Generate offer signature

let contactAddr = '0xFa4D5258804D7723eb6A934c11b1bd423bC31623' // X2Y2 loan contract address
let chainId = 1 // Ethereum chain mainnet
const ethers = require('ethers')
// Prepare the encode function
function encode(isCollection, {
  nonce,
  expiry,
  lender,
  amount,
  repayment,
  nftAddress,
  tokenId,
  duration,
  extra,
  erc20Address,
  createTime,
}){
  try {
    const offerPack = ethers.utils.solidityPack(
      ['address', 'uint256', 'uint256', 'address', 'uint32', 'uint256', 'bytes'],
      [erc20Address, amount, repayment, nftAddress, duration, createTime, extra])

    const signaturePack = ethers.utils.solidityPack(
      ['address', 'uint256', 'uint256'],
      [lender, nonce, expiry])

    let message = null
    if (isCollection) {
      message = ethers.utils.solidityKeccak256(
        [ 'bytes', 'bytes', 'address', 'uint256'],
        [ offerPack, signaturePack, contactAddr, chainId])
    } else {
      message = ethers.utils.solidityKeccak256(
        [ 'bytes', 'uint256', 'bytes', 'address', 'uint256'],
        [ offerPack, tokenId, signaturePack, contactAddr, chainId])
    }
    return message
  } catch(e) {
    console.error('signature encde', e.message)
  }
  return ''
}

// Construct an offer
const offer={
  nonce: , // Globally unique nonce String
  expiry: , // Expiry time of the offer, Unix timestamp in seconds
  createTime: , // Create time of the offer, Unix timestamp in seconds
  lender:, // Lender address
  amount:, // Borrow amount (in wei unit),String
  repayment:, // Repayment (in wei unit),String
  nftAddress:'0xbc4ca0eda7647a8ab7c2061c2e118a18a936f13d', // Contract address of the NFT 
  tokenId:'', // NFT id, set to '' when you are making a collection offer
  duration:2592000, // Duration of the offer
  extra:'0x00', //Reserved for future usage. Use 0x00 for now.
  erc20Address:'0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2', // wETH contract address
}

// Sign the encoded offer with user's wallet
const wallet = new ethers.Wallet(privateKey)
const signature = await wallet.signMessage(ethers.utils.arrayify(encode(offer.tokenId === '',offer))) // Signature for the offer