What is it?

snapshot4s is a tool that updates your tests on command.

You can use it with MUnit, Weaver and ScalaTest. It supports tests in Scala 2, Scala 3 and Scala.js.

A snapshot test is a unit test that asserts that a found value is equal to a previously snapshotted value.

test("should get all pets") {
  val pets = getPets()
  assertInlineSnapshot(
    pets,
    List(
      Pet(name = "Mistoffelees", age = 6),
      Pet(name = "Grizabella", age = 14),
      Pet(name = "Bombalurina", age = 5)
    )
  )
}

assertInlineSnapshot is like assert with superpowers. It takes control of its expected value and updates it on command.

Why use it?

It'll save you a ton of time and effort when updating your tests.

Suppose we intentionally change getPets: with our new logic, all pets are one year older.

We would have to update our test with the new ages:

test("should get all pets") {
  val pets = getPets()
  assertInlineSnapshot(
    pets,
    List(
      Pet(name = "Mistoffelees", age = 7),
      Pet(name = "Grizabella", age = 15),
      Pet(name = "Bombalurina", age = 6)
    )
  )
}

This is easy enough to do manually for one single test. It quickly becomes boring when there are many, especially when the expected values are large.

snapshot4s updates all your tests for you with a single sbt command.

Following the test failure, we can run snapshot4sPromote to replace the expected value with the actual value.

sbt test                  # Fails as snapshots do not match.
sbt snapshot4sPromote     # Updates snapshots
sbt test                  # Succeeds!

Instead of updating the tests manually yourself, you only need to review them using your favourite difftool.