How can I avoid out of gas on multiple eth transfers when looping addresses?

Question

Hello everyone good day,

I'm working on an automated eth distribution function that sends eth to all the nft holders of a determined collection (using ERC721A as standard for the nfts), it works well with small iterations, but when I tested it with 500 holders it fails with the out of gas error message.

I'm currently using the gnosis chain mainnet network for the tests, tried many things but I guess the idea of looping through wallets to automatically send the eth is unimaginable?

Can some gas optimization help to implement this? Because 500 addresses aren't that much, thinking in distributing to at least 3,000~5,000.

Should I have to implement some enqueue system at contract level to process smaller batches? Are there any eips or already made examples you could share with me please?

Here's the distribution contract:

interface IERC721A {
    /**
     * The caller must own the token or be an approved operator.
     */
    error ApprovalCallerNotOwnerNorApproved();

    /**
     * The token does not exist.
     */
    error ApprovalQueryForNonexistentToken();

    /**
     * Cannot query the balance for the zero address.
     */
    error BalanceQueryForZeroAddress();

    /**
     * Cannot mint to the zero address.
     */
    error MintToZeroAddress();

    /**
     * The quantity of tokens minted must be more than zero.
     */
    error MintZeroQuantity();

    /**
     * The token does not exist.
     */
    error OwnerQueryForNonexistentToken();

    /**
     * The caller must own the token or be an approved operator.
     */
    error TransferCallerNotOwnerNorApproved();

    /**
     * The token must be owned by `from`.
     */
    error TransferFromIncorrectOwner();

    /**
     * Cannot safely transfer to a contract that does not implement the
     * ERC721Receiver interface.
     */
    error TransferToNonERC721ReceiverImplementer();

    /**
     * Cannot transfer to the zero address.
     */
    error TransferToZeroAddress();

    /**
     * The token does not exist.
     */
    error URIQueryForNonexistentToken();

    /**
     * The `quantity` minted with ERC2309 exceeds the safety limit.
     */
    error MintERC2309QuantityExceedsLimit();

    /**
     * The `extraData` cannot be set on an unintialized ownership slot.
     */
    error OwnershipNotInitializedForExtraData();

    // =============================================================
    //                            STRUCTS
    // =============================================================

    struct TokenOwnership {
        // The address of the owner.
        address addr;
        // Stores the start time of ownership with minimal overhead for tokenomics.
        uint64 startTimestamp;
        // Whether the token has been burned.
        bool burned;
        // Arbitrary data similar to `startTimestamp` that can be set via {_extraData}.
        uint24 extraData;
    }

    // =============================================================
    //                         TOKEN COUNTERS
    // =============================================================

    /**
     * @dev Returns the total number of tokens in existence.
     * Burned tokens will reduce the count.
     * To get the total number of tokens minted, please see {_totalMinted}.
     */
    function totalSupply() external view returns (uint256);

    // =============================================================
    //                            IERC165
    // =============================================================

    /**
     * @dev Returns true if this contract implements the interface defined by
     * `interfaceId`. See the corresponding
     * [EIP section](https://eips.ethereum.org/EIPS/eip-165#how-interfaces-are-identified)
     * to learn more about how these ids are created.
     *
     * This function call must use less than 30000 gas.
     */
    function supportsInterface(bytes4 interfaceId) external view returns (bool);

    // =============================================================
    //                            IERC721
    // =============================================================

    /**
     * @dev Emitted when `tokenId` token is transferred from `from` to `to`.
     */
    event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);

    /**
     * @dev Emitted when `owner` enables `approved` to manage the `tokenId` token.
     */
    event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);

    /**
     * @dev Emitted when `owner` enables or disables
     * (`approved`) `operator` to manage all of its assets.
     */
    event ApprovalForAll(address indexed owner, address indexed operator, bool approved);

    /**
     * @dev Returns the number of tokens in `owner`'s account.
     */
    function balanceOf(address owner) external view returns (uint256 balance);

    /**
     * @dev Returns the owner of the `tokenId` token.
     *
     * Requirements:
     *
     * - `tokenId` must exist.
     */
    function ownerOf(uint256 tokenId) external view returns (address owner);

