Getting the USD/ETH price from MakerDAO feed [off-chain only]

getEthPriceInUsd.js

const Web3 = require("web3");
const abi = require("./MedianEthUsd.json");

const providerUrl = `https://mainnet.infura.io/v3/${process.env.INFURA_API_KEY}`;

const provider = new Web3.providers.HttpProvider(providerUrl);

const web3 = new Web3(provider);

const contract = new web3.eth.Contract(
  abi,
  "0x64de91f5a373cd4c28de3600cb34c7c6ce410c85"
);

async function getPriceFromLatestBlocks({ fromBlock }) {
  const events = await contract.getPastEvents("LogMedianPrice", {
    fromBlock,
  });

  if (events.length === 0) {
    throw Object.create(new Error(`No updates in the  last ${blocks} blocks`), {
      code: {
        value: "ENOTFOUND",
      },
    });
  }

  const lastEvent = events[events.length - 1];
  const price = Number(Web3.utils.fromWei(lastEvent.returnValues.val));

  if (Number.isNaN(price)) {
    throw new Error(`Invalid value for price ${lastEvent.returnValues.val}`);
  }

  return price;
}

/**
 * In case the feed stops being updated for a while,
 * we can still try to go further in the history to
 * get a price, even though it would be outdated.
 */
async function getPrice() {
  const maxAttempts = 5;
  let attempts = 0;

  const currentBlockNumber = await web3.eth.getBlockNumber();
  let searchBlocks = 1000;

  while (attempts < maxAttempts) {
    try {
      return await getPriceFromLatestBlocks({
        fromBlock: currentBlockNumber - searchBlocks,
      });
    } catch (err) {
      if (err.code !== "ENOTFOUND") {
        throw err;
      }

      attempts += 1;
      searchBlocks *= 10;
    }
  }
}

(async () => {
  console.log(await getPrice());
})();

