How to Fork Testnet on Latest Block

Forking lets you interact with contracts and run tests as if on a real network. Learn how to fork Hedera Testnet on the latest block and test your contracts with the latest state of the network.

What you will accomplish

• Deploy your smart contract to Hedera Testnet using forge create

• Use cast send to sign and publish a transaction that changes the state.

• Use cast call to perform a call without changing state.

• Fork Hedera Testnet on the latest block & run your tests against your deployed contract

Prerequisites

Before you begin, you should be familiar with the following:

• JavaScript

• Solidity

• Foundry

Also, you should have the following set up on your computer ⬇

• git installed

  • Minimum version: 2.37

  • Recommended: Install Git (Github)

• A code editor or IDE

  • Recommended: VS Code. Install VS Code (Visual Studio)

• NodeJs + npm installed

  • Minimum version of NodeJs: 18

  • Minimum version of npm: 9.5

  • Recommended for Linux & Mac: nvm

  • Recommended for Windows: nvm-windows

• foundry forge and cast installed

  • forge Minimum version: 0.2.0 (3cdee82 2024-02-15T00:19:38.655803000Z)

  • cast Minimum version: 0.2.0 (3cdee82 2024-02-15T00:19:38.543163000Z)

Check your prerequisites set up ⬇

Open your terminal, and enter the following commands.

git --version
code --version
node --version
npm --version
forge --version
cast --version

Each of these commands should output some text that includes a version number, for example:

git --version
git version 2.39.2 (Apple Git-143)

code --version
1.81.1
6c3e3dba23e8fadc360aed75ce363ba185c49794
arm64

node --version
v20.6.1

npm --version
9.8.1

forge --version
0.2.0 (3cdee82 2024-02-15T00:19:38.655803000Z)

cast --version
0.2.0 (3cdee82 2024-02-15T00:19:38.543163000Z)

If the output contains text similar to command not found, please install that item.

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Get started

Set up project

To follow along, start with the main branch, which is the default branch of this repository. This gives you the initial state from which you can follow along with the steps as described in the tutorial.

forge manages dependencies by using git submodules. Clone the following project and pass --recurse-submodules to the git clone command to automatically initialize and update the submodule in the repository.

git clone --recurse-submodules [e-mail]:hedera-dev/fork-hedera-testnet.git

Install the submodule dependencies

forge install

Copy Your ECDSA Hex Encoded Private Key

Grab your ECDSA hex encoded private key by logging into Hedera Portal.

If you need to create a Hedera account, follow the create a hedera portal profile faucet tutorial.

Deploy your contract to Hedera Testnet

In your terminal, replace the value of "HEX_Encoded_Private_Key" with your ECDSA account's private key in the command below:

Next, Replace "RPC_URL" with a Tesnet URL by choosing one of the options over at How to Connect to Hedera Networks Over RPC

forge create --rpc-url "RPC_URL" --private-key "HEX_Encoded_Private_Key" src/TodoList.sol:TodoList

Your hex encoded private key must be prefixed with `0x`.

You should see output similar to the following:

[⠒] Compiling...
[⠔] Compiling 22 files with 0.8.23
[⠑] Solc 0.8.23 finished in 3.44s
Compiler run successful!
No files changed, compilation skipped
Deployer: 0xdfAb7899aFaBd146732c84eD83250889C40d6A00
Deployed to: 0xc1E551Eb1B3430A8D373C43e8804561fca5ce90D
Transaction hash: 0x8709443db7b60df7b563c83514ce8b03e54c341a5fe9844e01c72b05fc50950e

Open the Hashscan link to your deployed contract by copying the Deployed to EVM address and replacing <Deployed_Contract_EVM_Address> in the link below:

https://hashscan.io/testnet/contract/<Deployed_Contract_EVM_Address>

Example: https://hashscan.io/testnet/contract/0xc1E551Eb1B3430A8D373C43e8804561fca5ce90D

Execute a contract call and create a new todo

Use Foundry's cast send command to sign and publish a transaction to Hedera Tesnet.

In your terminal, you will need to replace the following items in the command below:

• Replace "deployed-contract-EVM-address" with your deployed contracts EVM address

• Replace"HEX_Encoded_Private_Key" with your hex encoded private key.

