You do have 'instanceof' at runtime, so unless you mean some other type of 'class', casting to a class is really a non-issue (and you don't really 'cast'... since dynamic typing and all). If you mean some more advanced concept (e.g., structural type runtime checking), think how you would do it in statically typed languages (is it really better?).

    function Foo(s) {
        this.s = s
    }

    Foo.prototype.bar = function() {
        console.log(this.s)
    }

    const baz = new Foo("Baz")
    baz.bar()

(1) imperative style mutation of the prototype is runtime dependent, and typescript can't really rely on runtime behavior.

(2) Foo has two incompatible types in JS, it is both `(s) => void` and `new (s) => Foo`, typescript by default will assume the first signature (which is usually what people want).

In any case, you can specify the types (though in 99% of the cases, just use a class):

   interface Foo {
       s: string;
       bar(): void;
   }

   const Foo = function(this: Foo, s: string) {
       this.s = s;
   } as {
     new (s: string): Foo;
     (this: Foo, s: string): void;
   };
   
   Foo.prototype.bar = function(this: Foo) {
       console.log(this.s);
   }
   
   const baz = new Foo('Hello');
   baz.bar()

While that is a fair statement, I've been reading a lot of Typescript code lately and it seems in 99% of cases the selected solution is to use top level functions and globals to avoid the nesting hell class introduces, albeit introducing the global hell in the process. In practice, 'just use a class' doesn't overcome the human element.

Javascript was on the right track originally, but then veered off into the weeds for some reason. I appreciate you pointing out how it can be done. Although the solution is very much second class citizen, sadly. This certainly isn't going to wean developers away from global hell. Hopefully Javascript/Typescript can make some strides in improving this going forward.

We are talking about runtime class creation with dynamic methods, while still adding proper static types. There aren't many language allow this level of flexibility.

You can probably disallow prototype use with existing tools if that's desired, but it is an advanced tool for advanced use cases (which still exist).

Given:

    class Foo {
        constructor(x) {
            this.x = x
        }

        bar() {
            return this.x
        }
    }

    object Foo(x) {
        this.x = x
    }

    function Foo.bar() {
        return this.x
    }

This is exactly what bugs me about TypeScript. The language itself is fine—some annoying things, but generally they’re due to having to run on top of JS. But Microsoft had the chance to fix JS’s terrible standard library. And they didn’t!

https://caniuse.com/?search=import

I know that's less of a concern nowadays with modern http, but guess who still supports ie11

It feels like terrible advice to give someone, given ESM is far superior to AMD, and this would be going backwards in time, but in your case you are already blighted by the disease and it is advice that might staunch some of the bleeding.