Just declare your (any)object or variable with

'let' key word -> for "constan/Immutable" array, dictionary, variable, object..etc.

and

'var' key word -> for "Mutable" array, dictionary, variable, object..etc.

For more deeply information

“Use let to make a constant and var to make a variable. The value of a constant doesn’t need to be known at compile time, but you must assign it a value exactly once. This means you can use constants to name a value that you determine once but use in many places."

var myVariable = 42
myVariable = 50
let myConstant = 42

Read “The Swift Programming Language.”

There is only one Array and one Dictionary type in Swift. The mutability depends on how you construct it:

var mutableArray = [1,2,3]
let immutableArray = [1,2,3]

i.e. if you create an assign to a variable it is mutable, whereas if you create an assign to constant it is not.

WARNING: Immutable arrays are not entirely immutable! You can still change their contents, just not their overall length!

Arrays

Create immutable array

First way:

let array = NSArray(array: ["First","Second","Third"])

Second way:

let array = ["First","Second","Third"]

Create mutable array

var array = ["First","Second","Third"]

Append object to array

array.append("Forth")

Dictionaries

Create immutable dictionary

let dictionary = ["Item 1": "description", "Item 2": "description"]

Create mutable dictionary

var dictionary = ["Item 1": "description", "Item 2": "description"]

Append new pair to dictionary

dictionary["Item 3"] = "description"

_{More information on Apple Developer}

Swift does not have any drop in replacement for NSArray or the other collection classes in Objective-C.

There are array and dictionary classes, but it should be noted these are "value" types, compared to NSArray and NSDictionary which are "object" types.

The difference is subtle but can be very important to avoid edge case bugs.

In swift, you create an "immutable" array with:

let hello = ["a", "b", "c"]

And a "mutable" array with:

var hello = ["a", "b", "c"]

Mutable arrays can be modified just like NSMutableArray:

var myArray = ["a", "b", "c"]


myArray.append("d") // ["a", "b", "c", "d"]

However you can't pass a mutable array to a function:

var myArray = ["a", "b", "c"]


func addToArray(myArray: [String]) {
myArray.append("d") // compile error
}

But the above code does work with an NSMutableArray:

var myArray = ["a", "b", "c"] as NSMutableArray


func addToArray(myArray: NSMutableArray) {
myArray.addObject("d")
}


addToArray(myArray)


myArray // ["a", "b", "c", "d"]

You can achieve NSMutableArray's behaviour by using an inout method parameter:

var myArray = ["a", "b", "c"]


func addToArray(inout myArray: [String]) {
myArray.append("d")
}


addToArray(&myArray)


myArray // ["a", "b", "c", "d"]

Re-wrote this answer 2015-08-10 to reflect the current Swift behaviour.

If you want to work with Array (Swift) as with NSArray, you can use a simple bridge function. Example:

var arr1 : Array = []


arr1.bridgeToObjectiveC().count

It works the same for let.

From Apple's own docs:

Mutability of Collections

If you create an array, a set, or a dictionary and assign it to a variable, the collection that is created will be mutable. This means that you can change (or mutate) the collection after it is created by adding, removing, or changing items in the collection. Conversely, if you assign an array, a set, or a dictionary to a constant, that collection is immutable, and its size and contents cannot be changed.

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/CollectionTypes.html#//apple_ref/doc/uid/TP40014097-CH8-ID105

Other uses of immutable/mutable collections depend on why you want them to be mutable/immutable. Collections are value types in Swift, which means their contents is copied when they are assigned to another value, or passed to another function/method. Therefore, you do not need to worry about whether a receiving method function might change the original array. Therefore you don't need to ensure to return an immutable collection if your class is holding a mutable collection, for instance.

Swift Mutable/Immutable collection

[Unmodifiable and Immutable]

Swift's array can be muted

[let vs var, Value vs Reference Type]

Immutable collection^[About] - is a collection structure of which can not be changed. It means that you can not add, remove, modify after creation

let + struct(like Array, Set, Dictionary) is more suitable to be immutable

There are some classes(e.g. NSArray) which doesn't provide an interface to change the inner state

but

class A {
var value = "a"
}


func testMutability() {
//given
let a = A()
    

let immutableArr1 = NSArray(array: [a])
let immutableArr2 = [a]
    

//when
a.value = "aa"
    

//then
XCTAssertEqual("aa", (immutableArr1[0] as! A).value)
XCTAssertEqual("aa", immutableArr2[0].value)
}

It would rather is unmodifiable array