Compiling a Contract

To add a contract and compile it using Hardhat, do the following:

1. In the project directory, create a contracts directory.

   mkdir contracts
   

2. Create a new file in the contracts directory called HelloWorld.sol.

3. Copy and paste the following code into HelloWorld.sol.

   // SPDX-License-Identifier: MIT
   pragma solidity >=0.7.3;
   
   // Defines a contract named `HelloWorld`.
   contract HelloWorld {
   
       // Event emitted when update function is called
       event UpdatedMessages(string oldStr, string newStr);
   
       // Declares a state variable `message` of type `string`.
       string public message;
   
       // A constructor is a special function that is only executed upon contract creation.
       constructor(string memory initMessage) {
           message = initMessage;
       }
   
       // A public function that accepts a string argument and updates the `message` storage variable.
       function update(string memory newMessage) public {
           string memory oldMsg = message;
           message = newMessage;
           emit UpdatedMessages(oldMsg, newMessage);
       }
   }
   

4. Compile the contract by running the following command.

   npx hardhat compile
   

   You should see a message saying Compiled 1 Solidity file successfully.

   The command also creates a directory called artifacts that has some JSON files. The HelloWorld.json has the contract's code and application binary interface (ABI). These will be used for contract deployment and interaction.