In typescript, how to define type of async function

Found this searching how to declare a “typedef” for an async arrow function.

It works if you just declare the return type of the function to be a Promise:

interface SearchFn {
    (subString: string): Promise<boolean>;
}

or as a type declaration:

type SearchFn = (subString: string) => Promise<boolean>;

Microsoft’s TS Linter will recommend this second syntax.