web3-testing

Use Hardhat with Chai and ethers.js to write tests that deploy contracts via fixtures, assert state changes, and verify events. The loadFixture helper avoids redeploying contracts between tests, speeding up execution.

Hardhat uses JavaScript/TypeScript tests with a rich plugin ecosystem, while Foundry (Forge) writes tests directly in Solidity with built-in fuzzing and cheatcodes like vm.prank. Foundry runs faster; Hardhat integrates more easily with existing JS tooling.

Use Hardhat's forking configuration or Forge's vm.createSelectFork with an RPC URL to clone mainnet state locally. This lets tests interact with real deployed contracts like Uniswap or DAI at a pinned block number.

Fuzz testing in Forge automatically generates random inputs for test functions, such as testFuzzTransfer(uint256 amount). Use vm.assume to constrain inputs to valid ranges and verify properties hold across thousands of generated cases.

Enable hardhat-gas-reporter in the config to log gas costs per function call, or read receipt.gasUsed from transaction receipts in tests. Compare gas between contract implementations to validate optimization efforts.

Unit tests alone miss cross-contract interactions and real network conditions. Combine them with mainnet fork tests, fuzzing, and coverage analysis, and treat test results as one input alongside audits before deployment.