Extensions are used to extend the functionality of existing types in Swift. Extensions can add subscripts, functions, initializers, and computed properties. They can also make types conform to protocols.

Suppose you want to be able to compute the factorial of an Int. You can add a computed property in an extension:

extension Int {
    var factorial: Int {
        return (1..<self+1).reduce(1, combine: *)
    }
}

Then you can access the property just as if it had been included in original Int API.

let val1: Int = 10

val1.factorial  // returns 3628800

Extensions can contain functions and computed/constant get variables. They are in the format

extension ExtensionOf {
    //new functions and get-variables
}

To reference the instance of the extended object, self can be used, just as it could be used

To create an extension of String that adds a .length() function which returns the length of the string, for example

extension String {
    func length() -> Int {
        return self.characters.count
    }
}

"Hello, World!".length() // 13

Extensions can also contain get variables. For example, adding a .length variable to the string which returns the length of the string

extension String {
    var length: Int {
        get {
            return self.characters.count
        }
    }
}

"Hello, World!".length // 13

Extensions can contain convenience initializers. For example, a failable initializer for Int that accepts a NSString:

extension Int {
    init?(_ string: NSString) {
        self.init(string as String)  // delegate to the existing Int.init(String) initializer
    }
}

let str1: NSString = "42"
Int(str1) // 42

let str2: NSString = "abc"
Int(str2) // nil

A very useful feature of Swift 2.2 is having the ability of extending protocols.

It works pretty much like abstract classes when regarding a functionality you want to be available in all the classes that implements some protocol (without having to inherit from a base common class).

protocol FooProtocol {
    func doSomething()
}

extension FooProtocol {
    func doSomething() {
        print("Hi")
    }
}

class Foo: FooProtocol {
    func myMethod() {
        doSomething() // By just implementing the protocol this method is available
    }
}

This is also possible using generics.

It is possible to write a method on a generic type that is more restrictive using where sentence.

extension Array where Element: StringLiteralConvertible {
  func toUpperCase() -> [String] {
      var result = [String]()
      for value in self {
          result.append(String(value).uppercaseString)
      }
      return result
    }        
}

Example of use

let array = ["a","b","c"]
let resultado = array.toUpperCase()

Extensions add new functionality to an existing class, structure, enumeration, or protocol type. This includes the ability to extend types for which you do not have access to the original source code.

Extensions in Swift can:

• Add computed properties and computed type properties
• Define instance methods and type methods
• Provide new initializers
• Define subscripts
• Define and use new nested types
• Make an existing type conform to a protocol

When to use Swift Extensions:

• Additional functionality to Swift
• Additional functionality to UIKit / Foundation
• Additional functionality without messing with other persons code
• Breakdown classes into: Data / Functionality / Delegate

When not to use:

• Extend your own classes from another file

Simple example:

extension Bool {
    public mutating func toggle() -> Bool {
        self = !self
        return self
    }
}

var myBool: Bool = true
print(myBool.toggle()) // false

Source

Extensions can add new subscripts to an existing type.

This example gets the character inside a String using the given index:

extension String {
    subscript(index: Int) -> Character {
        let newIndex = startIndex.advancedBy(index)
        return self[newIndex]
    }
}

var myString = "StackOverFlow"
print(myString[2]) // a
print(myString[3]) // c

extension String {
    subscript(offset: Int) -> Character {
        let newIndex = self.index(self.startIndex, offsetBy: offset)
        return self[newIndex]
    }
}

var myString = "StackOverFlow"
print(myString[2]) // a
print(myString[3]) // c