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Apart from extension functions, Kotlin also offers an extension properties feature. For example, we can declare a property for the List<T> class that will hold information about the last element index value:

val <T> List<T>.lastIndex: Int  get() = size - 1

Extensions are a widely used pattern across Kotlin standard library classes. They work seamlessly with Java, Kotlin, JavaScript, and native classes defined within the project and in external dependencies as well.