Read/Write Smart Contracts

By knowing the functions available to a contract, we can programmatically use them — in situations where the project websites are down or when you need to automate certain transactions.

1. Setting up a Node.js project

In a new folder, initiate a new Node.js project with the command npm init -y to accept all default project parameters.

A package.json file will be created for you, which contains all your packages and project information.

2. Obtaining a contract's ABI

Create a new file named script.js.

In JavaScript, we'll be writing our code to make a request to the endpoint "Get Contract ABI for Verified Contract Source Codes", which you will need to specify your verified contract address.

https://api.gnosisscan.io/api?module=contract&action=getabi&address=0xFc7a5BD22dFc48565D6f04698E566Dd0C71d3155&apikey=YourApiKeyToken

For this example, we'll be using a contract specifically deployed for this tutorial on the Gnosis mainnet.

const ethers = require("ethers");

async function main() {
      // make an API call to the ABIs endpoint 
    const response = await fetch('https://api.gnosisscan.io/api?module=contract&action=getabi&address=0xFc7a5BD22dFc48565D6f04698E566Dd0C71d3155&apikey=YourApiKeyToken');
    const data = await response.json();

    // print the JSON response 
    let abi = data.result;
    console.log(abi);

}

main();

3. Connecting to a node

To interact with smart contracts, we will need a connection to a Gnosis node.

In this case we're using a public RPC endpoint hosted by Gnosis. You can find other public RPC endpoints on ChainList.

4. Integrating Ethers.js

We'll need to integrate a JavaScript library, known as Ethers.js that will be used to interact with the Gnosis blockchain.

To do so, run the command npm i ethers from a terminal within this project directory to install it.

Ethers.js provides several classes such as a Provider, which represents the state of the Gnosis blockchain. We can create a new Provider using the syntax below, and pass in our node URL to initiate a connection to the Ethereum network.

const ethers = require("ethers");

async function main() {
      // make an API call to the ABIs endpoint 
    const response = await fetch('https://api.gnosisscan.io/api?module=contract&action=getabi&address=0xFc7a5BD22dFc48565D6f04698E566Dd0C71d3155&apikey=YourApiKeyToken');
    const data = await response.json();

    // print the JSON response 
    let abi = data.result;
    console.log(abi);

    // creating a new Provider, and passing in our node URL 
    const node = "https://rpc.gnosischain.com";
    const provider = new ethers.providers.JsonRpcProvider(node);
}

main();

5. Creating a wallet

Another class that Ethers.js allows us to create is a Wallet, which will allow us to specify a private key and use a Gnosis address.

Performing write operations will incur gas costs, as such you may get some xDAI from a faucet to pay for transaction fees.

const ethers = require("ethers");

async function main() {
      // make an API call to the ABIs endpoint 
    const response = await fetch('https://api.gnosisscan.io/api?module=contract&action=getabi&address=0xFc7a5BD22dFc48565D6f04698E566Dd0C71d3155&apikey=YourApiKeyToken');
    const data = await response.json();

    // print the JSON response 
    let abi = data.result;
    console.log(abi);

    // creating a new Provider, and passing in our node URL 
    const node = "https://rpc.gnosischain.com";
    const provider = new ethers.providers.JsonRpcProvider(node);

     // initiating a new Wallet, passing in our private key to sign transactions
     let privatekey = "<Insert_Private_key>";
     let wallet = new ethers.Wallet(privatekey, provider);
 
     // print the wallet address
     console.log("Using wallet address " + wallet.address);
}

main();

6. Reading a contract

Finally, to interact with a smart contract we'll need to create a new Contract class.

The Contract class accepts an input of a contract address, an ABI (which we retrieved from the API earlier), and a wallet address to pay gas for any contract interactions.

const ethers = require("ethers");

async function main() {
      // make an API call to the ABIs endpoint 
    const response = await fetch('https://api.gnosisscan.io/api?module=contract&action=getabi&address=0xFc7a5BD22dFc48565D6f04698E566Dd0C71d3155&apikey=YourApiKeyToken');
    const data = await response.json();

    // print the JSON response 
    let abi = data.result;
    console.log(abi);

    // creating a new Provider, and passing in our node URL 
    const node = "https://rpc.gnosischain.com";
    const provider = new ethers.providers.JsonRpcProvider(node);

     // initiating a new Wallet, passing in our private key to sign transactions
     let privatekey = "<Insert_Private_key>";
     let wallet = new ethers.Wallet(privatekey, provider);
 
     // print the wallet address
     console.log("Using wallet address " + wallet.address);

     // specifying the deployed contract address 
    let contractaddress = "0xFc7a5BD22dFc48565D6f04698E566Dd0C71d3155";
    
    // initiating a new Contract
    let contract = new ethers.Contract(contractaddress, abi, wallet);
}

main();

Having a closer look at the ABI we retrieved in Step 2, we can see that the contract has a function named read, that doesn't accept an input however does return a uint256 number as an output.

