Ethereum Smart Contract Development: Using Infura and web3.js to Call Contracts Without Running Your Own Node

Infura provides public Ethereum mainnet and testnet nodes. Simply sign up on Infura's website with basic details and email to receive an API key.

Querying Contract-Stored State via RPC

A common query is checking token balances. Let's use the EOS token contract as an example.

Obtaining Contract Information

Find contract details on Etherscan. Essential details include:

Contract Address: e.g., 0x86Fa049857E0209aa7D9e616F7eb3b3B78ECfdb0
Function Signature: For ERC20's balanceOf(address), the signature is 70a08231.

Calculating Function Signatures

SHA3 hash the function name with parameters: balanceOf(address) → 0x70a08231...
Extract the first 8 characters after 0x: 70a08231.

web3.js Example:

var functionSig = web3.sha3("balanceOf(address)").substr(2, 8);

Making RPC Calls with `eth_call`

For read-only functions, use eth_call. Example POST data:

{
  "jsonrpc": "2.0",
  "method": "eth_call",
  "params": [{
    "to": "0x86Fa049857E0209aa7D9e616F7eb3b3B78ECfdb0",
    "data": "0x70a0823100000000000000000000000033b8287511ac7F003902e83D642Be4603afCd876"
  }, "latest"],
  "id": 123
}

Response:

{
  "jsonrpc": "2.0",
  "id": 123,
  "result": "0x000000000000000000000000000000000000000000000000b1d16ec625a59d3e"
}

The hex value 0x...3e converts to ~12.8 EOS tokens.

Using web3.js (JavaScript API)

For state updates, transactions require signing with a private key and Ether for gas fees. We'll deploy a test contract on Ropsten for demonstration.

Setup

npm install web3 ethereumjs-tx

Initialize web3

var Web3 = require('web3');
var web3 = new Web3(new Web3.providers.HttpProvider("https://ropsten.infura.io/"));

Deployed Test Contract

Contract Address: 0x5fb30123b9efedcd15094266948fb7c862279ee1
Function Signatures:

data(): 73d4a13a
set(uint256): 60fe47b1

Querying Contract State

var result = web3.eth.call({
  to: "0x5fb30123b9efedcd15094266948fb7c862279ee1",
  data: "0x73d4a13a"
});
console.log(parseInt(result, 16)); // Output: 0 (initial state)

Updating Contract State

1. Define Raw Transaction

var rawTx = {
  nonce: '0x14',
  gasPrice: '0x3B9ACA00', // 1 Gwei
  gasLimit: '0xC20A',
  to: '0x5fb30123b9efedcd15094266948fb7c862279ee1',
  value: '0x00',
  data: '0x60fe47b1000000000000000000000000000000000000000000000000000000000000000a' // Sets data=10
};

2. Sign and Send

var Tx = require('ethereumjs-tx');
var tx = new Tx(rawTx);
tx.sign(privateKey);
web3.eth.sendRawTransaction('0x' + tx.serialize().toString('hex'), (err, hash) => {
  if (!err) console.log(hash); // Transaction hash
});

Success Output:
0x2a9d89b0f329853b5c0f83c83cea4cfa2e38ddd1041d9abd0afcc9af5ed1bf1b

👉 Track your transaction on Etherscan

FAQ

Q1: Why use Infura instead of running my own node?
A1: Infura eliminates the need for extensive hardware/bandwidth, providing instant access to Ethereum networks.

Q2: How do I estimate gas limits accurately?
A2: Tools like Remix IDE or Tenderly simulate transactions to suggest optimal gas values.

Q3: Can I use this for mainnet contracts?
A3: Yes! Replace Ropsten endpoints with https://mainnet.infura.io/ and ensure sufficient ETH for gas.

References

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