Testing

By the end of this lesson you'll write real Go tests with the built-in testing package — single tests, table-driven tests with subtests, and benchmarks — and run them with go test . No third-party libraries needed; it's all in the standard library.

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Part of the free Go course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn in This Lesson

1️⃣ Your First Test

Go's test runner is built into the language — there's no framework to install. Three conventions are all you need: the file name ends in _test.go , each test function is named Test + a capitalised name, and it takes a single t *testing.T . Go has no assert keyword: you compare the values yourself and call a method on t when something's wrong. Read this worked example, then run it.

2️⃣ Reporting Failures: t.Errorf vs t.Fatalf

When a check fails you tell the test runner with a method on t . t.Errorf records the failure but keeps running the rest of the function, so one run can surface several issues. t.Fatalf records it and stops the test right there — use it after setup or an error check, when the lines below would otherwise crash. Both take a printf -style format string.

Now you try. The test below is almost complete — fill in the two blanks marked ___ using the hints, then run it. You're writing the expected value and the comparison.

3️⃣ Table-Driven Tests with Subtests

This is the pattern you'll write most in Go. Instead of copy-pasting a near-identical test for every input, you describe the cases as data in a slice of structs, then loop over them. Wrapping each case in t.Run(name, func) turns it into a named subtest , so the report tells you precisely which case broke and you can re-run just that one. Adding a new case is a single line.

Your turn again. The table below tests a Max function but is missing a case. Add one row that covers two equal numbers, then run it.

4️⃣ Running Tests: go test , -v , -run , -cover

You drive everything from the command line. go test runs the package's tests and prints ok or FAIL . Add -v for a line per test, -run to filter by a name regex (great with subtests), and -cover for the percentage of your code the tests exercise. ./... means "this package and everything below it".

5️⃣ Benchmarks (a quick note)

Once your code is correct , you may want to know if it's fast . A benchmark is named Benchmark + capital, takes a b *testing.B , and runs your code in a for i := 0; i b.N; i++ loop. The framework raises b.N until the timing is stable. Benchmarks don't run with a plain go test — you opt in with -bench .

📋 Quick Reference — Go Testing

No blanks this time — just a brief and an outline. The Reverse function is written for you; write a table-driven test for it from scratch, run it, and check each case passes. This is exactly the kind of test you'll write every day in real Go projects.

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