How to declare a module in TypeScript with an object as default export

In a TypeScript project, I would like to use a third-party library that does not provide type definitions. So I need to come up with my own module declaration. What the module basically exports looks like this:

const foo = {
  bar () {
    return 23;
  }
};

module.exports = foo;

So, the question now is how to come up with the module declaration. First of all, it's obvious that the module uses a default export, not a named one. This default export is an object, which has methods. So I tried this:

declare module 'foo' {
  export default {
    bar(): number;
  };
}

However, the TypeScript compiler complains that:

The expression of an export statement must be an identifier or a qualified name in an ambient context.

What does this mean? Using bar, I have used an identifier, haven't I? And what does "qualified name in an ambient context" mean?

Using bar, I have used an identifier, haven't I?

The error is talking about the object of the export clause. While you have used bar as an identifier, specifically as a method name, you are not exporting that identifier, you are exporting an object literal contains it.

In

declare module 'foo' {
  export default {bar(): number};
}

the identifier bar refers to a method of the exported value not the exported value itself.

To correct this, write

declare module 'foo' {
  const foo: {bar(): number};
  export default foo;
}

A qualified name is one which is referred to by qualifying its name with its enclosing scope as in a.b

declare module 'foo' {
  namespace ns {
    const foo: {bar(): number};
  }
  export default ns.foo; 
}