empty class acts as "any" type: by design or bug?

[ziriax]

The following typechecks fine:

class Empty {
}

// From a strongly-typed point of view, this feels strange
var e1: Empty = "Strange";
var e2: Empty = 42;

This also compiles

class Foo {
    name: string;

    constructor(name: string) {
        this.name = name;
    }

    hello() {
        return "456";
    }
}

class Bar {
    name: string;

    hello() {
        return "123";
    }
}

var foo = new Foo("Jo");

// From a strongly-typed point of view, this feels strange
var bar: Bar = foo;

Is this by design?

[RyanCavanaugh]

Yes. Note that the empty type does not behave as any - it is a valid target for all assignments, but is only a valid source for assignments to other empty types:

var e: Empty, f: Foo;
f = e; // Error

TypeScript uses a structural type system, see also:
http://stackoverflow.com/questions/22385775/typescript-type-check-failed-to-catch-wrong-assignment/22386927#22386927
http://stackoverflow.com/questions/32106253/why-does-typescript-implicitly-convert-between-unrelated-classes/32106416#32106416
http://stackoverflow.com/questions/28080106/typescript-type-confusion-variable-doesnt-mind-type/28080519#28080519

[binarez]

I'm using typescript in an educational situation and would find useful to have a stricter mode that doesn't allow this special case. Is there any way to disable this? I found a work-around : adding a dummy private member makes not empty (obviously).

class Empty
{
    private dummy : any;
}

let e : Empty = 2;

Type 'number' is not assignable to type 'Empty'. Property 'dummy' is missing in type 'Number'.

I use '_' instead of 'dummy' in my lib (empty File class, it's really just a handle for the user, no attributes or methods) but you get the idea.

[RyanCavanaugh]

There isn't a way to disable this behavior using a flag or other setting. Adding a member to each class is the best solution.

[binarez]

Is adding one an option or is it too tied to the type system? I haven't read the type system docs you provided above, yet.

[RyanCavanaugh]

This isn't something we'd add an option for. It's a structural type system, so empty types are equivalent by definition.