Key Components

1. User Model

interface User {
  id: string;
  credits: number;
  // ... other fields
}

2. Stripe Product Setup

const PRICE_IDS = {
  "100": process.env.PRICE_ID_100, // 100 credits
  "250": process.env.PRICE_ID_250, // 250 credits
  "500": process.env.PRICE_ID_500, // 500 credits
} as const;

// Map credits to their price IDs
type Credits = keyof typeof PRICE_IDS;

3. Checkout Session Creation

async function createCheckoutSession(userId: string, credits: Credits) {
  const priceId = PRICE_IDS[credits];
  if (!priceId) throw new Error("Invalid credits amount");

  const session = await stripe.checkout.sessions.create({
    mode: "payment",
    metadata: {
      userId,
      credits, // Store credits in metadata for webhook
    },
    line_items: [
      {
        price: priceId,
        quantity: 1,
      },
    ],
    success_url: `${process.env.HOST_URL}/dashboard`,
    cancel_url: `${process.env.HOST_URL}/pricing`,
  });

  return session;
}

4. Webhook Handler

async function handleStripeWebhook(body: unknown, signature: string) {
  const event = stripe.webhooks.constructEvent(
    body,
    signature,
    process.env.STRIPE_WEBHOOK_SECRET!
  );

  if (event.type === "checkout.session.completed") {
    const session = event.data.object as Stripe.Checkout.Session;
    const { userId, credits } = session.metadata;

    // Add credits to user
    await updateUserCredits(userId, parseInt(credits));
  }

  return { success: true };
}

5. Credit Management

// Add credits (after purchase)
async function updateUserCredits(userId: string, amount: number) {
  const user = await getUser(userId);
  const newCredits = user.credits + amount;

  await updateUser(userId, { credits: newCredits });
}

// Use credits (before action)
async function useCredits(userId: string, amount = 1) {
  const user = await getUser(userId);

  if (user.credits < amount) {
    throw new Error("Insufficient credits");
  }

  const newCredits = user.credits - amount;
  await updateUser(userId, { credits: newCredits });
}

Usage Pattern

1. User clicks "Buy Credits" button

2. Create checkout session with metadata

3. Redirect to Stripe checkout

4. Webhook triggers on successful payment

5. Parse metadata and add credits to user

6. Check credits before expensive operations:

async function generateSomething(userId: string) {
  try {
    await useCredits(userId);
    // Do expensive operation
  } catch (error) {
    throw new Error("Please purchase more credits");
  }
}

Tips

• Always use database transactions when modifying credits

• Add credits field to your auth token/session for quick checks

• Consider adding a credit history/log for transparency

• Make credit operations idempotent using session IDs