How to build a public Expensify integration: Building the Auth Flow

Aug 7, 2024 • 8 minute read

Hey there, fellow JavaScript enthusiasts! Ready to dive into the world of Expensify integrations? Today, we're going to tackle one of the most crucial parts of building a public integration: the authorization flow. Buckle up, because we're about to make your Expensify integration dreams come true!

Introduction

Expensify's API is a powerful tool that allows us to tap into their expense management ecosystem. But before we can start playing with receipts and reports, we need to get our authorization ducks in a row. Trust me, it's not as daunting as it sounds!

Prerequisites

Before we jump in, make sure you've got:

An Expensify Partner account (if you don't have one, go grab it!)
A solid grasp on OAuth 2.0 (don't worry, we'll refresh your memory)
A Node.js and Express.js setup ready to go

Got all that? Great! Let's get this party started.

Setting up the Expensify API credentials

First things first, we need to get cozy with Expensify's developer portal:

Log into your Expensify Partner account
Navigate to the API credentials section
Grab your Client ID and Client Secret (treat these like your firstborn)
Set up your redirect URI (this is where Expensify will send your users after they log in)

Pro tip: Keep these credentials safe and sound. They're the keys to your Expensify kingdom!

Implementing the Authorization Flow

Alright, here's where the magic happens. We're going to break this down into three main steps:

Initiating the auth request

const authUrl = `https://www.expensify.com/oauth2/authorize?client_id=${CLIENT_ID}&redirect_uri=${REDIRECT_URI}&response_type=code&scope=expenses`;

res.redirect(authUrl);

This little snippet will send your users on a trip to Expensify's login page. They'll log in, grant permissions, and then Expensify will redirect them back to your app with a shiny new authorization code.

Handling the callback

app.get('/callback', async (req, res) => {
  const { code } = req.query;
  
  // Exchange the code for tokens
  const tokenResponse = await axios.post('https://www.expensify.com/oauth2/token', {
    client_id: CLIENT_ID,
    client_secret: CLIENT_SECRET,
    code,
    redirect_uri: REDIRECT_URI,
    grant_type: 'authorization_code'
  });

  const { access_token, refresh_token } = tokenResponse.data;

  // Store these tokens securely!
  // ...

  res.send('Authorization successful!');
});

When Expensify sends the user back, we grab that authorization code and trade it in for access and refresh tokens. It's like exchanging tickets for awesome prizes at the arcade!

Storing tokens securely

Now, I can't stress this enough: store these tokens like they're the nuclear launch codes. Use secure storage solutions, encrypt them if possible, and never, ever expose them client-side.

Refreshing Access Tokens

Access tokens don't last forever, so we need to be ready to refresh them:

async function refreshAccessToken(refreshToken) {
  const response = await axios.post('https://www.expensify.com/oauth2/token', {
    grant_type: 'refresh_token',
    client_id: CLIENT_ID,
    client_secret: CLIENT_SECRET,
    refresh_token: refreshToken
  });

  return response.data.access_token;
}

Keep an eye on those expiration times and refresh before they expire. It's like changing the oil in your car – do it regularly, and you'll avoid a lot of headaches!

Making Authenticated Requests

Now that we've got our access token, let's use it:

const response = await axios.get('https://integrations.expensify.com/Integration-Server/ExpensifyIntegrations', {
  headers: {
    Authorization: `Bearer ${accessToken}`
  },
  params: {
    // Your API request parameters here
  }
});

Remember to handle any authentication errors gracefully. Nobody likes a crashy app!

Best Practices

Here are some golden rules to live by:

Store tokens securely (I know, I'm a broken record)
Log errors, but don't expose sensitive info
Respect Expensify's rate limits (they're not just suggestions)

Testing the Auth Flow

Set up a test environment and try to break your auth flow. Test edge cases like expired tokens, network errors, and user cancellations. The more you test now, the fewer surprises you'll have later!

Conclusion

And there you have it, folks! You've just built a rock-solid authorization flow for your Expensify integration. Pat yourself on the back – you've earned it!

Remember, this is just the beginning. With this auth flow in place, you're now ready to start building out the rest of your integration. The sky's the limit!

Additional Resources

Now go forth and build amazing things with Expensify! And remember, if you get stuck, the developer community is always here to help. Happy coding!