Test, deploy to testnet, then consider an audit
A smart contract deployed to mainnet is effectively immutable and often controls real money the moment it goes live, so the development lifecycle looks stricter than typical web development. Writing automated tests, using frameworks like Hardhat or Foundry, before trusting a contract with anything is non-negotiable: tests should cover the expected happy paths, but more importantly, they should cover edge cases and adversarial scenarios, like what happens if someone calls a function with zero as an argument, or tries to call it before the contract is properly initialized.
Testnets like Sepolia let you deploy and interact with a contract using free, worthless test ETH, on the same EVM semantics as mainnet, catching bugs that only show up under real transaction conditions, gas limits, and multi-contract interactions, without any financial risk. A professional security audit, where independent specialists review the code line by line looking for exactly the kinds of issues covered in this course, reentrancy, access control, overflow edge cases, and more, is the next layer for any contract that will hold meaningful value, though an audit reduces risk, it never eliminates it entirely.