Swift: if vs. guard

if_vs_guard.swift

// Declare three optional variables
var firstName: String?
var lastName: String?
var age: Int?

// EXAMPLE: if-Statement
if let firstName = firstName {
    if let lastName = lastName {
        if let age = age, age >= 18 {
            doSomething(firstName, lastName, age)
        }
    }
}

// EXAMPLE: guard-Statement
guard 
    let firstName = firstName, 
    let lastName = lastName, 
    let age = age, age >= 18 
else {
    return
}

doSomething(firstName, lastName, age)