typescript 2.2 interfaces extend object type

Question

I have function using an object as parameter like this:

interface Options {
    foo?: string;
    bar?: number;
};

function fooNction(opts: Options): void {
}

This works fine in some cases but not all:

fooNction({foo: "s"});   // OK
fooNction({a: "x"});     // fine as TS gives an Error as expected
fooNction("hello");      // no Error...

I tried to extend my interface from TS 2.2 object type like this

interface Options extends object {
    foo?: string;
    bar?: number;
};

to disallow basic types but typescript tells "Cannot fine name 'object'".

Is there any way to define an interfaces has to be an object but has no mandatory field?

Bentz

Answer 1

For some reason, it's not allowed to have an interface that extends built-in type like object or string. You may try to bypass that by declaring type alias for object like this

type ObjectAlias = object;

interface Options extends ObjectAlias {
   ...
}

but it still does not work because structural compatibility is used for typechecking interfaces:

function fooNction(opts: Options): void {
}

fooNction("hello"); //ok

So it seems like object is usable only with intersection types, and you must declare Options as a type, not an interface to make it work:

type  Options = object & {
    foo?: string;
    bar?: number;
};

function fooNction(opts: Options): void {
}

fooNction("hello"); //error

Haleakala