    /**
     * @dev Safely transfers `tokenId` token from `from` to `to`,
     * checking first that contract recipients are aware of the ERC721 protocol
     * to prevent tokens from being forever locked.
     *
     * Requirements:
     *
     * - `from` cannot be the zero address.
     * - `to` cannot be the zero address.
     * - `tokenId` token must exist and be owned by `from`.
     * - If the caller is not `from`, it must be have been allowed to move
     * this token by either {approve} or {setApprovalForAll}.
     * - If `to` refers to a smart contract, it must implement
     * {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
     *
     * Emits a {Transfer} event.
     */
    function safeTransferFrom(
        address from,
        address to,
        uint256 tokenId,
        bytes calldata data
    ) external payable;

    /**
     * @dev Equivalent to `safeTransferFrom(from, to, tokenId, '')`.
     */
    function safeTransferFrom(
        address from,
        address to,
        uint256 tokenId
    ) external payable;

    /**
     * @dev Transfers `tokenId` from `from` to `to`.
     *
     * WARNING: Usage of this method is discouraged, use {safeTransferFrom}
     * whenever possible.
     *
     * Requirements:
     *
     * - `from` cannot be the zero address.
     * - `to` cannot be the zero address.
     * - `tokenId` token must be owned by `from`.
     * - If the caller is not `from`, it must be approved to move this token
     * by either {approve} or {setApprovalForAll}.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(
        address from,
        address to,
        uint256 tokenId
    ) external payable;

    /**
     * @dev Gives permission to `to` to transfer `tokenId` token to another account.
     * The approval is cleared when the token is transferred.
     *
     * Only a single account can be approved at a time, so approving the
     * zero address clears previous approvals.
     *
     * Requirements:
     *
     * - The caller must own the token or be an approved operator.
     * - `tokenId` must exist.
     *
     * Emits an {Approval} event.
     */
    function approve(address to, uint256 tokenId) external payable;

    /**
     * @dev Approve or remove `operator` as an operator for the caller.
     * Operators can call {transferFrom} or {safeTransferFrom}
     * for any token owned by the caller.
     *
     * Requirements:
     *
     * - The `operator` cannot be the caller.
     *
     * Emits an {ApprovalForAll} event.
     */
    function setApprovalForAll(address operator, bool _approved) external;

    /**
     * @dev Returns the account approved for `tokenId` token.
     *
     * Requirements:
     *
     * - `tokenId` must exist.
     */
    function getApproved(uint256 tokenId) external view returns (address operator);

    /**
     * @dev Returns if the `operator` is allowed to manage all of the assets of `owner`.
     *
     * See {setApprovalForAll}.
     */
    function isApprovedForAll(address owner, address operator) external view returns (bool);

    // =============================================================
    //                        IERC721Metadata
    // =============================================================

    /**
     * @dev Returns the token collection name.
     */
    function name() external view returns (string memory);

    /**
     * @dev Returns the token collection symbol.
     */
    function symbol() external view returns (string memory);

    /**
     * @dev Returns the Uniform Resource Identifier (URI) for `tokenId` token.
     */
    function tokenURI(uint256 tokenId) external view returns (string memory);

    // =============================================================
    //                           IERC2309
    // =============================================================

    /**
     * @dev Emitted when tokens in `fromTokenId` to `toTokenId`
     * (inclusive) is transferred from `from` to `to`, as defined in the
     * [ERC2309](https://eips.ethereum.org/EIPS/eip-2309) standard.
     *
     * See {_mintERC2309} for more details.
     */
    event ConsecutiveTransfer(uint256 indexed fromTokenId, uint256 toTokenId, address indexed from, address indexed to);
}

interface IERC721AQueryable is IERC721A {
    /**
     * Invalid query range (`start` >= `stop`).
     */
    error InvalidQueryRange();

    /**
     * @dev Returns the `TokenOwnership` struct at `tokenId` without reverting.
     *
     * If the `tokenId` is out of bounds:
     *
     * - `addr = address(0)`
     * - `startTimestamp = 0`
     * - `burned = false`
     * - `extraData = 0`
     *
     * If the `tokenId` is burned:
     *
     * - `addr = <Address of owner before token was burned>`
     * - `startTimestamp = <Timestamp when token was burned>`
     * - `burned = true`
     * - `extraData = <Extra data when token was burned>`
     *
     * Otherwise:
     *
     * - `addr = <Address of owner>`
     * - `startTimestamp = <Timestamp of start of ownership>`
     * - `burned = false`
     * - `extraData = <Extra data at start of ownership>`
     */
    function explicitOwnershipOf(uint256 tokenId) external view returns (TokenOwnership memory);

