# Purpose

As part of the Basechain upgrade, we'll be updating the Transfer Gateway which will include redeployment of the Gateway contract to Ethereum Mainnet. To make the transition as smooth as possible, we've added a new API to loom-js for interacting with the Ethereum Gateway contracts. This page outlines the changes you'll need to make in order to ensure your DApp continues to work after the Transfer Gateway update goes live.

# 1. Update loom-js to 1.70.1

To use the new API, you have to make sure that you're running loom-js version 1.70.1 or later.

In a terminal window, type the following command to see your loom-js version:

npm ls --depth=0 loom-js

If you're running a version earlier than 1.70.1, you can update it by entering the following command:

npm install [email protected]

# 2. Changes to the loom-js API

You should update your code to use the new loom-js Ethereum Gateway wrapper API to ensure minimal disruption when the Ethereum Gateway contract is updated on Ethereum Mainnet. Otherwise, users of your DApp will not be able to transfer assets from Loom to Ethereum.

# 2.1 Use the createEthereumGatewayAsync function to Instantiate the Ethereum Transfer Gateway

The method signatures of the withdraw methods on the latest version of the Ethereum Gateway contract have changed, loom-js provides a thin wrapper over the Ethereum Gateway contract that provides a uniform interface regardless of the version of the underlying Gateway contract. The wrapper can be instantiate using the createEthereumGatewayAsync function like this:

import {
  LocalAddress,
  CryptoUtils,
  Address,
  Contracts,
  createEthereumGatewayAsync, // Include this line!
  getMetamaskSigner
} from 'loom-js'

// Returns a new wrapper for the Ethereum Gateway contract
async function getEthereumGatewayContract() {
  const networkId = await web3.eth.net.getId()

  let version
  switch (networkId) {
    case 1: // Ethereum Mainnet
      version = 1
      break

    case 4: // Rinkeby
      version = 2
      break

    default:
      throw new Error('Ethereum Gateway is not deployed on network ' + networkId)
  }

  /*
  * If you're running `loom-js` in Node.js, change the following line to something like:
  * const signer = new OfflineWeb3Signer(web3, account)
  */
  const signer = getMetamaskSigner(web3.currentProvider)
  const ethereumGatewayContract = await createEthereumGatewayAsync(
    version,
    this._RinkebyGatewayAddress(), // In this example, we're instantiating the Rinkeby transfer gateway
    signer
  )
}

# 2.2 Use the withdrawalReceiptAsync Function to Get the Withdrawal Receipt

Prior to loom-js 1.70.1, you probably did something like this to retrieve a withdrawal receipt:

const receipt = await loomGatewayContract.withdrawalReceiptAsync(userLocalAddr)
if (!receipt) {
  return null
}
const signature = CryptoUtils.bytesToHexAddr(receipt.oracleSignature)
return {
  signature: signature,
  amount: receipt.value.toString(10),
  tokenContract: receipt.tokenContract.local.toString()
}

In loom-js 1.70.1 you no longer have to worry about what's in the receipt, nor do you need to figure out which of the withdraw functions you need to pass the receipt to, the wrapper will handle that for you:

const receipt = await loomGatewayContract.withdrawalReceiptAsync(userLoomAddr)
const tx = await ethereumGatewayContract.withdrawAsync(receipt, { gasLimit: gas })
return tx.hash

Please note that you must also call the withdrawAsync function to resume a withdrawal.

# Sample Code

We've updated our examples to reflect the changes to the API. If you're running loom-js in Node.js, please refer to the truffle-dappchain-example repository for more details. If you're running loom-js in the browser, please see the loom-examples repository.

If you get stuck at some point, feel free to contact us and we'll help you out!