An extension is defined by using the extension keyword followed by the name of the type you are extending. We then put the functionality that we are adding to the type between curly brackets. The following example shows how to define an extension:

extension String { 
  // Add functionality here 
} 

The previous example would add an extension to the String type from the Swift standard library. Since we can extend any type, we can use extensions to add functionality to types from the Swift standard library, types from frameworks, or our own custom types. While we can use extensions to add functionality to our own custom types, it is usually better to add the functionality directly to the type itself. The reason for this is that our code is easier to maintain if all the functionality (code) for our custom types is located together.

If we are adding functionality to a framework and we have the code for that framework, it is still better to add the functionality with an extension...