_Follow along the course with this video._ --- ### Intro to Function Selectors Continuing from the last lesson, when we call the `fund` function our MetaMask is going to pop up with a bunch of information about the transaction. ::image{src='/html-fundme/3-function-selector/function-selector1.png' style='width: 75%; height: auto;'} By clicking the `Hex` tab, we can confirm the raw data for this transaction and exactly which function is being called. ::image{src='/html-fundme/3-function-selector/function-selector2.png' style='width: 75%; height: auto;'} We'll go into `function selectors` a lot more later, but the important thing to understand is that when a Solidity contract is compiled, our functions are converted into a low-level bytecode called a `function selector`. When we call our `fund` function, this is converted to a `function selector` that we can actually verify using Foundry's `cast` command. ```bash cast sig "fund()" ``` The above should result in the output `0xb60d4288` and when we compare this to the `Hex` data in our MetaMask, we see that it does indeed match! Were the function being called something secret/nefarious like `stealMoney()`. This function selector would be completely different. Running our cast command again confirms this clearly with a return of `0xa7ea5e4e`. We can use this knowledge to verify any function we're calling through our browser wallet by comparing the expected and actual `function selectors` for the transaction. There's even a way to decode calldata using the cast command. Let's say our function was a little different and it required an argument. ```js function fund(uint256 amount) public payable { require(amount.getConversionRate(s_priceFeed) >= MINIMUM_USD, "You need to spend more ETH!"); // require(PriceConverter.getConversionRate(msg.value) >= MINIMUM_USD, "You need to spend more ETH!"); s_addressToAmountFunded[msg.sender] += amount; s_funders.push(msg.sender); } ``` If we were to call this function, the information MetaMask gives us is a little different. ::image{src='/html-fundme/3-function-selector/function-selector3.png' style='width: 75%; height: auto;'} In this instance, we can use the command `cast --calldata-decode <SIG> <CALLDATA>` to provide us the parameters being passed in this function call! ```bash cast --calldata-decode "fund(uint256)" 0xca1d209d000000000000000000000000000000000000000000000000016345785d8a0000 ``` The above decodes to: ```bash 100000000000000000 [1e17] ``` 0.1 Eth! The same amount being passed as an argument to our `fund` call. It seems this function is safe! ### Wrap Up This more or less summarizes how transactions work through our browser wallet and what we can expect to see from a low-level with respect to the encoded `function selectors` and `calldata`, we'll go over those in more detail later. I encourage you to experiment with the remaining functions on the front end. A few things to try: - Funding and Withdrawing with an account - Funding with Account A and Withdrawing with Account B - what happens? - Verify the `function selectors` of our other functions In our next lesson we'll recap everything we've learnt so far 💪