Bidder.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;
contract Bidder {
 uint256 highestBid;
 address currentHighestBidder;
function bid() external payable {
 require(msg.value > highestBid, "Bid higher");
 require(payable(currentHighestBidder).transfer(highestBid)); // ensure the bid is sent back, else revert the transaction
 highestBid = msg.value;
 currentHighestBidder = msg.sender;
 }
}