client library attaches that user's JWT to subsequent requests, allowing RLS policies to evaluate permissions based on the specific user ID.
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