Go Snaps

Jest-like snapshot testing in Golang

Contents

• Installation
• MatchSnapshot
• MatchJSON
  • Matchers
    • match.Any
    • match.Custom
    • match.Type[ExpectedType]
• MatchStandaloneSnapshot
• Configuration
• Update Snapshots
  • Clean obsolete Snapshots
  • Sort Snapshots
  • Skipping Tests
• Running tests on CI
• No Color
• Snapshots Structure
• Acknowledgments
• Contributing
• Appendix

Installation

To install go-snaps, use go get:

go get github.com/gkampitakis/go-snaps

Import the go-snaps/snaps package into your code:

package example

import (
  "testing"

  "github.com/gkampitakis/go-snaps/snaps"
)

func TestExample(t *testing.T) {
  snaps.MatchSnapshot(t, "Hello World")
}

MatchSnapshot

MatchSnapshot can be used to capture any type of data structured or unstructured.

You can pass multiple parameters to MatchSnapshot or call MatchSnapshot multiple times inside the same test. The difference is in the latter, it will create multiple entries in the snapshot file.

// test_simple.go

func TestSimple(t *testing.T) {
  t.Run("should make multiple entries in snapshot", func(t *testing.T) {
    snaps.MatchSnapshot(t, 5, 10, 20, 25)
    snaps.MatchSnapshot(t, "some value")
  })
}

go-snaps saves the snapshots in __snapshots__ directory and the file name is the test file name with extension .snap.

So for example if your test is called test_simple.go when you run your tests, a snapshot file will be created at ./__snapshots__/test_simple.snaps.

MatchJSON

MatchJSON can be used to capture data that can represent a valid json.

You can pass a valid json in form of string or []byte or whatever value can be passed successfully on json.Marshal.

func TestJSON(t *testing.T) {
  type User struct {
    Age   int
    Email string
  }

  snaps.MatchJSON(t, `{"user":"mock-user","age":10,"email":"mock@email.com"}`)
  snaps.MatchJSON(t, []byte(`{"user":"mock-user","age":10,"email":"mock@email.com"}`))
  snaps.MatchJSON(t, User{10, "mock-email"})
}

JSON will be saved in snapshot in pretty format for more readability and deterministic diffs.

Matchers

MatchJSON's third argument can accept a list of matchers. Matchers are functions that can act as property matchers and test values.

You can pass the path of the property you want to match and test.

More information about the supported path syntax from gjson.

Currently go-snaps has three build in matchers

• match.Any
• match.Custom
• match.Type[ExpectedType]

match.Any

Any matcher acts as a placeholder for any value. It replaces any targeted path with a placeholder string.

Any("user.name")
// or with multiple paths
Any("user.name", "user.email")

Any matcher provides some methods for setting options

match.Any("user.name").
  Placeholder(value). // allows to define a different placeholder value from the default "<Any Value>"
  ErrOnMissingPath(bool) // determines whether the matcher will err in case of a missing, default true

match.Custom

Custom matcher allows you to bring your own validation and placeholder value

match.Custom("user.age", func(val any) (any, error) {
		age, ok := val.(float64)
		if !ok {
				return nil, fmt.Errorf("expected number but got %T", val)
		}

		return "some number", nil
})

The callback parameter value for JSON can be on of these types:

bool // for JSON booleans
float64 // for JSON numbers
string // for JSON string literals
nil // for JSON null
map[string]any // for JSON objects
[]any // for JSON arrays

If Custom matcher returns an error the snapshot test will fail with that error.

Custom matcher provides a method for setting an option

match.Custom("path",myFunc).
  Placeholder(value). // allows to define a different placeholder value from the default "<Any Value>"
  ErrOnMissingPath(bool) // determines whether the matcher will err in case of a missing path, default true

match.Type

Type matcher evaluates types that are passed in a snapshot and it replaces any targeted path with a placeholder in the form of <Type:ExpectedType>.

match.Type[string]("user.info")
// or with multiple paths
match.Type[float64]("user.age", "data.items")

Type matcher provides a method for setting an option

match.Type[string]("user.info").
  ErrOnMissingPath(bool) // determines whether the matcher will err in case of a missing path, default true

You can see more examples.

MatchStandaloneSnapshot

MatchStandaloneSnapshot will create snapshots on separate files as opposed to MatchSnapshot which adds multiple snapshots inside the same file.