MedianEthUsd.json

[
  {
    "constant": false,
    "inputs": [
      {
        "internalType": "address",
        "name": "usr",
        "type": "address"
      }
    ],
    "name": "deny",
    "outputs": [],
    "payable": false,
    "stateMutability": "nonpayable",
    "type": "function"
  },
  {
    "constant": false,
    "inputs": [
      {
        "internalType": "address[]",
        "name": "a",
        "type": "address[]"
      }
    ],
    "name": "diss",
    "outputs": [],
    "payable": false,
    "stateMutability": "nonpayable",
    "type": "function"
  },
  {
    "constant": false,
    "inputs": [
      {
        "internalType": "address",
        "name": "a",
        "type": "address"
      }
    ],
    "name": "diss",
    "outputs": [],
    "payable": false,
    "stateMutability": "nonpayable",
    "type": "function"
  },
  {
    "constant": false,
    "inputs": [
      {
        "internalType": "address[]",
        "name": "a",
        "type": "address[]"
      }
    ],
    "name": "drop",
    "outputs": [],
    "payable": false,
    "stateMutability": "nonpayable",
    "type": "function"
  },
  {
    "constant": false,
    "inputs": [
      {
        "internalType": "address[]",
        "name": "a",
        "type": "address[]"
      }
    ],
    "name": "kiss",
    "outputs": [],
    "payable": false,
    "stateMutability": "nonpayable",
    "type": "function"
  },
  {
    "constant": false,
    "inputs": [
      {
        "internalType": "address",
        "name": "a",
        "type": "address"
      }
    ],
    "name": "kiss",
    "outputs": [],
    "payable": false,
    "stateMutability": "nonpayable",
    "type": "function"
  },
  {
    "constant": false,
    "inputs": [
      {
        "internalType": "address[]",
        "name": "a",
        "type": "address[]"
      }
    ],
    "name": "lift",
    "outputs": [],
    "payable": false,
    "stateMutability": "nonpayable",
    "type": "function"
  },
  {
    "anonymous": false,
    "inputs": [
      {
        "indexed": false,
        "internalType": "uint256",
        "name": "val",
        "type": "uint256"
      },
      {
        "indexed": false,
        "internalType": "uint256",
        "name": "age",
        "type": "uint256"
      }
    ],
    "name": "LogMedianPrice",
    "type": "event"
  },
  {
    "anonymous": true,
    "inputs": [
      {
        "indexed": true,
        "internalType": "bytes4",
        "name": "sig",
        "type": "bytes4"
      },
      {
        "indexed": true,
        "internalType": "address",
        "name": "usr",
        "type": "address"
      },
      {
        "indexed": true,
        "internalType": "bytes32",
        "name": "arg1",
        "type": "bytes32"
      },
      {
        "indexed": true,
        "internalType": "bytes32",
        "name": "arg2",
        "type": "bytes32"
      },
      {
        "indexed": false,
        "internalType": "bytes",
        "name": "data",
        "type": "bytes"
      }
    ],
    "name": "LogNote",
    "type": "event"
  },
  {
    "constant": false,
    "inputs": [
      {
        "internalType": "uint256[]",
        "name": "val_",
        "type": "uint256[]"
      },
      {
        "internalType": "uint256[]",
        "name": "age_",
        "type": "uint256[]"
      },
      {
        "internalType": "uint8[]",
        "name": "v",
        "type": "uint8[]"
      },
      {
        "internalType": "bytes32[]",
        "name": "r",
        "type": "bytes32[]"
      },
      {
        "internalType": "bytes32[]",
        "name": "s",
        "type": "bytes32[]"
      }
    ],
    "name": "poke",
    "outputs": [],
    "payable": false,
    "stateMutability": "nonpayable",
    "type": "function"
  },
  {
    "constant": false,
    "inputs": [
      {
        "internalType": "address",
        "name": "usr",
        "type": "address"
      }
    ],
    "name": "rely",
    "outputs": [],
    "payable": false,
    "stateMutability": "nonpayable",
    "type": "function"
  },
  {
    "constant": false,
    "inputs": [
      {
        "internalType": "uint256",
        "name": "bar_",
        "type": "uint256"
      }
    ],
    "name": "setBar",
    "outputs": [],
    "payable": false,
    "stateMutability": "nonpayable",
    "type": "function"
  },
  {
    "constant": true,
    "inputs": [],
    "name": "age",
    "outputs": [
      {
        "internalType": "uint32",
        "name": "",
        "type": "uint32"
      }
    ],
    "payable": false,
    "stateMutability": "view",
    "type": "function"
  },
  {
    "constant": true,
    "inputs": [],
    "name": "bar",
    "outputs": [
      {
        "internalType": "uint256",
        "name": "",
        "type": "uint256"
      }
    ],
    "payable": false,
    "stateMutability": "view",
    "type": "function"
  },
  {
    "constant": true,
    "inputs": [
      {
        "internalType": "address",
        "name": "",
        "type": "address"
      }
    ],
    "name": "bud",
    "outputs": [
      {
        "internalType": "uint256",
        "name": "",
        "type": "uint256"
      }
    ],
    "payable": false,
    "stateMutability": "view",
    "type": "function"
  },
  {
    "constant": true,
    "inputs": [
      {
        "internalType": "address",
        "name": "",
        "type": "address"
      }
    ],
    "name": "orcl",
    "outputs": [
      {
        "internalType": "uint256",
        "name": "",
        "type": "uint256"
      }
    ],
    "payable": false,
    "stateMutability": "view",
    "type": "function"
  },
  {
    "constant": true,
    "inputs": [],
    "name": "peek",
    "outputs": [
      {
        "internalType": "uint256",
        "name": "",
        "type": "uint256"
      },
      {
        "internalType": "bool",
        "name": "",
        "type": "bool"
      }
    ],
    "payable": false,
    "stateMutability": "view",
    "type": "function"
  },
  {
    "constant": true,
    "inputs": [],
    "name": "read",
    "outputs": [
      {
        "internalType": "uint256",
        "name": "",
        "type": "uint256"
      }
    ],
    "payable": false,
    "stateMutability": "view",
    "type": "function"
  },
  {
    "constant": true,
    "inputs": [
      {
        "internalType": "uint8",
        "name": "",
        "type": "uint8"
      }
    ],
    "name": "slot",
    "outputs": [
      {
        "internalType": "address",
        "name": "",
        "type": "address"
      }
    ],
    "payable": false,
    "stateMutability": "view",
    "type": "function"
  },
  {
    "constant": true,
    "inputs": [
      {
        "internalType": "address",
        "name": "",
        "type": "address"
      }
    ],
    "name": "wards",
    "outputs": [
      {
        "internalType": "uint256",
        "name": "",
        "type": "uint256"
      }
    ],
    "payable": false,
    "stateMutability": "view",
    "type": "function"
  },
  {
    "constant": true,
    "inputs": [],
    "name": "wat",
    "outputs": [
      {
        "internalType": "bytes32",
        "name": "",
        "type": "bytes32"
      }
    ],
    "payable": false,
    "stateMutability": "view",
    "type": "function"
  }
]