node-web3-js

Setup

1. Populate .env file:
   1. WALLET_PRIVATE_KEY
      • How to export a Metamask account's private key
   2. ALCHEMY_API_KEY
      • Alchemy API Key
   3. ALCHEMY_WEBHOOK_SIGNING_KEY
      • Alchemy Custom Webhooks
2. Run npm install to install dependencies
3. Run npm run build to build the project and compile Typescript in Javascript
4. Run npm run start to start the Express server and listen to API calls

Creating an Alchemy Webhook

You can follow the docs by Alchemy on Custom Webhooks .

On Alchemy when creating a Webhook you'll be asked to write a GraphQL query to filter the contract addresses and the events you are interested it.

Make sure your Webhook is pointing to the right network. In the default case, it should be pointing to the Polygon Mumbai network.

How can I test this locally?

You can use Ngrok to expose your local environment to the internet. Open a terminal window and type ngrok http 3000 and you will get a URL that you can use to test your webhook.

Example URL by Ngrok: https://bc9d-2a01-11-9210-5a40-5c18-d3b5-7dcd-b34e.ngrok-free.app

So on Alchemy you can put https://bc9d-2a01-11-9210-5a40-5c18-d3b5-7dcd-b34e.ngrok-free.app/webhooks/events

Test the project

Once everything is set up, you can test the project by sending a POST request to the following endpoint:

• POST http://localhost:3000/tokens/send

  {
    "address": "0x8E8C44C72cee669EBcb0933360F194C9b7cC8BEd",
    "amount": 10
  }

Step-by-step guide

Follow this guide to recreate the repo step-by-step.

1. Assuming you are in a new (and clean) folder, the first thing to do is initializing a new npm project. You can do that using the CLI npm init -y. This command will create a package.json file with a simple setup and configuration that you will be able to edit later.
2. Before starting to code, let's first install all the packages that we will need along our journey: npm install --save express ethers web3 web3-eth web3-eth-accounts bignumber.js dotenv
   1. express: to easily setup a REST API
   2. ethers, web3, web3-eth, web3-eth-accounts: needed for our web3 shenanigans (interacting with the blockchain)
   3. bignumber.js: the blockchain uses this format to represent numbers, so we need it to properly decode some data
3. Let's start with our Express app! Create an index.ts file inside the src folder and set up a simple Express with one endpoint POST /tokens/send. We also need to create a controller for this endpoint, so let's create a controllers folder with a tokens.ts file in it. In tokens.ts we will write some code that allows transferring an amount of tokens from the owner address to another address. Try it out on Postman and check Polygon scan if the transaction went well.
4. Next in line, we want to be able to listen to our Token smart contract events. Meaning that everytime somebody transfers tokens to another address, we want to be notified about: who transferred the tokens to whom, together with the amount transferred. To do that we can integrate with the newly released Alchemy Custom Webhooks. We will create a new endpoint in our express app: POST /webhooks/events, and a new controller that processes the event and logs the data in which we are interested.
5. To complete our exercise, let's try the whole flow again. Trigger an HTTP call to send some tokens, and let's see if our webhook endpoint is received and decoding the event data properly.