_You can follow along with the video course from here._ ### Introduction In this lesson, we will delve into how do _transaction reverts_ work, what is _gas_ where is used. ### Revert Let's start by adding some logic to the `fund` function: ```js uint256 public myValue = 1; function fund() public { myValue = myValue + 2; } ``` A _revert_ action **undoes** all prior operations and returns the remaining gas to the transaction's sender. In this `fund` function, `myValue` increases by two (2) units with each successful execution. However, if a revert statement is encountered right after, all actions performed from the start of the function are undone. `myValue` will then reset to its initial state value, or one. ```js uint256 public myValue = 1; function fund() public { myValue = myValue + 2; require(msg.value > 1e18, "didn't send enough ETH"); // a function revert will undo any actions that have been done. // It will send the remaining gas back } ``` ### Gas Usage > 🔥 **CAUTION**:br > The gas used in the transaction will not be refunded if the transaction fails due to a revert statement. The gas has already been **consumed** because the code was executed by the computers, even though the transaction was ultimately reverted. Users can specify how much gas they're willing to allocate for a transaction. In the case where the `fund` function will contain a lot of lines of code after the `require` and we did indeed set a limit, the gas which was previously allocated but not used will not be charged to the user > 🗒️ **NOTE**:br > If a transaction reverts, is defined as failed ### Transaction Fields During a **value** transfer, a transaction will contain the following fields: - **Nonce**: transaction counter for the account - **Gas price (wei)**: maximum price that the sender is willing to pay _per unit of gas_ - **Gas Limit**: maximum amount of gas the sender is willing to use for the transaction. A common value could be around 21000. - **To**: _recipient's address_ - **Value (Wei)**: amount of cryptocurrency to be transferred to the recipient - **Data**: 🫙 _empty_ - **v,r,s**: components of the transaction signature. They prove that the transaction is authorised by the sender. During a **_contract interaction transaction_**, it will instead be populated with: - **Nonce**: transaction counter for the account - **Gas price (wei)**: maximum price that the sender is willing to pay _per unit of gas_ - **Gas Limit**: maximum amount of gas the sender is willing to use for the transaction. A common value could be around 21000. - **To**: _address the transaction is sent to (e.g. smart contract)_ - **Value (Wei)**: amount of cryptocurrency to be transferred to the recipient - **Data**: 📦 _the content to send to the **To** address_, e.g. a function and its parameters. - **v,r,s**: components of the transaction signature. They prove that the transaction is authorised by the sender. ### Conclusion **Reverts** and **gas usage** help maintain the integrity of the blockchain state. _Reverts_ will undo transactions when failures occur, while _gas_ enables transactions execution and runs the EVM. When a transaction fails, the gas consumed is not recoverable. To manage this, Ethereum allows users to set the maximum amount of gas they're willing to pay for each transaction. ### 🧑‍💻 Test yourself 1. 📕 Describe the two types of transactions listed in this lesson. 2. 📕 Why are reverts used? 3. 🧑‍💻 Bob sets his gas price to 20 Gwei and his gas limit to 50,000 units. The transaction consumes 30,000 units of gas before a revert occurs. How much ETH will be effectively charged?