How to Use the testing/fstest Package for File System Tests

The testing/fstest package provides utilities to create in-memory file systems for testing code that interacts with the io/fs interface. Use fstest.MapFS to define a virtual file system with specific content and pass it to your code instead of the real disk.

package main

import (
	"io/fs"
	"testing"
	"testing/fstest"
)

func TestMyReader(t *testing.T) {
	// Create a virtual file system with test data
	fs := fstest.MapFS{
		"data.txt": &fstest.MapFile{
			Data: []byte("hello world"),
		},
	}

	// Pass the virtual FS to the function under test
	// Example: content, err := ReadFile(fs, "data.txt")
	if _, err := fs.Open("data.txt"); err != nil {
		t.Fatalf("failed to open file: %v", err)
	}

	// Verify behavior without touching the real disk
	info, err := fs.Stat("data.txt")
	if err != nil {
		t.Fatalf("failed to stat file: %v", err)
	}
	if info.Size() != 11 {
		t.Errorf("expected size 11, got %d", info.Size())
	}
}

The testing/fstest package lets you create a fake file system that lives only in your computer's memory while tests run. It is useful when you need to test code that reads or writes files but want to avoid creating messy temporary files on your actual hard drive. Think of it as a sandbox where you can set up exactly the files you need for a test, run your code, and then the sandbox disappears instantly.