Testing shared code

Testing shared code in KMP is similar to writing code in KMP: in your shared code, you will have to write platform-agnostic code. This means no third-party testing frameworks or libraries that target a specific platform, JVM, JS, or iOS, such as XCTest or JUnit. Thankfully, KMP already provides a library that targets JVM, JS, and Native: https://kotlinlang.org/api/latest/kotlin.test/.

kotlin.test provides an Asserter abstraction with a DefaultAsserter that is dependency-free, but it also provides JUnitAsserter, JUnit5Asserter, and TestNGAsserter so that you can choose the one you'd like to use in your JVM or Android targets.

You can also implement your own Asserter implementations for the different platforms if you wish. The same expect/actual mechanism can be used in your tests as well.

But what about which test double (https://en.wikipedia.org/wiki/Test_double) you should choose to test your shared code – Stubs, Mocks, Spies, Fakes, or Dummies...