How can I add an async function to a type def file?

Question

I am trying to add an async function definition to for async-busboy;

I have created a file "async-busboy.d.ts"

declare module 'async-busboy' {
    export default async function asyncBusby
        (subString: any): Promise<any>;
}

But this gives me the error: "error TS1040: 'async' modifier cannot be used in an ambient context."

How do I write async function defs?
Thanks

EDIT Thanks @Titian Cernicova-Dragomir and @Aaron for your answer. But when I call it I get an error

const dddd = await asyncBusboy(22);
[ts] 'await' expression is only allowed within an async function.

Any suggestions?

Answer 1

All you need is this:

declare module 'async-busboy' {
    export default function asyncBusby
        (subString: any): Promise<any>;
}

The fact that it returns a Promise is what makes it of async type (nothing related to the async modifier). Consumers are expected to await the any value now.

On the other hand, the async modifier is used to augment the function (body) implementation (what it returns becomes wrapped in a promise). Since the type-def doesn't deal with the implementation, therefore the async isn't allowed in the type-def.

In other words, from the perspective of the method/function interface the async modifier is not saying/changing anything and therefore is not needed in the interface declaration.