How is an extension added and what exactly can be added?

Extensions are added to classes, properties, functions, or receiver types.

You can add:

• New functions to an existing class.
• New properties to an existing class.
• New helper functions for a specific receiver type.

Example of adding an extension function:

fun String.toUpperCaseFirstChar(): String {  
    // Returns a string where the first character is in uppercase
    return this.substring(0, 1).toUpperCase() + this.substring(1)  
}

Example of usage:

val myString = "hello"
val capitalizedString = myString.toUpperCaseFirstChar()
// Now capitalizedString = "Hello"