[BE] [Practice] Make a backend service to get a users balance of an erc20 token

[cupOJoseph]

Goal

Learn about generating services with redwood and getting data.

Deliverable

A simple redwood service which gets a particular balance from the blockchain (any network).

Suggested tasks

• If you don't have one already, create a service eg. yarn rw generate service user which will create a file in api/src/services where you will place your logic.
• Use ethers.js v5 to load a wallet token or coin balance. You will probably need to look for docs for this! Pick any network, any coin or token, and any user address. These can be hardcoded.

The service could look like this:

import { Contract, getDefaultProvider, providers, utils } from 'ethers'

function getProvider(chain) {
  const provider =
    chain === 'ETH'
      ? getDefaultProvider()
      : new providers.JsonRpcProvider(
          'https://www.ethercluster.com/etc',
          'classic'
        )
  return provider
}

const contractAbiFragment = [
    {
      name: 'balanceOf',
      type: 'function',
      inputs: [
        {
          name: '_owner',
          type: 'address',
        },
      ],
      outputs: [
        {
          name: 'balance',
          type: 'uint256',
        },
      ],
      constant: true,
      payable: false,
    },
  ]

  const provider = getProvider('ETH')
  const contract = new Contract(token, contractAbiFragment, provider)

//do stuff