    /**
     * @dev Returns an array of `TokenOwnership` structs at `tokenIds` in order.
     * See {ERC721AQueryable-explicitOwnershipOf}
     */
    function explicitOwnershipsOf(uint256[] memory tokenIds) external view returns (TokenOwnership[] memory);

    /**
     * @dev Returns an array of token IDs owned by `owner`,
     * in the range [`start`, `stop`)
     * (i.e. `start <= tokenId < stop`).
     *
     * This function allows for tokens to be queried if the collection
     * grows too big for a single call of {ERC721AQueryable-tokensOfOwner}.
     *
     * Requirements:
     *
     * - `start < stop`
     */
    function tokensOfOwnerIn(
        address owner,
        uint256 start,
        uint256 stop
    ) external view returns (uint256[] memory);

    /**
     * @dev Returns an array of token IDs owned by `owner`.
     *
     * This function scans the ownership mapping and is O(`totalSupply`) in complexity.
     * It is meant to be called off-chain.
     *
     * See {ERC721AQueryable-tokensOfOwnerIn} for splitting the scan into
     * multiple smaller scans if the collection is large enough to cause
     * an out-of-gas error (10K collections should be fine).
     */
    function tokensOfOwner(address owner) external view returns (uint256[] memory);
}

contract Ownable {
    address private _owner;
    address private _previousOwner;
    uint256 private _lockTime;

    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    constructor () {
        address msgSender = msg.sender;
        _owner = msgSender;
        emit OwnershipTransferred(address(0), msgSender);
    }

    function owner() public view returns (address) {
        return _owner;
    }   
    
    modifier onlyOwner() {
        require(_owner == msg.sender, "Ownable: caller is not the owner");
        _;
    }
    
    function renounceOwnership() public virtual onlyOwner {
        emit OwnershipTransferred(_owner, address(0));
        _owner = address(0);
    }

    function transferOwnership(address newOwner) public virtual onlyOwner {
        require(newOwner != address(0), "Ownable: new owner is the zero address");
        emit OwnershipTransferred(_owner, newOwner);
        _owner = newOwner;
    }

    function getUnlockTime() public view returns (uint256) {
        return _lockTime;
    }
    
    function getTime() public view returns (uint256) {
        return block.timestamp;
    }

    function lock(uint256 time) public virtual onlyOwner {
        _previousOwner = _owner;
        _owner = address(0);
        _lockTime = block.timestamp + time;
        emit OwnershipTransferred(_owner, address(0));
    }
    
    function unlock() public virtual {
        require(_previousOwner == msg.sender, "You don't have permission to unlock");
        require(block.timestamp > _lockTime , "Contract is locked until 7 days");
        emit OwnershipTransferred(_owner, _previousOwner);
        _owner = _previousOwner;
    }
}

abstract contract ReentrancyGuard {
    uint256 private constant _NOT_ENTERED = 1;
    uint256 private constant _ENTERED = 2;

    uint256 private _status;

    constructor() {
        _status = _NOT_ENTERED;
    }

    modifier nonReentrant() {
        _nonReentrantBefore();
        _;
        _nonReentrantAfter();
    }

    function _nonReentrantBefore() private {
        require(_status != _ENTERED, "ReentrancyGuard: reentrant call");
        _status = _ENTERED;
    }

    function _nonReentrantAfter() private {
        _status = _NOT_ENTERED;
    }
}

contract EthDistributionOnNFTs is Ownable, ReentrancyGuard {

    IERC721AQueryable public _nft;

    uint256 public totalDistributedRewards = 0;

    mapping (address => uint256) public holdersClaimedReward;
    mapping (address => bool) private payedHolder;

    address[] private holders;

    constructor (address nft_) {
        _nft = IERC721AQueryable(nft_);
    }

    function distribute() internal nonReentrant {

        for(uint256 i=0; i < holders.length; i++){
            payedHolder[holders[i]] = false;  
        }

        delete holders;

        uint256 totalSupplyNFT = _nft.totalSupply();
        uint256 totalETH = address(this).balance;
        uint256 totalDistributed = 0;

        for(uint256 i=1; i <= totalSupplyNFT; i++){

            IERC721AQueryable.TokenOwnership memory tokenOwner = _nft.explicitOwnershipOf(i);

