Unit Testing Basics

Write tests with the standard testing package, run them with go test, and measure coverage.

8 min readProgramming Foundations (Go)Code

Writing Your First Test

Test files end with _test.go. Each test function takes *testing.T and starts with Test. The go test command finds and runs them automatically.

Goa-basic-test-file.go

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`t.Errorf` marks the test as failed and logs a formatted message, but continues running. `t.Fatalf` fails and stops immediately. Use `Errorf` when you want to see all failures at once.

Running tests


# Run all tests in the current module
go test ./...

# Run tests in a specific package
go test ./internal/task/...

# Verbose output — see each test name
go test -v ./...

# Run a specific test
go test -run TestNewTask ./internal/task/...

./... means all packages recursively. -v shows each test by name, which helps when debugging failures in a large test suite.

Table-Driven Tests

Table-driven tests use a slice of anonymous structs — one row per scenario. A single test function loop verifies them all. This is the dominant testing pattern in the Go standard library.

Gotable-driven-test-for-a-filter-function.go

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`t.Run(name, func)` creates a **subtest** that can be run individually with `-run TestFilterByTag/work_tag`. Each subtest gets its own `t`, so a failure in one does not skip the others.

Testing with Temp Files

Use t.TempDir() to create a directory that is automatically removed after the test. This keeps tests self-contained and prevents state leaking between test runs.

Gotesting-file-based-storage.go

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`t.TempDir()` creates a unique temp directory per test and registers cleanup automatically. `t.Fatalf` immediately stops the test on setup failure — there is no point continuing if the save itself fails.

Coverage and go test

Measuring test coverage


# Print coverage percentage per package
go test -cover ./...

# Generate an HTML coverage report
go test -coverprofile=coverage.out ./...
go tool cover -html=coverage.out -o coverage.html

# Show uncovered lines in the terminal
go tool cover -func=coverage.out

Coverage shows which lines are exercised by your tests. 100% coverage is not the goal — focus on testing behaviour at boundaries: zero values, empty inputs, error paths, and concurrency.

Tests are documentation

A well-named table-driven test is a specification: it lists exactly what inputs are accepted, what outputs are expected, and what error conditions exist. When you read a Go test file, you should understand what the code does without reading the implementation.

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