An async permit signing function

permitFunction.ts

// Uses a default infinite expiration time
async function getPermitSignature(
  token: ERC20Permit,
  sourceAddr: string,
  spenderAddr: string,
  spenderAmount: BigNumberish,
  version: string
) {

  // To be replaced in front end with signer type from ethers
  const signer = ethers.provider.getSigner(0);

  const name = await token.name();
  const chainId = await signer.getChainId();
  const domain = {
    name: name,
    version: version,
    chainId: chainId,
    verifyingContract: token.address
};

  const types = { "Permit": [{
    "name": "owner",
    "type": "address"
    },
    {
      "name": "spender",
      "type": "address"
    },
    {
      "name": "value",
      "type": "uint256"
    },
    {
      "name": "nonce",
      "type": "uint256"
    },
    {
      "name": "deadline",
      "type": "uint256"
    }
  ]}

  const nonce = await token.nonces(sourceAddr);

  const data = {
    owner:  sourceAddr,
    spender: spenderAddr,
    value: spenderAmount,
    nonce: nonce,
    deadline: ethers.constants.MaxUint256
  };


  let sigStringPromise = signer._signTypedData(domain, types, data);
  return sigStringPromise.then((value: string) => {
    return parseSigString(value)
  })
}