[Typescript] Mind map: Never and Unknown

 

You cannot assign anything to never, except for never itself.

// red squiggly lines under everything in parens
fn("hello");
fn(42);
fn(true);
fn({});
fn([]);
fn(() => {});

// no error here, since we're assigning `never` to `never` 
fn(getNever());

 

However, you can assign never to anything:

const str: string = getNever();
const num: number = getNever();
const bool: boolean = getNever();
const arr: string[] = getNever();

 

For the function can throw, you don't need to add return type, it defualts to never

type S = string | never // string

import { Equal, Expect } from '@total-typescript/helpers';

const throwError = (message: string) => {
  throw new Error(message);
};

const handleSearchParams = (params: { id?: string }) => {
  const id = params.id || throwError('No id provided');

  type test = Expect<Equal<typeof id, string>>;

  return id;
};