Mocking a filesystem call

files.go

package findgo

import (
	"io/fs"
	"path/filepath"
)

func Files(folder fs.FS) int {
	var count int
	fs.WalkDir(folder, ".", func(p string, d fs.DirEntry, err error) error {
		if filepath.Ext(p) == ".go" {
			count++
		}
		return nil
	})

	return count
}

files_test.go

package findgo

import (
	"testing"
	"testing/fstest"
)

func TestFilesInMemory(t *testing.T) {
	t.Parallel()
	fsys := fstest.MapFS{
		"file.go":                {},
		"subfolder/subfolder.go": {},
		"subfolder/another.go":   {},
		"subfolder/file.go":      {},
	}

	want := 4
	got := Files(fsys)
	if want != got {
		t.Errorf("want %d, got %d", want, got)
	}
}