In this article, we’ll explore a familiar scenario to Blockchain developers and users: sending the maximum amount of Ether from a wallet. If you’ve ever used MetaMask, you’ve likely noticed the “Send Max” feature when making a transfer. Inspired by this useful functionality, we’ll demonstrate how to implement a similar feature using ethers.js, a popular Ethereum JavaScript library.

MetaMask’s “Send Max” Feature

MetaMask, a widely-used Ethereum wallet and browser extension, has a “Send Max” feature that automatically calculates the maximum amount of Ether (or ERC-20 tokens) you can send from your wallet, taking into account the gas fee for the transaction. This feature is extremely handy when you want to empty an address or send as much as possible in one go.

The ethers.js Library

To implement our version of this feature, we’ll use ethers.js. This is a comprehensive Ethereum library that simplifies interactions with the Ethereum blockchain from JavaScript. It provides an intuitive interface for tasks like sending transactions, managing accounts, interacting with smart contracts, and more.

The Challenge

Just like MetaMask’s “Send Max” feature, our task is to send the maximum amount of Ether from a wallet. However, since every Ethereum transaction requires gas (a measure of computational workload), and gas costs Ether, we need to subtract the gas fee from the total balance to calculate the maximum amount that can be sent.

Understanding Gas and Calculating Fees

In the Ethereum network, operations cost ‘gas’, and this ‘gas’ has a price. It’s crucial to understand this concept before we dive into our code. Don’t worry if you’re new to Ethereum, we’ll walk you through it step-by-step!

What is Gas?

Think of Ethereum as a giant, global computer. When you want this computer to do something, you need to pay for it. The payment is made in ‘gas’. Just like you need fuel (or ‘gas’) to drive a car, you need ‘gas’ to make things happen on the Ethereum network.

How is Gas Priced?

The price of ‘gas’ on Ethereum isn’t fixed. It’s usually given in a unit called ‘Gwei’. To fetch the current gas price, we can use ethers.js like this:

const gasPrice = await wallet.provider.getGasPrice();

This will return the current gas price in Gwei. And just for your information, 1 Ether equals 1,000,000,000 (one billion) Gwei!

Calculating the Gas Fee

For a simple Ether transfer, the gas cost is typically 21000 units. To calculate the gas fee, we multiply this number by the current gas price. Here’s how we do it in code:

const gasEstimate = ethers.BigNumber.from(21000); // Gas units for a simple Ether transfer.
const gasFee = gasPrice.mul(gasEstimate);

This will give you the gas fee in Ether.

How Does Gas Affect the Maximum Transferable Amount?
When you’re sending all your Ether to someone else, you need to remember to save some for the gas fee. Here’s how we calculate the maximum transferable amount in code:

const balance = await wallet.getBalance(); // Get the balance of the wallet.
const maxAmount = balance.sub(gasFee); // Subtract the gas fee from the balance.

In this code, maxAmount is the maximum amount of Ether that you can send from your wallet.

With this understanding of gas and its calculation, you’re now ready to understand the rest of the code we’re going to work with. Let’s dive in…

Our “Send Max” Solution
Inspired by MetaMask, here is a TypeScript code snippet that demonstrates how to calculate and send the maximum amount of Ether from a wallet:

// RPC provider for interacting with blockchain.
// put your rpc url instead of <rpc-url-here>
const provider = new ethers.providers.JsonRpcProvider("<rpc-url-here>");

async function sendMaxAmount() {
  const wallet = new ethers.Wallet(
    '0x........',                      // senders private key
    provider
  );
  const recipient = '0x...';  // recipients address

  // Get the current gas price from the network.
  const gasPrice = await wallet.provider.getGasPrice();

  // Estimate the gas cost for the transaction.
  const gasEstimate = ethers.BigNumber.from(21000); // This is a rough estimate for a simple Ether transfer.

  // Calculate the gas fee.
  const gasFee = gasPrice.mul(gasEstimate);

  // Get the balance of the wallet.
  const balance = await wallet.getBalance();

  // Make sure the balance is greater than the gas fee.
  if (balance.lt(gasFee)) {
    throw new Error('Balance is not enough to cover the gas fee.');
  }

  // Calculate the maximum amount of Ether that can be sent by subtracting the gas fee from the balance.
  const maxAmount = balance.sub(gasFee);

  // Send the transaction.
  const transactionHash = await wallet.sendTransaction({
    from: wallet.address,
    to: recipient,
    value: maxAmount,
    gasPrice: gasPrice,
    gasLimit: gasEstimate,
  });

  return transactionHash.hash;
}

Let’s dissect this code:

1. Set up the wallet and recipient: The wallet is initialized with the private key and provider. The recipient’s address is specified as a string. Remember to replace rpc url, sender’s private key, and recipient’s address in the code.
2. Get the current gas price: This is the current price per unit of gas, obtained from the network via the provider.
3. Estimate the gas cost for the transaction: This is a rough estimate for a simple Ether transfer, set to 21000 units of gas.
4. Calculate the gas fee: This is done by multiplying the gas price by the estimated gas cost. This amount of Ether will be used to pay for the transaction.
5. Get the balance of the wallet: This is the total amount of Ether in the wallet.
6. Check if the balance can cover the gas fee: If the balance is less than the gas fee, it throws an error.
7. Calculate the maximum amount of Ether that can be sent: This is done by subtracting the gas fee from the balance.
8. Send the transaction: The transaction is sent using the sendTransaction method of the wallet. The transaction hash is returned.

Conclusion

Just as MetaMask’s “Send Max” feature improves user experience by automating the calculation of transaction costs, our solution allows developers to implement a similar feature using ethers.js. Understanding how to calculate and deduct gas fees is fundamental to Ethereum development, as it enables us to determine the maximum amount of Ether that can be sent in a transaction.

Remember, always handle private keys with utmost care and never expose them in your code or version control systems.