Write Tests

Reference

Full pattern details: .claude/docs/reference/testing.md

Steps

Step 1: Determine test file and package

• •File: <source_file>_test.go in the same directory
• •External test package (package foo_test) for exported API
• •Internal test package (package foo) only for unexported functions
• •OS-specific: add //go:build unix or //go:build windows

Step 2: Write test function

func Test_FunctionName(t *testing.T) {
	t.Parallel()

	tests := []struct {
		name string
		// input and expected output fields
	}{
		// test cases
	}

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			t.Parallel()
			// assertions
		})
	}
}

Step 3: Define test cases

Name with Japanese and [正常系]/[異常系] prefix:

{name: "[正常系] 正しい入力", input: "valid", want: expected},
{name: "[異常系] 空文字列",   input: "",      wantErr: &pkg_err.SomeError{}},

For filesystem tests, use setupFs func() afero.Fs:

{
	name: "[正常系] ファイルが存在する",
	setupFs: func() afero.Fs {
		fs := afero.NewMemMapFs()
		_ = afero.WriteFile(fs, "/file", []byte("data"), 0o644)
		return fs
	},
},

Step 4: Write assertions

Step 5: Add imports

import (
	"reflect"
	"testing"

	"github.com/google/go-cmp/cmp"
	"github.com/spf13/afero"

	"github.com/cffnpwr/nix-prefetch-pnpm-deps/internal/<package>"
	pkg_err "github.com/cffnpwr/nix-prefetch-pnpm-deps/internal/<package>/errors"
)

Error subpackage alias: <pkg>_err (e.g., lockfile_err, pnpm_err).

Checklist

• • t.Parallel() on test function AND each subtest
• • Table-driven with []struct and for _, tt := range tests
• • Names in Japanese with [正常系]/[異常系] prefix
• • Both normal and error cases covered
• • Structs compared with cmp.Diff, errors with reflect.TypeOf
• • afero.NewMemMapFs() for filesystem tests (not real FS)
• • Import order: stdlib, external, local
• • t.Setenv() for env vars (not with t.Parallel() in same subtest)