Extensions are used to add new functionality to an existing class, enumeration, or structure. They are like categories in Objective-C, but in Swift we have two differences:

In Swift, extensions can do many things. Check this list:

To create an extension, use this form:

extension someType{
    //New functionalities go here
}

In extensions, as we said, you can add new functionalities, but you can't override an existing one.

extension Double{
    
    var km : Double{
        return self / 1000.0;
    }
    var cm : Double{
        return self * 100.0
    }
}

var distance = 1500...