EIP712 for DAI style permit()

prepareDataStruct.js

async preparePermitDataStruct (holder, allowed) {
    const netId = await this.web3.eth.net.getId()
    let nonce = await this.stableTokenContract.methods.nonces(holder).call({
      from: holder
    })
    const expiration = Math.floor(Date.now() / 1000) + 900000
    const EIP712Domain = [
      {
        name: 'name',
        type: 'string'
      },
      {
        name: 'version',
        type: 'string'
      },
      {
        name: 'chainId',
        type: 'uint256'
      },
      {
        name: 'verifyingContract',
        type: 'address'
      }
    ]
    const Permit = [
      {
        name: 'holder',
        type: 'address'
      },
      {
        name: 'spender',
        type: 'address'
      },
      {
        name: 'nonce',
        type: 'uint256'
      },
      {
        name: 'expiry',
        type: 'uint256'
      },
      {
        name: 'allowed',
        type: 'bool'
      }
    ]

    const domain = {
      name: process.env.VUE_APP_TOKEN_NAME,
      version: '1',
      chainId: parseInt(netId),
      verifyingContract: verifyingContract
    }
    var message = {
      holder: holder,
      spender: depositManagerAddress,
      nonce,
      expiry: expiration,
      allowed: allowed
    }
    const data = {
      types: {
        EIP712Domain,
        Permit
      },
      domain,
      primaryType: 'Permit',
      message
    }
    return Promise.resolve(data)
  }
}

signIt.js

// npm install eth-sig-util --save-dev

const sigUtil = require('eth-sig-util')

let px = '0x123123...'
let privateKey = Buffer.from(pk.slice(2), 'hex')
let msgParams = {
  data: typedData
}

try {
  const signature = sigUtil.signTypedData(privateKey, msgParams)
  resolve(signature)
} catch (e) {
  console.log(e)
  reject(e)
}