I am using window.fetch in Typescript, but I cannot cast the response directly to my custom type:

I am hacking my way around this by casting the Promise result to an intermediate 'any' variable.

What would be the correct method to do this?

import { Actor } from './models/actor';

fetch(`http://swapi.co/api/people/1/`)
      .then(res => res.json())
      .then(res => {
          // this is not allowed
          // let a:Actor = <Actor>res;

          // I use an intermediate variable a to get around this...
          let a:any = res; 
          let b:Actor = <Actor>a;
      })

---

A few examples follow, going from basic through to adding transformations after the request and/or error handling:

Basic:

// Implementation code where T is the returned data shape
function api<T>(url: string): Promise<T> {
  return fetch(url)
    .then(response => {
      if (!response.ok) {
        throw new Error(response.statusText)
      }
      return response.json<T>()
    })

}

// Consumer
api<{ title: string; message: string }>('v1/posts/1')
  .then(({ title, message }) => {
    console.log(title, message)
  })
  .catch(error => {
    /* show error message */
  })

Data transformations:

Often you may need to do some tweaks to the data before its passed to the consumer, for example, unwrapping a top level data attribute. This is straight forward:

function api<T>(url: string): Promise<T> {
  return fetch(url)
    .then(response => {
      if (!response.ok) {
        throw new Error(response.statusText)
      }
      return response.json<{ data: T }>()
    })
    .then(data => { /* <-- data inferred as { data: T }*/
      return data.data
    })
}

// Consumer - consumer remains the same
api<{ title: string; message: string }>('v1/posts/1')
  .then(({ title, message }) => {
    console.log(title, message)
  })
  .catch(error => {
    /* show error message */
  })

Error handling:

I'd argue that you shouldn't be directly error catching directly within this service, instead, just allowing it to bubble, but if you need to, you can do the following:

function api<T>(url: string): Promise<T> {
  return fetch(url)
    .then(response => {
      if (!response.ok) {
        throw new Error(response.statusText)
      }
      return response.json<{ data: T }>()
    })
    .then(data => {
      return data.data
    })
    .catch((error: Error) => {
      externalErrorLogging.error(error) /* <-- made up logging service */
      throw error /* <-- rethrow the error so consumer can still catch it */
    })
}

// Consumer - consumer remains the same
api<{ title: string; message: string }>('v1/posts/1')
  .then(({ title, message }) => {
    console.log(title, message)
  })
  .catch(error => {
    /* show error message */
  })

---

If you take a look at @types/node-fetch you will see the body definition

export class Body {
    bodyUsed: boolean;
    body: NodeJS.ReadableStream;
    json(): Promise<any>;
    json<T>(): Promise<T>;
    text(): Promise<string>;
    buffer(): Promise<Buffer>;
}

That means that you could use generics in order to achieve what you want. I didn't test this code, but it would looks something like this:

import { Actor } from './models/actor';

fetch(`http://swapi.co/api/people/1/`)
      .then(res => res.json<Actor>())
      .then(res => {
          let b:Actor = res;
      });

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