Build a Decentralized Lending App With Compound – ICOVO

Click on the Write button and send a transaction.

You can see that you returned cSai and received Sai instead.

You should also be able to see that there is still a tiny amount of cSai in your account. That’s the interest you accumulated.

All right, let’s get started with our application. Again, we will build a simple JavaScript application that lends and withdraws Sai. You can see this repository for the finished code.

Obtain Your Private Key and Address
First, obtain your private key and address in vanity-eth. Click on the “Generate” button at the bottom. Keep them somewhere for now. Please do NOT use them other than testing purposes. It’s not safe.

Get Some Eth and Sai
Send some Eth and Sai to the address you just obtained. 0.01 Eth should be enough to pay for the gas. I will supply only 1 Sai in this post.

*We will use mainnet cSai as, unfortunately, it is difficult to get testnet Sai tokens.

Get the Infura Endpoint
We will use Infura as a provider to communicate with the Ethereum network. Make an Infura account and get the endpoint at Mainnet by making a new project. It should look something like this:

https://mainnet.infura.io/v3/[PROJECT_ID]

Start a New npm Project

mkdir eht-compound-demo
cd eht-compound-demo/
npm init --yes

*If you don’t have npm and nodejs, install them from here.

Initialize Git

git init

Install the web3 Package

npm i — save web3@1.2.1

web3 is the Ethereum JavaScript API. It is an interface to Ethereum networks. It communicates with an Ethereum node or transacts with smart contracts deployed on the blockchain from your JavaScript application.

Install the ethereumjs-tx Package

npm i --save ethereumjs-tx@1.3.7

We use ethereumjs-tx to create and sign transactions.

Add Scripts to package.json

“scripts”: ,

mjs is a file extension that allows you to write with ES6 syntax.

Make saiTokenContract.mjs file and cSaiContract.mjs file
You can just copy the files from here and here.

In these files, I am simply declaring and exporting the abi and the address of sai token contract and the cSai contract.

Now we will start writing code. Let’s start from a lending feature.

We will make two separate files:

1. approveSaiMain.mjs
2. supplySaiMain.mjs

1. approveSaiMain.mjs

Make a New File
Make a new file approveSaiMain.mjs.

Import Packages

import Web3 from “web3”;
import EthTx from “ethereumjs-tx”;

Import cSaiContract and saiTokenContract

import cSaiContract from ‘./cSaiContract’;
import saiTokenContract from ‘./saiTokenContract’;

Set Up Web3
Set up Web3 to use Infura as your web3 provider:

const web3 = new Web3(
new Web3.providers.HttpProvider("https://mainnet.infura.io/v3/[YOUR_PROJECT_ID]"
)
);

Declare Your Address and Private Key Variables

const addressFrom = “[YOUR_ADDRESS]”;
const privKey = “[YOUR_PRIVATE_KEY]”;

Instantiate the Sai Token Contract
You use the Contract method of web3 with the address and abi to instantiate the Sai token contract.

const saiTokenContractInstance = new web3.eth.Contract(
JSON.parse(saiTokenContract.saiTokenContractAbi),
saiTokenContract.saiTokenContractAddress
);

Now we are ready to call the sai token contract methods.

Declare Const Variables to Pass to the approve Function

const ADDRESS_SPENDER = cSaiContract.cSaiContractAddress;
const TOKENS = web3.utils.toHex(-1);

You can pass 1 * 10 ** 18 to the second argument of the approve function, but in practice, it’s a common convention to pass -1 so that your users never have to approve again. If you want to understand how this works, read this.

Encode the Abi for the approve Function

const approveEncodedABI = saiTokenContractInstance.methods
.approve(ADDRESS_SPENDER, TOKENS)
.encodeABI();

Declare the sendSignedTx Function
Make a function to sign a transaction object and then propagate that to an Ethereum network:

function sendSignedTx(transactionObject, cb) `, cb);
}

Construct a Transaction Object and Then Execute sendSignedTx

web3.eth.getTransactionCount(addressFrom).then(transactionNonce => ;sendSignedTx(transactionObject, function(error, result))
}
);

Set the gasLimit to 100000. You can change gasPrice if you want to pay less. Check out the current optimal gasPrice in EthGasStation. You can convert its gwei value to wei in Ethereum unit converter.

Run the File

npm run approve-sai-main

Once you get a transaction hash like this 0xa18d71767ba95b95831fbf3a441494165002bc5f4104afdb0406511ebce7cbe3, you can go to Etherscan to see the detail of your transaction. Below is my transaction as an example. If your transaction doesn’t go through, increase your gasPrice and send another transaction.

const MINT_AMOUNT = web3.utils.toHex(1 * 10 ** 18);const mintEncodedABI = cSaiContractInstance.methods
.mint(MINT_AMOUNT)
.encodeABI();

web3.eth.getTransactionCount(addressFrom).then(transactionNonce => ;sendSignedTx(transactionObject, function(error, result))
}
);

npm run supply-sai-main

const REDEEM_AMOUNT = web3.utils.toHex(1 * 10 ** 18);const redeemUnderlyingEncodedABI = cSaiContractInstance.methods
.redeemUnderlying(REDEEM_AMOUNT)
.encodeABI();

web3.eth.getTransactionCount(addressFrom).then(transactionNonce => ;sendSignedTx(transactionObject, function(error, result))
}
);

npm run withdraw-sai-main