Express.js SDK

Installation

Install the SDK via npm:

npm install lancer-express-sdk

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Getting Started

1. Initialize the Lancer SDK

Create a reusable instance of the lancer function with your signingSecret for signature verification. Store this in a shared module for easy access across your API routes.

import lancer from "lancer-express-sdk";

const lancerInstance = lancer({
  signingSecret: "<your-lancer-signing-secret>",
});

export default lancerInstance;

Parameter	Type	Required	Description
signingSecret	string	Yes	Secret key used to verify webhook signatures.

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2. Set Up API Routes

Authentication Middleware

Use the auth method to handle session authentication in an Express.js route. Implement your custom logic within the handler.

import express from "express";
import lancerInstance from "@/lib/lancer";

const router = express.Router();

router.post(
  "/auth",
  lancerInstance.auth(async ({ token, session }) => {
    console.log("Session Payload:", session);
    
    // Custom authentication logic
    return {
      ownerId: "<user-id>", // Replace with actual user/owner ID
      status: 200, // HTTP status code
    };
  })
);

export default router;

Parameter	Type	Description
token	string	Lancer session token from the Authorization header.
session	SessionRequest	Payload containing session details from Lancer.

Example Request

POST /auth
Authorization: Bearer <token>
{
  "sessionId": "abc123"
}

Example Response

{
  "ownerId": "user123"
}

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Webhook Middleware

Handle webhook events sent by Lancer with the webhook method. Enable verification to ensure payload integrity using your signingSecret.

import express from "express";
import lancerInstance from "@/lib/lancer";

const router = express.Router();

router.post(
  "/webhook",
  lancerInstance.webhook(async ({ event, payload }) => {
    console.log("Webhook Event:", event);
    
    // Handle event data
    return true;
  }, true)
);

export default router;

Parameter	Type	Description
handler	(event: WebhookEvent) => Promise<boolean>	Callback function to process webhook events.
verification	boolean	Enables payload verification (default: true).

Verification Workflow

• The SDK verifies the x-timestamp and x-signature headers.
• The payload is signed using HMAC SHA-256 and compared to the provided signature.
• If the verification fails, the SDK responds with 400 Bad Request.

Example Webhook Payload

{
  "id": "evt_123",
  "type": "file.uploaded",
  "payload": {
    "fileId": "file_abc",
    "userId": "user123"
  }
}

Example Response

{
  "status": 200,
  "message": "Webhook processed successfully"
}

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3. Directory Structure

Organize your Express.js project for modularity:

/lib/
  lancer.js     # Lancer SDK instance
/routes/
  auth.js       # Authentication route
  webhook.js    # Webhook route

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API Reference

Function: lancer

Constructor

lancer({ signingSecret: string });

Methods

1. auth(handler: Function): Middleware

   • Handles session authentication using custom logic.
   • Parameters:
     • handler({ token, session }): Promise<{ ownerId: string; status: number }>

2. webhook(handler: Function, verification?: boolean): Middleware

   • Processes Lancer webhook events.
   • Parameters:
     • handler({ event, payload }): Promise<boolean>
     • verification (optional): boolean

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Types

SessionAuthGrant

interface SessionAuthGrant {
  ownerId: string;
  status: number;
}

WebhookEvent

type WebhookEvent<T> = {
  type: string;
  payload: T;
};

SessionRequest

interface SessionRequest {
  sessionId: string;
  [key: string]: any;
}

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Security Best Practices

1. Protect Your Signing Secret: Ensure your signingSecret is stored securely in environment variables.
2. Verify Signatures: Always enable verification for sensitive webhook endpoints.
3. Rate Limit Your API: Use rate-limiting middleware to prevent abuse.

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License

MIT License © 2025 Weekend Dev Labs