[reissbaker]

Not what I was thinking of, but I should've been more explicit. Using generics you can type the keys and values of Maps in any language with generic support that I'm familiar with: certainly C++ and Java can. But you can't make type assertions about certain values existing under certain keys: that's what structs/classes/etc are for. But, those constructs can't easily be generated at runtime, and are typed nominally: even just as a caller you have to explicitly say they inherit (or in Java, implement) a specific type. However, with TypeScript-like structural subtyping, you can do:

    interface BreadIngredientOptions {
      flourType?: String; // this is the syntax for optional strings
      sugarAmount?: String; // ditto: it's the ? that makes it optional
      // ...
    }

    function bakeBread(ingredientOverrides: BreadIngredientOptions) {
      // ...
    }

    // callers don't need to explicitly inherit or implement to be type checked
    // however, since all properties are optional, this is less interesting
    bakeBread({
      flourType: 'white'
    });

But you can do even better than that example shows. One common problem with maps-as-named-arguments is that you can't easily determine which arguments are required and which are optional. With typed optional properties and structural subtyping you can enforce that at compile time, as follows:

    interface MyArgumentInterface {
      requiredArg: number;
      optionalArg?: number;
    }

    function f(args: MyArgumentInterface) {
      // ...
    }

    // This works:
    f({
      requiredArg: 10,
      optionalArg: 5
    });

    // This also works:
    f({ requiredArg: 50 });

    // This statically throws at compile time:
    f({ optionalArg: 10 });

It's a combination of the simple object literal syntax from raw JS that makes it easy to create objects of arbitrary types, with structural subtyping. I'm not aware of any language with the same features (but would love to be corrected!).