?? operator in Swift

In the "The Swift Programming Language" book (page 599), I came across this code snippet that kind of confused me. It went like this:

func buyFavoriteSnack(person:String) throws {
let snackName = favoriteSnacks[person] ?? "Candy Bar"
try vend(itemName:snackName)
}

Its explanation was:

> The buyFavoriteSnack(_:) function looks up the given person's favorite snack and tries to buy it for them. If they don't have a favorite snack listed, it tries to buy a candy bar. If they...

How can this explanation map to the "??" operator in the code given. When should/can we use this syntax in our own code?

It is "nil coalescing operator" (also called "default operator"). `a ?? b` is value of `a` (i.e. `a!`), unless `a` is `nil`, in which case it yields `b`. I.e. if `favouriteSnacks[person]` is missing, return assign `"Candy Bar"` in its stead.

The nil-coalescing operator `a ?? b` is a shortcut for `a != nil ? a! : b`

let something = a ?? b

means

let something = a != nil ? a! : b

This:

let snackName = favoriteSnacks[person] ?? "Candy Bar"

Is equals this:

if favoriteSnacks[person] != nil {
let snackName = favoriteSnacks[person]
} else {
let snackName = "Candy Bar"
}

Explaining in words, if the `let` statement fail to grab `person` from `favoriteSnacks` it will assigned Candy Bar to the `snackName`

One addition to @Icaro's answer you can declare values without initialize them. In my opinion this is better:

func buyFavoriteSnack(person:String) throws {
// let snackName = favoriteSnacks[person] ?? "Candy Bar"
let snackName: String

if let favoriteSnackName = favoriteSnacks[person] {
snackName = favoriteSnackName
} else {
snackName = "Candy Bar"
}

try vend(itemName:snackName)
}