Combined with snaps.Ext you can have proper syntax highlighting and better readability

// test_simple.go

func TestSimple(t *testing.T) {
  snaps.MatchStandaloneSnapshot(t, "Hello World")
  // or create an html snapshot file
  snaps.WithConfig(snaps.Ext(".html")).
    MatchStandaloneSnapshot(t, "<html><body><h1>Hello World</h1></body></html>")
}

go-snaps saves the snapshots in __snapshots__ directory and the file name is the test file name with extension .snap.

So for example if your test is called test_simple.go when you run your tests, a snapshot file will be created at ./__snapshots__/TestSimple_1.snaps.

Configuration

go-snaps allows passing configuration for overriding

• the directory where snapshots are stored, relative or absolute path
• the filename where snapshots are stored
• the snapshot file's extension (regardless the extension the filename will include the .snaps inside the filename)
• programmatically control whether to update snapshots. You can find an example usage at examples

t.Run("snapshot tests", func(t *testing.T) {
  snaps.WithConfig(snaps.Filename("my_custom_name"), snaps.Dir("my_dir")).MatchSnapshot(t, "Hello Word")

  s := snaps.WithConfig(
    snaps.Dir("my_dir"),
    snaps.Filename("json_file"),
    snaps.Ext(".json")
    snaps.Update(false)
  )

  s.MatchJSON(t, `{"hello":"world"}`)
})

You can see more on examples

Update Snapshots

You can update your failing snapshots by setting UPDATE_SNAPS env variable to true.

UPDATE_SNAPS=true go test ./...

If you don't want to update all failing snapshots, or you want to update only one of them you can you use the -run flag to target the test(s) you want.

For more information on go test flags you can run

go help testflag

Clean obsolete snapshots

go-snaps can identify obsolete snapshots.

In order to enable this functionality you need to use TestMain(m *testing.M) to call snaps.Clean(t) after your tests have run. This will also print a Snapshot Summary. (if running tests with verbose flag -v)

If you want to remove the obsolete snap files and snapshots you can run tests with UPDATE_SNAPS=clean env variable.

The reason for using TestMain is because go-snaps needs to be sure that all tests are finished so it can keep track of which snapshots were not called.

Example:

func TestMain(m *testing.M) {
  v := m.Run()

  // After all tests have run `go-snaps` can check for unused snapshots
  snaps.Clean(m)

  os.Exit(v)
}

For more information around TestMain.

Sort Snapshots

By default go-snaps appends new snaps to the snapshot file and in case of parallel tests the order is random. If you want snaps to be sorted in deterministic order you need to use TestMain per package:

func TestMain(m *testing.M) {
  v := m.Run()

  // After all tests have run `go-snaps` will sort snapshots
  snaps.Clean(m, snaps.CleanOpts{Sort: true})

  os.Exit(v)
}

Skipping Tests

If you want to skip one test using t.Skip, go-snaps can't keep track if the test was skipped or if it was removed. For that reason go-snaps exposes a wrapper for t.Skip, t.Skipf and t.SkipNow, which keep tracks of skipped files.

You can skip, or only run specific tests by using the -run flag. go-snaps can identify which tests are being skipped and parse only the relevant tests for obsolete snapshots.

Running Tests on CI

When go-snaps detects that it is running in CI it will automatically fail when snapshots are missing. This is done to ensure new snapshots are committed alongside the tests and assertions are successful.

go-snaps uses ciinfo for detecting if it runs on CI environment.

No Color

go-snaps supports disabling color outputs by running your tests with the env variable NO_COLOR set to any value.

NO_COLOR=true go test ./...

For more information around NO_COLOR.

Snapshots Structure

Snapshots have the form

[TestName - Number]
<data>
---

TestID is the test name plus an increasing number to allow multiple calls of MatchSnapshot in a single test.

[TestSimple/should_make_a_map_snapshot - 1]
map[string]interface{}{
    "mock-0": "value",
    "mock-1": int(2),
    "mock-2": func() {...},
    "mock-3": float32(10.399999618530273),
}
---

Note

If your snapshot data contain characters --- at the start of a line followed by a new line, go-snaps will "escape" them and save them as /-/-/-/ to differentiate them from termination characters.

Acknowledgments

This library used Jest Snapshoting and Cupaloy as inspiration.

• Jest is a full-fledged Javascript testing framework and has robust snapshoting features.
• Cupaloy is a great and simple Golang snapshoting solution.
• The logo was made by MariaLetta.

Appendix

Warning

When running a specific test file by specifying a path go test ./my_test.go, go-snaps can't track the path so it will mistakenly mark snapshots as obsolete.

Important

Snapshots should be treated as code. The snapshot artifact should be committed alongside code changes, and reviewed as part of your code review process

Note

go-snaps doesn't handle CRLF line endings. If you are using Windows, you may need to convert the line endings to LF.