            if (tokenOwner.addr != address(0) && !tokenOwner.burned) {
                uint256 holderTotalNFT = _nft.tokensOfOwner(tokenOwner.addr).length;
                uint256 holderShare = (holderTotalNFT*100*100)/_nft.totalSupply();
                uint256 holderETH = (totalETH*holderShare)/10000;

                if(!payedHolder[tokenOwner.addr]) {
                    (bool sent,) = payable(tokenOwner.addr).call{ value: holderETH }('');
                    require(sent, "ETH transfer failed");
                    payedHolder[tokenOwner.addr] = true;
                    holdersClaimedReward[tokenOwner.addr] += holderETH;
                    holders.push(tokenOwner.addr);
                    totalDistributed += holderETH;
                }
            }
        }

        uint256 remainingETH = address(this).balance;

        if(remainingETH > 0) {
            (bool sent,) = payable(msg.sender).call{ value: remainingETH }('');
            require(sent, "ETH transfer failed");
        }

        totalDistributedRewards += totalDistributed;
    }

    function setNFT(address nftAddress_) public onlyOwner {
        _nft = IERC721AQueryable(nftAddress_);
    }

    receive() external payable { 
        if(msg.value > 0) {
            distribute(); 
        }
    }
}

Here's the demo NFT address: 0x89ED5E0f6E3A6F34DF837770AbAf2A31b7840C27

Here's the demo NFT abi:

