Infer one of generic types from function argument

Question

Consider the following example. fetchItems function returns response or response body depending on passed onlyBody argument which defaults to

Answer 1

Function overloads can do what you need:

function fetchItems(url: string, onlyBody: false): Promise>
function fetchItems(url: string, onlyBody?: true): Promise
function fetchItems(url: string, onlyBody: boolean = true) {
  return Promise
    .resolve({body: 'some data'} as any)
    .then(res => onlyBody ? res.body : res);
}

Playground

Solution with conditional types does not work due to TypeScript "design limitation" described here.