• Replace "RPC_URL" with a Tesnet URL.

cast send "deployed-contract-EVM-address" --private-key "HEX_Encoded_Private_Key" "createTodo(string)(uint256)" "Buy camping supplies"  --rpc-url "RPC_URL" 

You should see output similar to the following:

blockHash               0x958f246cc010aa074c81eae1988abc16c16bc12d2246397320d163b5ea748a89
blockNumber             7261495
contractAddress         0xc1E551Eb1B3430A8D373C43e8804561fca5ce90D
cumulativeGasUsed       91838
effectiveGasPrice       1050000000000
gasUsed                 91838
logs                    []
logsBloom               0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
root                    
status                  1
transactionHash         0x18286c79ae735dd08fccbe676ee2d7335147d7a1a6c7f2d0e99099c2802c397c
transactionIndex        2
type                    2

Use cast call to execute TodoList.sol's numberOfTodos().

cast call "deployed-contract-EVM-address" "numberOfTodos()(uint256)" --rpc-url "RPC_URL"

You should see numberOftodos has incremented by 1.

cast send signs/publishes a transaction and changes the state.

cast call performs a call without changing state. Essentially a read transaction.

Write your test

An almost-complete test has already been prepared for you. It's located at test/TodoList.t.sol. You will only need to make a few modifications (outlined below) for it to run successfully.

Look for a comment in the code to locate the specific lines of code which you will need to edit. For example, in this step, look for this: // Step (1) in the accompanying tutorial You will need to delete the inline comment that looks like this: /* ... */. Replace it with the correct code.

Step 1: Target your deployed contract

Open the TodoList.t.sol file and copy and paste the line below.

Replace "Deployed_Contract_EVM_Address" with your deployed contract's EVM address.

    TodoList public todoList =
        TodoList("Deployed_Contract_EVM_Address");

Step 2: Perform an assertion

This test saves the current number of todos as numberOfTodos, then executes createTodo(), and finally performs an assertion to check that the new number of todos has increased.

Write a statement which asserts that the todoCountAfterCreate is equal to the numberOfTodos + 1.

Copy the below line and paste it in TodoList.t.sol

assertEq(todoCountAfterCreate, (numberOfTodos + 1));

Fork test Hedera Testnet and run your test

Using the --fork-url flag you will run your test against a forked Hedera Testnet environment at the latest block.

In your terminal, replace "RPC_URL" with a Tesnet URL

forge test --fork-url "RPC_URL" -vvvv

You should see output similar to the following:

[⠢] Compiling...
[⠰] Compiling 1 files with 0.8.23
[⠔] Solc 0.8.23 finished in 1.15s
Compiler run successful!

Running 1 test for test/TodoList.t.sol:TodoListTest
[PASS] test_createTodo_returnsNumberOfTodosIncrementedByOne() (gas: 59188)
Traces:
  [59188] TodoListTest::test_createTodo_returnsNumberOfTodosIncrementedByOne()
    ├─ [2325] 0xc1E551Eb1B3430A8D373C43e8804561fca5ce90D::getNumberOfTodos() [staticcall]
    │   └─ ← 1
    ├─ [51026] 0xc1E551Eb1B3430A8D373C43e8804561fca5ce90D::createTodo("A new todo for you!")
    │   └─ ← 2
    └─ ← ()

Test result: ok. 1 passed; 0 failed; 0 skipped; finished in 2.13s
 
Ran 1 test suites: 1 tests passed, 0 failed, 0 skipped (1 total tests)

Your output will show you the state of numberOfTodos before you created a new todo and after. It also shows whether the test passed, failed or was skipped.

If you'd like to test a contract deployed on mainnet use a Mainnet RPC URL. Currently fork testing at the latest block is only supported. Be aware everytime you run your test it is against the latest state of the network.

Complete

Congratulations, on completing the tutorial on how to fork Hedera Testnet on the latest block.

You have learned how to:

• Deploy your smart contract to Hedera Testnet using forge create

• Use cast send to sign and publish a transaction that changes the state.

• Use cast call to perform a call without changing state.

• Fork Hedera Testnet on the latest block & run your tests against your deployed contract

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Writer: Abi Castro, DevRel Engineer

GitHub | LinkedIn

Editor: Brendan, DevRel Engineer

GitHub | Blog