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IERC721A.TokenOwnership","name":"ownership","type":"tuple"}],"stateMutability":"view","type":"function"},{"inputs":[{"internalType":"uint256[]","name":"tokenIds","type":"uint256[]"}],"name":"explicitOwnershipsOf","outputs":[{"components":[{"internalType":"address","name":"addr","type":"address"},{"internalType":"uint64","name":"startTimestamp","type":"uint64"},{"internalType":"bool","name":"burned","type":"bool"},{"internalType":"uint24","name":"extraData","type":"uint24"}],"internalType":"struct IERC721A.TokenOwnership[]","name":"","type":"tuple[]"}],"stateMutability":"view","type":"function"},{"inputs":[{"internalType":"uint256","name":"tokenId","type":"uint256"}],"name":"getApproved","outputs":[{"internalType":"address","name":"","type":"address"}],"stateMutability":"view","type":"function"},{"inputs":[],"name":"getTime","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"stateMutability":"view","type":"function"},{"inputs":[],"name":"getUnlockTime","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"stateMutability":"view","type":"function"},{"inputs":[{"internalType":"address","name":"owner","type":"address"},{"internalType":"address","name":"operator","type":"address"}],"name":"isApprovedForAll","outputs":[{"internalType":"bool","name":"","type":"bool"}],"stateMutability":"view","type":"function"},{"inputs":[],"name":"isCollectionRevealed","outputs":[{"internalType":"bool","name":"","type":"bool"}],"stateMutability":"view","type":"function"},{"inputs":[],"name":"isPaused","outputs":[{"internalType":"bool","name":"","type":"bool"}],"stateMutability":"view","type":"function"},{"inputs":[],"name":"isTeamAlreadyMinted","outputs":[{"internalType":"bool","name":"","type":"bool"}],"stateMutability":"view","type":"function"},{"inputs":[{"internalType":"uint256","name":"time","type":"uint256"}],"name":"lock","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[],"name":"maxSupply","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"stateMutability":"view","type":"function"},{"inputs":[{"internalType":"uint256","name":"_mintAmount","type":"uint256"}],"name":"mint","outputs":[],"stateMutability":"payable","type":"function"},{"inputs":[{"internalType":"address","name":"_to","type":"address"},{"internalType":"uint256","name":"_amount","type":"uint256"}],"name":"mintForAddress","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"address","name":"_teamAddress","type":"address"}],"name":"mintForTeam","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[],"name":"mintPrice","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"stateMutability":"view","type":"function"},{"inputs":[],"name":"multisigWallet","outputs":[{"internalType":"address","name":"","type":"address"}],"stateMutability":"view","type":"function"},{"inputs":[],"name":"name","outputs":[{"internalType":"string","name":"","type":"string"}],"stateMutability":"view","type":"function"},{"inputs":[],"name":"owner","outputs":[{"internalType":"address","name":"","type":"address"}],"stateMutability":"view","type":"function"},{"inputs":[{"internalType":"uint256","name":"tokenId","type":"uint256"}],"name":"ownerOf","outputs":[{"internalType":"address","name":"","type":"address"}],"stateMutability":"view","type":"function"},{"inputs":[],"name":"renounceOwnership","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"address","name":"_token","type":"address"}],"name":"rescueTrappedFunds","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[],"name":"royaltyFee","outputs":[{"internalType":"uint96","name":"","type":"uint96"}],"stateMutability":"view","type":"function"},{"inputs":[{"internalType":"uint256","name":"_tokenId","type":"uint256"},{"internalType":"uint256","name":"_salePrice","type":"uint256"}],"name":"royaltyInfo","outputs":[{"internalType":"address","name":"","type":"address"},{"internalType":"uint256","name":"","type":"uint256"}],"stateMutability":"view","type":"function"},{"inputs":[{"internalType":"address","name":"from","type":"address"},{"internalType":"address","name":"to","type":"address"},{"internalType":"uint256","name":"tokenId","type":"uint256"}],"name":"safeTransferFrom","outputs":[],"stateMutability":"payable","type":"function"},{"inputs":[{"internalType":"address","name":"from","type":"address"},{"internalType":"address","name":"to","type":"address"},{"internalType":"uint256","name":"tokenId","type":"uint256"},{"internalType":"bytes","name":"_data","type":"bytes"}],"name":"safeTransferFrom","outputs":[],"stateMutability":"payable","type":"function"},{"inputs":[{"internalType":"address","name":"operator","type":"address"},{"internalType":"bool","name":"approved","type":"bool"}],"name":"setApprovalForAll","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"string","name":"_newURI","type":"string"}],"name":"setBaseURI","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"string","name":"_hiddenMetadataUri","type":"string"}],"name":"setHiddenMetadataUri","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"uint256","name":"_newSupply","type":"uint256"}],"name":"setMaxSupply","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"uint256","name":"_newPrice","type":"uint256"}],"name":"setMintPrice","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"address","name":"_newAddress","type":"address"}],"name":"setMultisigWallet","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"bool","name":"_state","type":"bool"}],"name":"setPaused","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"bool","name":"_state","type":"bool"}],"name":"setRevealed","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"uint96","name":"_newFee","type":"uint96"}],"name":"setRoyaltyFee","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"uint256","name":"_newAmount","type":"uint256"}],"name":"setTeamAllocation","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"bytes4","name":"interfaceId","type":"bytes4"}],"name":"supportsInterface","outputs":[{"internalType":"bool","name":"","type":"bool"}],"stateMutability":"view","type":"function"},{"inputs":[],"name":"symbol","outputs":[{"internalType":"string","name":"","type":"string"}],"stateMutability":"view","type":"function"},{"inputs":[],"name":"teamAllocation","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"stateMutability":"view","type":"function"},{"inputs":[{"internalType":"uint256","name":"tokenId","type":"uint256"}],"name":"tokenURI","outputs":[{"internalType":"string","name":"","type":"string"}],"stateMutability":"view","type":"function"},{"inputs":[{"internalType":"address","name":"owner","type":"address"}],"name":"tokensOfOwner","outputs":[{"internalType":"uint256[]","name":"","type":"uint256[]"}],"stateMutability":"view","type":"function"},{"inputs":[{"internalType":"address","name":"owner","type":"address"},{"internalType":"uint256","name":"start","type":"uint256"},{"internalType":"uint256","name":"stop","type":"uint256"}],"name":"tokensOfOwnerIn","outputs":[{"internalType":"uint256[]","name":"","type":"uint256[]"}],"stateMutability":"view","type":"function"},{"inputs":[],"name":"totalSupply","outputs":[{"internalType":"uint256","name":"","type":"uint256"}],"stateMutability":"view","type":"function"},{"inputs":[{"internalType":"address","name":"from","type":"address"},{"internalType":"address","name":"to","type":"address"},{"internalType":"uint256","name":"tokenId","type":"uint256"}],"name":"transferFrom","outputs":[],"stateMutability":"payable","type":"function"},{"inputs":[{"internalType":"address","name":"newOwner","type":"address"}],"name":"transferOwnership","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[],"name":"unlock","outputs":[],"stateMutability":"nonpayable","type":"function"}] 

Thank you very much fam!

Answer 1

Generally, looping through large arrays isn't good practice in Solidity since the gas cost can become unbounded, and then the function call will always fail with the out-of-gas error.

