How can I calculate gas fee when swapping tokens?

Question

This is my code.

const amountIn = ethers.utils.parseUnits('0.1', 'ether'); //ether is the measurement, not the coin
const amounts = await router.getAmountsOut(amountIn, [buyToken, sellToken]);

const amountOutMin = amounts[1].sub(amounts[1].div(10)); // math for Big numbers in JS
console.log(`
~~~~~~~~~~~~~~~~~~~~
Buying new token
~~~~~~~~~~~~~~~~~~~~
buyToken: ${amountIn.toString()} ${buyToken} (WBNB)
sellToken: ${amountOutMin.toString()} ${sellToken}
`);
const tx = await router.swapExactTokensForTokens(
    amountIn,
    amountOutMin,
    [buyToken, sellToken],
    addresses.me,
    Date.now() + 1000 * 60 * 5 //5 minutes
);
const receipt = await tx.wait();
console.log('Transaction receipt');
console.log(receipt);

But there's nowhere that gas is added to this. I only knew how to send signed transactions but now I find the swapExactTokensForTokens function and I really can't work out how to send gas when using it.

Answer 1

I guess you're using Ethers.js, so you can use overrides, contract.METHOD_NAME( ...args [ , overrides ] );

in your case:

const tx = await router.swapExactTokensForTokens(
    amountIn,
    amountOutMin,
    [buyToken, sellToken],
    addresses.me,
    Date.now() + 1000 * 60 * 5, //5 minutes
    {
       gasPrice: 1 //(wei)
    }
);

You can use it for:

• overrides.from - the msg.sender (or CALLER) to use during the execution of the code
• overrides.value - the msg.value (or CALLVALUE) to use during the execution of the code overrides.
• overrides.gasPrice , etc...

More info here https://docs.ethers.io/v5/api/contract/contract/#Contract-functionsCall