> You can't do that e.g. with c#, where one class will never be exchangeable with an unrelated class , no matter how similar the definitions.
Agreed. Duck typing is a killer feature. I can't say how many time's I've rolled a new class for no reason other than data structuring, which would have been far preferable as something just defined inline.
Const x = [{x: 3, y: "hi"}, {x: 9, y: "bye"}];
Automatically gets you x: "Array of {x : number, y: string }". And if you declare the same type somewhere else, as long as they're "structurally equivalent" (i.e. same x and y types) you can actually use them interchangeably. You can't do that e.g. with c#, where one class will never be exchangeable with an unrelated class , no matter how similar the definitions.
This lets you do some funny stuff like"subset type detection ": https://gist.github.com/hraban/66c1778cdd31868034b12db93fcce...
All in all, it's more of an oddity than an actual strength, if you ask me. It's necessary to emulate JS semantics, but I wouldn't consider it an advantage in a new language.