Extending Data Object in TypeScript

I'm trying to extending a data object in TypeScript, by adding some new fields. While I guess it's a quite common pattern in JavaScript, I can't make it be able to compile, without making bar as optional in the snippet as below.

I'm wondering is there any way to avoid making bar optional. Any suggestion would be welcome, thank you.

interface BaseDataObject {
    foo: string;
}

interface ExtendedDataObject extends BaseDataObject {
    bar?: string;
}

function extendData(input : BaseDataObject) : ExtendedDataObject {
    var output : ExtendedDataObject = input;
    output.bar = input.foo + ' some suffix';
    return output;
}