Creating an airdrop in the way you are describing would require you to send a large number of transactions regardless of how many batches you split them into, a much cheaper method of hosting an airdrop would be to use something like a Merkle Tree. This allows you to effectively store a lot of data, aka addresses and amounts to be airdropped, in a single bytes32 variable. Using this root variable anyone can create a proof that their leaf is contained inside the Merkle Tree without having to loop through the entire tree.

To showcase this in code I will be using an example repo I've created called foundry-merkle.

First, you'll need a script to create a Merkle tree using whatever data you want:

const { StandardMerkleTree } = require("@openzeppelin/merkle-tree");

async function createTree() {
// leaf consists of `holder`, `tokenId`, `eventId`, `heldUntil`
  const leaves = [
    ["0xe4A98D2bFD66Ce08128FdFFFC9070662E489a28E", "0", "0", "110"],
    ["0xe4A98D2bFD66Ce08128FdFFFC9070662E489a28E", "1", "0", "105"],
    ["0xe4A98D2bFD66Ce08128FdFFFC9070662E489a28E", "2", "0", "100"],
  ];

  // Create the Merkle tree and explicitly state the structure of the leaves
  const tree = StandardMerkleTree.of(leaves, [
    "address",
    "uint256",
    "uint256",
    "uint256",
  ]);

 console.log("Merkle Root:", tree.root);
  // write the entire tree to a JSON file at the specified path
  fs.writeFileSync("trees/tree_00.json", JSON.stringify(tree.dump()));
}

Once you have the tree, you can make the root publicly known along with a description of how you created the tree. This way anyone can recreate the root and prove that it is authentic.

Next, a user would need to create a proof using their leaf information and the publicly known root:

const { StandardMerkleTree } = require("@openzeppelin/merkle-tree");
const fs = require("fs");

async function createProof() {
  // grab the merkle tree from the specified path, this could potentially become an input param
  const tree = StandardMerkleTree.load(
    JSON.parse(fs.readFileSync("trees/tree_00.json", "utf8")),
  );

  // loop through the tree and find entries that match the target address
  for (const [i, v] of tree.entries()) {
    if (v[0] === "0xe4A98D2bFD66Ce08128FdFFFC9070662E489a28E") {
      // obtain the proof at the target entry and log it
      const proof = tree.getProof(i);
      console.log("Value:", v);
      console.log("Proof:", proof);
    }
  }
}

In the above example I loop through the tree to find the index we care about, the one that contains my address, then we log the proof at each level.

Finally, using the obtained proof, the user can call whatever claim() function is in place:

function claimWithSignature(
        bytes32[] memory proof,
        bytes memory signature,
        ClaimInfo memory info
    ) public returns (uint) {
        // ensure the event has ended
        if (block.number < eventMap[info.eventId].endBlock) {
            revert Claim__RewardsPeriodHasntEnded(
                block.number,
                eventMap[info.eventId].endBlock
            );
        }
        // ensure the user hasn't already claimed for this event and tokenId
        if (claimMap[info.eventId][info.tokenId]) {
            revert Claim__AlreadyClaimed();
        }
        // Recover the signer's address
        // create the message hash
        bytes32 messageHash = keccak256(abi.encodePacked(info.to));
        // sign the message hash
        bytes32 signedMessageHash = keccak256(
            abi.encodePacked("\x19Ethereum Signed Message:\n32", messageHash)
        );
        // recover the signer
        address signer = recoverSigner(signedMessageHash, signature);

        // revert if signer address isn't the holder
        if (signer != info.holder) {
            revert Claim__InvalidSigner();
        }

        // Calculate the user's `leaf` hash
        bytes32 leaf = keccak256(
            bytes.concat(
                keccak256(
                    abi.encode(
                        info.holder,
                        info.tokenId,
                        info.eventId,
                        info.heldUntil
                    )
                )
            )
        );

        // Use MerkleProof lib to verify proof with root and leaf
        bool verified = MerkleProof.verify(
            proof,
            eventMap[info.eventId].merkleRoot,
            leaf
        );
// Revert if the verification returns false
        if (!verified) {
            revert Claim__InvalidProof();
        }
}

In my function example, I combine the Merkle Tree proof with ECDSA, so that the user can claim from any address after first signing a message, however, this isn't necessary for your airdrop.

You can easily implement a Merkle tree into your project with Open Zeppelin's Solidity and Javascript libraries. I also found Smart Contract Programmer's written and video guide to be extremely helpful.