Does a Swift constructor parentheses work the same way as a Javascript IIFE parentheses?

Question

I just started learning Javascript and ran across an Immediately Invoked Function Expression or IFFE for short. The parentheses at the end immediately invokes the function.

var greetingsObject = function(name){
  return 'Hello' + name;
}(); //will return Hello undefined

In Swift I'm also learning how to programmatically create objects. I create a UILabel

var greetingsObject: UILabel = {
 let label = UILabel()
 label.text = "Hello " + nameTextField.text!
 return label
}()

My question is does the parentheses at the end of the Swift object work the same way as the one at the end of the JS IFFE? If it doesn't call the function then what does it do?

Answer 1

I first to point out a bug with your example: your code reference nameTextField within the initializer for greetingObject. There's no guarantee what property will be initialized first so you can't reference another property within the init closure.

Having said that, it works the same in Swift as Javascript. greetingsObject will be initialized when you initialize the class instance. This pawttern is usually employed when you want the property to be initialzed only once and shared among multiple instances.

Swift also extended the concept with an optional lazy modifier. With lazy, it will initialize the property the first time you reference it.

class ViewController : UIViewController {
    lazy var greetingsObject: UILabel = {
        print("Initialzing greetingsObject")

        let label = UILabel()
        label.text = "Hello World"
        return label
    }()
}

let vc = ViewController() // `greetingsObject` is not yet initialized

print("First reference to greetingsObject")
vc.greetingsObject.text   // it will be initialized here