[
    {
    "inputs": [],
    "name": "read" // function named read
    
    "outputs": [
        {  
        "internalType": "uint256",
        "name": "",
        "type": "uint256" // returns a uint256 as output
        }
                ],
    "stateMutability": "view",
    "type": "function"
}, {
    "inputs": [
        {
        "internalType": "uint256",
        "name": "newScore",
        "type": "uint256"
        }
                ],
    "name": "write",
    "outputs": [],
    "stateMutability": "nonpayable",
    "type": "function"
    }
]

We can therefore call that function of the contract, read the value stored and print it out.

You may run this code from your console using the command node script.js.

Reading data stored in a contract incurs no gas cost, as it does not change the state of the Gnosis blockchain.

const ethers = require("ethers");

async function main() {
      // make an API call to the ABIs endpoint 
    const response = await fetch('https://api.gnosisscan.io/api?module=contract&action=getabi&address=0xFc7a5BD22dFc48565D6f04698E566Dd0C71d3155&apikey=YourApiKeyToken');
    const data = await response.json();

    // print the JSON response 
    let abi = data.result;
    console.log(abi);

    // creating a new Provider, and passing in our node URL 
    const node = "https://rpc.gnosischain.com";
    const provider = new ethers.providers.JsonRpcProvider(node);

     // initiating a new Wallet, passing in our private key to sign transactions
     let privatekey = "<Insert_Private_key>";
     let wallet = new ethers.Wallet(privatekey, provider);
 
     // print the wallet address
     console.log("Using wallet address " + wallet.address);

     // specifying the deployed contract address 
    let contractaddress = "0xFc7a5BD22dFc48565D6f04698E566Dd0C71d3155";
    
    // initiating a new Contract
    let contract = new ethers.Contract(contractaddress, abi, wallet);
    
    // calling the "retrieve" function to read the stored value
    let read = await contract.read();
    console.log("Value stored in contract is " + read.toString());
}

main();

7. Writing a contract

Referring to the ABI once again, we can see that the contract has another method write, which accepts a uint256 number as an input and does not return any output.

[
    {
    "inputs": [],  
    "name": "read" 
    
    "outputs": [
        {  
        "internalType": "uint256",
        "name": "",
        "type": "uint256" 
        }
                ],
    "stateMutability": "view",
    "type": "function"
}, {
    "inputs": [
        {
        "internalType": "uint256", // requires a uint256 input
        "name": "newScore", 
        "type": "uint256"
        }
                ],
    "name": "write", // function named as write
    "outputs": [], // does not return an output
    "stateMutability": "nonpayable",
    "type": "function"
    }
]

We can call that function and pass in any number as a parameter. To check that its updated, we'll wait for a 2 block confirmation, and read the contract again to confirm that the number has been updated.

You may run this code from your console using the command node script.js.

const ethers = require("ethers");

async function main() {
      // make an API call to the ABIs endpoint 
    const response = await fetch('https://api.gnosisscan.io/api?module=contract&action=getabi&address=0xFc7a5BD22dFc48565D6f04698E566Dd0C71d3155&apikey=YourApiKeyToken');
    const data = await response.json();

    // print the JSON response 
    let abi = data.result;
    console.log(abi);

    // creating a new Provider, and passing in our node URL 
    const node = "https://rpc.gnosischain.com";
    const provider = new ethers.providers.JsonRpcProvider(node);

     // initiating a new Wallet, passing in our private key to sign transactions
     let privatekey = "<Insert_Private_key>";
     let wallet = new ethers.Wallet(privatekey, provider);
 
     // print the wallet address
     console.log("Using wallet address " + wallet.address);

     // specifying the deployed contract address 
    let contractaddress = "0xFc7a5BD22dFc48565D6f04698E566Dd0C71d3155";
    
    // initiating a new Contract
    let contract = new ethers.Contract(contractaddress, abi, wallet);
    
    // calling the "retrieve" function to read the stored value
    let read = await contract.read();
    console.log("Value stored in contract is " + read.toString());

    // call the "store" function to update the value to 420
    let write = await contract.write(420);
    
    // wait for 2 blocks of confirmation 
    write.wait(2)
        .then(async () => {  
            // read the contract again, similar to above
            let read = await contract.read();
            console.log("Updated value stored in contract is " + read.toString());
        });

}

main();

Beyond the testing grounds

The full sample code is on Github, feel free to experiment and use it ( with caution ) on real world contracts out there.

Last updated