What is the meaning of "??" in swift?

Nil-Coalescing Operator

It's kind of a short form of this. (Means you can assign default value nil or any other value if something["something"] is nil or optional)

let val = (something["something"] as? String) != nil ? (something["something"] as! String) : "default value"

The nil-coalescing operator (a ?? b) unwraps an optional a if it contains a value, or returns a default value b if a is nil. The expression a is always of an optional type. The expression b must match the type that is stored inside a.

The nil-coalescing operator is shorthand for the code below:

a != nil ? a! : b The code above uses the ternary conditional operator and forced unwrapping (a!) to access the value wrapped inside a when a is not nil, and to return b otherwise. The nil-coalescing operator provides a more elegant way to encapsulate this conditional checking and unwrapping in a concise and readable form.

If the value of a is non-nil, the value of b is not evaluated. This is known as short-circuit evaluation.

The example below uses the nil-coalescing operator to choose between a default color name and an optional user-defined color name:

let defaultColorName = "red"
var userDefinedColorName: String?   // defaults to nil

var colorNameToUse = userDefinedColorName ?? defaultColorName
// userDefinedColorName is nil, so colorNameToUse is set to the default of "red"

See section Nil-Coalescing Operator here

This operator is generally used to provide a default value when an expression or variable produces an optional result. for ex:

let i: Int? = 5
let j: Int? = nil

let value1 = i ?? 9 //value1 will be 5 non-optional
let value2 = j ?? 9 //value2 will be 9 non-optional

You can chain multiple of these operators as such:

let value3 = j ?? i ?? 9 //value3 will be 5 non-optional

The chain terminates whenever a non nil value is found. The result of the expression will be a optional/non-optional depending on the type of last expression in the chain.

The example you've provided, although syntactically correct, is redundant, as omitting the ?? nil would have no result on the outcome.