What is "self" used for in Swift?

Yes it is the same as this in Java and self in Objective-C, but with Swift, self is only required when you call a property or method from a closure or to differentiate property names inside your code, such as initializers. So you can use almost all of your class components safely without using self unless you are making the call from a closure.

“The self Property Every instance of a type has an implicit property called self, which is exactly equivalent to the instance itself. You use the self property to refer to the current instance within its own instance methods.

The increment() method in the example above could have been written like this:

func increment() {
    self.count += 1
}

In practice, you don’t need to write self in your code very often. If you don’t explicitly write self, Swift assumes that you are referring to a property or method of the current instance whenever you use a known property or method name within a method. This assumption is demonstrated by the use of count (rather than self.count) inside the three instance methods for Counter.

The main exception to this rule occurs when a parameter name for an instance method has the same name as a property of that instance. In this situation, the parameter name takes precedence, and it becomes necessary to refer to the property in a more qualified way. You use the self property to distinguish between the parameter name and the property name.

Here, self disambiguates between a method parameter called x and an instance property that is also called x:”

Excerpt From: Apple Inc. “The Swift Programming Language (Swift 2 Prerelease).”

This is how Ray Wenderlich recommends the use of self in Swift for their tutorials:

Use of Self

For conciseness, avoid using self since Swift does not require it to access an object's properties or invoke its methods.

Use self when required to differentiate between property names and arguments in initializers, and when referencing properties in closure expressions as required by the compiler:

class BoardLocation {
  let row: Int, column: Int

  init(row: Int, column: Int) {
    self.row = row
    self.column = column

    let closure = {
      println(self.row)
    }
  }
}

And this is GitHub's recommendations on self for their applications:

Only explicitly refer to self when required

When accessing properties or methods on self, leave the reference to self implicit by default:

private class History {
    var events: [Event]

    func rewrite() {
        events = []
    }
}

Only include the explicit keyword when required by the language — for example, in a closure, or when parameter names conflict:

extension History {
    init(events: [Event]) {
        self.events = events
    }

    var whenVictorious: () -> () {
        return {
            self.rewrite()
        }
    }
}

Rationale: This makes the capturing semantics of self stand out more in closures, and avoids verbosity elsewhere.

You will also use self a lot when creating your extensions, example:

extension Int {
    func square() -> Int {
        return self * self
    }

    // note: when adding mutating in front of it we don't need to specify the return type
    // and instead of "return " whatever
    // we have to use "self = " whatever

    mutating func squareMe() {
        self = self * self
    }
}
let x = 3
let y = x.square()  
println(x)         // 3
printlx(y)         // 9

now lets say you want to change the var result itself you have to use the mutating func to make change itself

var z = 3

println(z)  // 3

now lets mutate it

z.squareMe()

println(z)  // 9

// now lets see another example using strings :

extension String {
    func x(times:Int) -> String {
        var result = ""
        if times > 0 {
            for index in 1...times{
                result += self
            }
            return result
        }
        return ""
    }

    // note: when adding mutating in front of it we don't need to specify the return type
    // and instead of "return " whatever
    // we have to use "self = " whatever

    mutating func replicateMe(times:Int){
        if times > 1 {
            let myString = self
            for index in 1...times-1{
                self = self + myString
            }
        } else {
            if times != 1 {
                self = ""
            }
        }
    } 
}


var myString1 = "Abc"
let myString2 = myString1.x(2)

println(myString1)         // "Abc"
println(myString2)         // "AbcAbc"

now lets change myString1

myString1.replicateMe(3)

println(myString1)         // "AbcAbcAbc"

In what situations it's necessary to use it

It is necessary to use it only when the name of a local variable overshadows the name of a property.

However, as a matter of style (and readability), I always use it:

• I use it with property names, because otherwise I am left wondering what this variable is (since it is neither locally declared nor an incoming parameter).

• I use it as the receiver of function (method) calls, in order to differentiate such methods from top-level or local functions.

This is why we need self.

When we define a class, like:

class MyClass {
    func myMethod()
}

We are creating a "Class Object". Yes, Class is an object too.

Then no matter how many instances are created using the class, all instances will have a reference pointer to its Class Object.

You can imagine that all instance methods defined by the Class are in the Class Object, and there will be only one copy of them.

That means all instances created using the Class are sharing the same method.

Now imagine you are the myMethod in the Class Object, and because you are shared for all instances, you must have a way to tell which instance you are working on.

When someone says instance1.myMethod(), it means "Hi! myMethod, please do your work and instance1 is the object you are working on".

To reference the object that the caller sent to you, use self.

“In practice, you don’t need to write self in your code very often. If you don’t explicitly write self, Swift assumes that you are referring to a property or method of the current instance whenever you use a known property or method name within a method.”

Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks. https://itun.es/tw/jEUH0.l

The reserved word self in Swift is similar to this but it's not the same as in Java or JavaScript.

As @Dave Gomez correctly quoted:

Every instance of a type has an implicit property called self, which is exactly equivalent to the instance itself.

Here lies one of the main differences, because:

1. "Every instance" in Swift (at least for now) is almost every-thing.
2. In Java, for example, you can only use the word this inside an instance scope, in Swift you can use it almost every-where.

Here are a few examples:

//Example 1:
var x="foo"
x.self="bar".self//compiles and run

//Example 2:
print.self(x);//compiles and run

//Example 3:
func myOther(self otherSelf:Person){}
myOther(self: personObject);//compiles and run

//Example 4:
class Foo{
      var bar=""
      init(){
          self.addSome()//this would be the same in Java
      }
      func addSome(){
          //But definitely not this:
          self.self.bar.self.self="some".self.self
      }
}
//Guess what - also compiles and run...
let f=Foo()
print(f.bar)

See : Why 'self.self' compiles and run in swift for more information.