Simulations

Simulations are a way to debug and validate your transactions without having to actually submit them and irreversibly updating your chain's state.

Typically wallets rely on external services such as Tenderly or, more recently, on the new'ish eth_simulateV1 RPC method (not yet supported on all chains). This allows wallets to give you a preview of what your transaction will do (e.g.: will it succeed? which token transfers will happen?).

However, 3rd-party services are not always free, and most importantly, they can't operate on your local anvil. However, the technology for enabling has been around for a while, and you use it all the time if you develop smart contracts with forge. It's just that it's not typically exposed in a way that frontend developers can leverage it.

ethui uses revm to simulate transactions just as forge itself would, allowing you to preview important details about it in the preview screen.