Sign message with etherjs using following ethereum format signature EIP-1271. If your signature contains complex data follow the EIP-712 to structure your signature & hash: this will allow end users to see the data they sign instead of an incomprehensible bytes32. For safe signatures follow these recommendations https://swcregistry.io/docs/SWC-121

sign_message.js

const { ethers } = require('ethers');
/** 
 * @notice Path to where your .env file is. If the .env is at the root of your project,
 *         you can simply write `require('dotenv').config()`
 */
require('dotenv').config({ path: '../../.env' });

async function main() {
    // instanciation can also be done using a private key 
    const wallet = new ethers.Wallet.fromMnemonic(process.env.SEED);
    // by default the address at index 0 is used
    walletAddr = await wallet.getAddress();
    console.log('walletAddr: ', walletAddr);

    /// @dev in solidity this will be written: `bytes memory rawEncodedMsg = abi.encode(param1, param2, ...)` 
    const rawEncodedMsg = ethers.utils.defaultAbiCoder.encode(
        ['uint256', 'uint256', 'address', 'address', 'uint256', 'uint256'],
        [
            parseInt(process.env.ID),
            ethers.BigNumber.from(process.env.AMOUNT),
            process.env.FROM,
            process.env.TO,
            parseInt(process.env.TIMESTAMP),
            parseInt(process.env.CHAIN_ID),
        ]
    );
    /**
     * @dev Create the hash of the original message.
     *      In solidity this will be written: `bytes32 hash = keccak256(rawEncodedMsg)`
     */
    const encodedMessage = ethers.utils.keccak256(rawEncodedMsg);

    /**
     * @dev If only a bytes32 is needed we can encode the message in one line (like below),
     * using: `ethers.utils.solidityKeccak256`
     */
    console.log('raw encoded msg: ', rawEncodedMsg);
    console.log('encoded msg: ', encodedMessage);

    /**
     * @dev Equivalent of OpenZeppelin: ECDSA.toEthSignedMessageHash(bytes memory s), which is:
     * `keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n", Strings.toString(s.length), s));`
     */
    let ethSignedMessageHash = ethers.utils.solidityKeccak256(
        ['string', 'string', 'bytes'],
        [
            '\x19Ethereum Signed Message:\n',
            ethers.utils.hexDataLength(rawEncodedMsg).toString(),
            rawEncodedMsg,
        ]
    );

    console.log('ethSignedMessageHash: ', ethSignedMessageHash);

    // etherjs specific format requirements for signature
    let messageHashBytes = ethers.utils.arrayify(rawEncodedMsg);

    /// @dev equivalent of: ethereum.request({method: 'personal_sign', params: [walletAddr, messageHash]})
    let signature = await wallet.signMessage(messageHashBytes);
    console.log('signature: ', signature);

    ///@dev Pass r,s & v in Rewards contract
    console.log('v: ', ethers.utils.splitSignature(signature).v);
    console.log('r: ', ethers.utils.splitSignature(signature).r);
    console.log('s: ', ethers.utils.splitSignature(signature).s);
}

main().then(console.log).catch(console.error);