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Web3.js (deprecated)

Web3.js has been sunset in 2024. Please consider using thirdweb or viem for your project.
Web3.js was established in 2014, making it the oldest web3 library. With extensive documentation, an active community and modular design, Web3.js is powerful and easy-to-use. It has support for Celo features (specifically Fee Abstraction via plugins since version 4.13.1. To learn more about building with Web3.js, check out their docs as they have excellent examples of how to use it in your project.

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Fee Abstraction with Web3.js

Fee Abstraction on Celo enables the use of stablecoins like cUSD, USDT and USDC as gas tokens.

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Requirements

  • Web3.js v. > 4.13.1

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Install the Celo Web3.js Plugin

For Celo’ specific features like Fee Abstraction transactions you need to install @celo/web3-plugin-transaction-types as well as web3@4.13.1 or higher. This also adds utils like getCoreContractAddress for fetching core contract address from onchain registry. This is useful to get the stablecoin addresses you are planning to use programmatically. Make sure they are listed in the FeeCurrencyDirectory.sol. 
yarn add @celo/web3-plugin-transaction-types web3
Now web3.celo is available and web3.eth is “celo-aware”. “aware” means that if the tx has the feeCurrency param, calling eth.sendTransaction will use the celo cip64 serializer and be able to recognize the fee currency. Here an example on how to use “feeCurrency” with cEUR. 
// see web3 docs for more info on setup

import { Web3 } from "web3";
import { CeloTransactionTypesPlugin } from "@celo/web3-plugin-transaction-types";

const web3 = new Web3("http://127.0.0.1:8545");
web3.registerPlugin(new CeloTransactionTypesPlugin());

// Now `web3.celo` is available and `web3.eth` is celo-aware. 
// Get all StableToken Addresses through the getCoreContractAddress
const cEUR = await web3.celo.getCoreContractAddress("StableTokenEUR");
const txData = {
  from: "0x123...",
  to: "0x456...",
  value: 123n,
  // find the list of accepted tokens in the Fee Abstraction documentation
  feeCurrency: cEUR, // optional
};
await web3.celo.populateTransaction(txData);
const tx = await web3.eth.sendTransaction(txData);

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Gas Price

When paying for transaction with an alternate feeCurrency token it is important to know the price of gas denominated in that token. You can use web3.celo.populateTransaction to make sure maxPriorityFeePerGas, maxFeePerGas, and gas are filled properly. 
await web3.celo.populateTransaction(txData);
You can find more examples in the github repository readme.