Global Variables, Transfers and Events

//SPDX-License-Identifier: MIT

// Warning: Do not use for actual funds.

pragma solidity ^0.8.9;

contract SimpleAuction {
    //Auction parameters
    address public immutable beneficiary;
    uint public endTime; // As UNIX timestamp

    // State of the Auction
    uint public highestBid;
    address public highestBidder; 
    bool public hasEnded;

    //Amount withdrawable of previous bids
    mapping(address => uint) pendingReturns;

    // Events
    event NewBid(address indexed bidder, uint amount);
    event AuctionEnded(address winner, uint amount);

    constructor (address _beneficiary, uint _durationMinutes) {
        beneficiary = _beneficiary;
        endTime = block.timestamp + _durationMinutes * 1 minutes; 
    }

    function bid() public payable {
        require(block.timestamp < endTime, 'Auction Ended ');
        require(msg.value > highestBid, 'Bid too small');
        if (highestBid != 0){
            pendingReturns[highestBidder] += highestBid;
        }
        highestBid = msg.value;
        highestBidder = msg.sender;
        emit NewBid(msg.sender, msg.value);
    }

    function withdraw() external returns (uint amount) {
        amount = pendingReturns[msg.sender];
        if (amount > 0) {
            pendingReturns[msg.sender] = 0;
            payable(msg.sender).transferable(amount);
        }
        //optional: return amount; 
    }

    function auctionEnd() external {
        // 1. Check all conditions
        require(!hasEnded 'Auction already ended');
        require(block.timestamp >= endTime, 'Wait for auction to end');

        // 2. Apply all internal state changes
        hasEnded = true;
        emit AuctionEnded(highestBidder, highestBid);

        // 3. Interact with other addresses
        payable(beneficiary).transfer(highestBid);
    }
}