Estimate gas fees for smart contract function call with Ethers.js

Question

Let's say you have a function in your smart contract, like:

function saveUser(address newUser) public {
    // Saving user
}

How can we estimate, with Ethers.js, what the gas price will be for calling this function?

Answer 1

First, you must get the estimated gas price to use in an empty transaction:

const gasPrice = await provider.getGasPrice();

Then, get the estimated gas price for your specific transaction, and multiply it by gasPrice:

const functionGasFees = await contract.estimateGas.saveUser(userAddress);
const finalGasPrice = gasPrice * functionGasFees;

Of course, finalGasPrice will never be exactly equal to the actual gas price you will end up paying, but it is a close enough estimate.