Smart Contract Testing Cheat Sheet

Updated 2026-07-31

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Smart contract testing is the discipline of proving a Solidity contract behaves correctly before it goes on-chain, using the test runners built into Hardhat and Foundry plus specialized fuzzers and symbolic execution engines. It matters more here than in most software: a deployed contract is immutable, so a bug caught in a test costs nothing while the same bug caught by an attacker can drain a protocol. A useful mental model is that no single technique is sufficient on its own — unit tests verify intended behavior, fuzzing and invariant testing hunt for behavior you never intended, and coverage tells you which of these you haven't even tried yet — so a mature contract repo layers all three rather than treating 100% line coverage as "done."

What This Cheat Sheet Covers

This topic spans 15 focused tables and 106 indexed concepts. Below is a complete table-by-table outline of this topic, spanning foundational concepts through advanced details.

Table 1: Testing Fundamentals and MethodologyTable 2: Foundry Unit Testing with ForgeTable 3: Hardhat Unit Testing with Mocha and ChaiTable 4: Fixtures and Reusable Test SetupTable 5: Assertions, Reverts, and EventsTable 6: Mocking and Test DoublesTable 7: Integration Testing and Mainnet ForkingTable 8: Code Coverage MeasurementTable 9: Fuzz Testing and Property-Based TestingTable 10: Stateful and Invariant TestingTable 11: Property Suites and Differential TestingTable 12: Symbolic Execution and Formal VerificationTable 13: Gas Usage TestingTable 14: Debugging and Diagnosing Test FailuresTable 15: Continuous Integration for Contract Repos

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Table 1: Testing Fundamentals and Methodology

Before touching a framework, it helps to know the vocabulary and structure that every mature contract test suite shares — the same layers used in general software testing, adapted to a world where deployed code cannot be patched.

Concept	Example	Description
Unit Testing	`function test_Increment() public { counter.increment(); assertEq(counter.number(), 1); }`	Tests a single function's behavior in isolation, covering every valid and invalid parameter combination it can be called with.
Integration Testing	Deploy a vault and a token together, then call `vault.deposit()` and assert the token balance moved	Verifies that two or more deployed contracts interact correctly, catching bugs that only appear when components are wired together.
Negative Testing (Revert Testing)	Assert a bid below the minimum reverts	Confirms the contract correctly rejects invalid inputs and unauthorized calls, not just that it accepts valid ones.
End-to-End (System) Testing	Simulate a full user journey: connect wallet → approve → deposit → withdraw	Exercises the whole system the way a real user would, confirming every component works together from a cold start.

Table 1: Testing Fundamentals and Methodology

Concept	Example	Description
Unit Testing	`function test_Increment() public { counter.increment(); assertEq(counter.number(), 1); }`	Tests a single function's behavior in isolation, covering every valid and invalid parameter combination it can be called with.
Integration Testing	Deploy a vault and a token together, then call `vault.deposit()` and assert the token balance moved	Verifies that two or more deployed contracts interact correctly, catching bugs that only appear when components are wired together.
Negative Testing (Revert Testing)	Assert a bid below the minimum reverts	Confirms the contract correctly rejects invalid inputs and unauthorized calls, not just that it accepts valid ones.
End-to-End (System) Testing	Simulate a full user journey: connect wallet → approve → deposit → withdraw	Exercises the whole system the way a real user would, confirming every component works together from a cold start.