When is Ethereum gas fee calculated during execution of contract. I was wondering if i take out flash loan and if the trade happened to be profitable, will the profit be able to offset transaction fee or I would have to pay upfront?
When you create a transaction you set two parameters:
gas: Maximum amount of gas to be used
gasPrice: Price to pay for each gas unit
To execute the transaction your account needs to have at least
gas * gasPrice ether.
Before executing your account is charged with
gas * gasPrice.
After the transaction has completed its execution unused gas and refunded gas are returned to your account.