ERC20 Viem Backend

A Node.js backend built with TypeScript and Express, designed to interact with an ERC20 token smart contract using the viem library. This project lets you deploy a token, interact with it (balance, approve, transfer, transferFrom), and test blockchain logic locally using Hardhat.

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Project Setup

Prerequisites

• Node.js v22
• Yarn or npm
• TypeScript installed globally or via ts-node

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Install Dependencies

npm install
# or
yarn install

Usage Instructions

Start Local Hardhat Node

npm run start:node

This will start a local Ethereum network at http://127.0.0.1:8545 and give you test accounts with private keys.

Copy a Private Key

From the Hardhat node logs, choose an account's private key and paste it into your .env file:

DEFAULT_ACCOUNT_PRIVATE_KEY=0x...

Compile the Smart Contract

npm run compile:contract

Deploy the Contract

npm run deploy:contract

After deployment, copy the deployed contract address from the terminal output and paste it into .env:

CONTRACT_ADDRESS=0xYourDeployedContractAddress

Start the Server

npm run start:server

Your backend will run at: http://localhost:3000

Environment Variables (.env)

# Blockchain
RPC_URL=http://127.0.0.1:8545
DEFAULT_ACCOUNT_PRIVATE_KEY=0x...

# Contract
CONTRACT_ADDRESS=0x...
CONTRACT_INFO_PATH=artifacts/contracts/Token.sol/CustomToken.json

# Server
PORT=3000
NODE_ENV=development

Available Scripts

Command	Description
start:node	Starts a local Hardhat blockchain
compile:contract	Compiles the Solidity smart contract
deploy:contract	Deploys the contract using TypeScript/viem
start:server	Starts the Express backend server
test	Runs backend tests (Jest)

API Usage Example

POST /api/transferFrom

Execute a token transfer on behalf of another address.

• URL: http://127.0.0.1:3000/api/transferFrom
• Method: POST
• Headers: Content-Type: application/json

Request Body

    {
      "from": "0x70997970C51812dc3A010C7d01b50e0d17dc79C8",
      "to": "0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266",
      "amount": 100
    }

Successful Response

{
    "success": true,
    "hash": "0x...",
    "from": "0x7099...",
    "to": "0xf39f...",
    "amount": "100",
    "message": "Transferred 100 tokens from 0x7099... to 0xf39f..."
}

Make sure that from has already approved the DEFAULT_ACCOUNT_PRIVATE_KEY to spend tokens on its behalf.

Design Decisions & Considerations

• Viem over Ethers.js: We chose viem for modern type-safety, performance, and better developer ergonomics.
• Modular Architecture: Routes, blockchain logic, and deployment scripts are separated for maintainability.
• Manual ABI Injection: The ABI is pulled dynamically from artifacts/ after compilation to avoid redundancy.
• TypeScript: Strong typing across the project for safer contract interaction and better DX.
• Middleware: Includes custom error handling and JSON parsing.