Kotlin Programming Cookbook

How it works…

The prefix to the extension function (the name that comes before the dot) is called receiver type, that is, the type being extended. This receiver object is accessed inside the function using the this keyword. The extension functions are resolved statically; it's like calling a static method. Since this is a static method, it's not needed to be defined under the class, but since it's a static method, it's hard to test. For example, Mockito (a testing framework) cannot test static methods, so to produce great code quality, use extension function only if that function doesn't require any testing.