TipJarOwnable – Solidity Smart Contract (Ownable Pattern + Withdrawals)

This project is a simple Tip Jar smart contract built with Solidity, Hardhat, OpenZeppelin, and Ethers v6. It was developed as part of my blockchain study roadmap to understand smart contract patterns, especially the Ownable access control model.

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📌 Project Goal

Build a small but realistic Solidity contract that:

• Accepts ETH tips from any user
• Stores tip messages + metadata
• Allows only the owner to withdraw funds
• Demonstrates the pull-payment pattern
• Uses OpenZeppelin’s Ownable for access control
• Is fully tested using Hardhat + Ethers v6

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📘 What I Learned

🔑 1. How OpenZeppelin’s Ownable Works

• The contract deployer becomes the default owner.

• The onlyOwner modifier protects sensitive functions like withdraw().

• Newer OZ versions use custom errors, not string reverts:

  OwnableUnauthorizedAccount(address)
  

• Tests must use:

  .to.be.revertedWithCustomError(contract, "OwnableUnauthorizedAccount")

💸 2. Pull-Based Withdrawals

Instead of sending funds during tipping, the contract holds ETH until the owner calls:

function withdraw() external onlyOwner { ... }

This avoids re-entrancy issues and matches Web3 best practices.

🧪 3. How to Write Ethers v6 Tests

• Learned how to deploy contracts with Hardhat.

• Wrote event tests (TipReceived, Withdraw).

• Fixed multiple issues involving incorrect signer usage and undefined variables.

• Learned why testing wallet balance changes can fail due to gas costs.

• Replaced balance-difference tests with the correct pattern:

  • Check contract balance before withdrawal
  • Check contract balance after withdrawal
  • Verify event emission

🛠 4. Debugging BigInt Mixing Errors

Ethers v6 returns BigInt, not BigNumber. I learned how to avoid:

TypeError: Cannot mix BigInt and other types

by ensuring all math stays strictly in BigInt.

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✨ Final Features

• sendTip(message) — send ETH + message

• receive() / fallback() — accept direct ETH transfers

• getAllTips() — retrieve full tip history

• withdraw() — owner-only ETH withdrawal

• Events:

  • TipReceived
  • Withdraw

All tests pass:

14 passing
0 failing

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📂 Project Structure

contracts/
  TipJarOwnable.sol

scripts/
  deploy.js

test/
  tipjar.test.js

hardhat.config.js

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▶ How to Run the Project

Start a local Hardhat blockchain

npx hardhat node

Run tests

npx hardhat test --network localhost

Deploy the contract locally

npx hardhat run scripts/deploy.js --network localhost

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