How to Modify State Variables

In this guide, we will set up a simple Hardhat project structure, add a contract with a state variable and a function to modify it. We will then write a quick test to make sure the function modifies the state variable as expected - let's get to it!


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Step 1: Set Up Project Structure Using Hardhat

  1. In a folder of your choice, run mkdir modify-contract-state && cd modify-contract-state
  2. Run npm init -y
  3. Run npm install --save-dev hardhat
  4. Run npm install @nomiclabs/hardhat-waffle ethereum-waffle chai @nomiclabs/hardhat-ethers ethers
  5. Run npx hardhat to initiate the Hardhat development environment - it will bring up some yes/no options, use the arrow keys to toggle the options and select Create an empty hardhat.config.js


  1. Your project directory should now contain the following: node modules, package.json, package-lock.json and the empty hardhat.config.js you just created - rolling on!
  2. Open your project's hardhat.config.js, delete all of its contents and copy-paste the following:

module.exports = {
  solidity: "0.8.4",

Make sure that the solidity compiler version in your project's hardhat-config.js is set to 0.8.4 so that it matches that of the following contract!


Step 2: Create Smart Contract

  1. From your project root directory, run mkdir contracts && cd contracts
  2. Run touch ModifyVariable.sol (creates a new file called ModifyVariable.sol in the current directory) and open the newly-created contract file
  3. Copy-paste the following:
//SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

contract ModifyVariable {
  uint public x;

  constructor(uint _x) {
    x = _x;

  function modifyToLeet() public {
    x = 1337;


We've implemented a very simple smart contract that contains one state variable x and a function modifyToLeet that, when called, changes the state of the variable to be 1337.

  1. Go ahead and save your contract - feel free to add more functions!

Step 3: Create Test

  1. Make sure to go back to your project root directory by running cd back from the contracts directory
  2. In your project root, run mkdir test to create a new /test directory that will contain all your testing files!
  3. In the /test directory, create a file called sample-test.js and copy-paste the following into it:
// import testing libraries: 
const { expect, assert } = require("chai");

// the `describe` scope encapsulates an entire test called `TestModifyVariable`
// the `it` says the behavior that should be expected from the test
describe("TestModifyVariable", function () {
  it("should change x to 1337", async function () {
    // this line creates an ethers ContractFactory abstraction:
    const ModifyVariable = await ethers.getContractFactory("ModifyVariable");

    // we then use the ContractFactory object to deploy an instance of the contract
    const contract = await ModifyVariable.deploy(10);

    // wait for contract to be deployed and validated!
    await contract.deployed();

    // modify x from 10 to 1337 via this function!
    await contract.modifyToLeet();
    // getter for state variable x
    const newX = await contract.x();
    assert.equal(newX.toNumber(), 1337);

This test, when executed, will deploy a contract instance contract and set x (that instance's state variable) to 10. It then calls the modifyToLeet() function on the instance which prompts a change to the state variable to 1337 and then uses assert.equal() to verify the change was successful.

  1. Go ahead and save the file - feel free to play around with the values and add more tests!

Step 4: Run the Test

  1. In your project root folder, run npx hardhat test

If you are still in /test in your terminal, just type in cd .. and that will push your directory one back! :)

  1. Your terminal output should look something like this:

You have successfully set up a whole project structure! With the help of Hardhat, you were able to test whether your functions modifying your smart contract's state variables were actually modified - nice job!

Extra Challenges:

  • Create a new type string state variable and modify it
  • Change the constructor argument
  • Add a new test
  • Create a scripts directory, deploy your contract and change the contract state

Learn More About Ethereum Development

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