Since Typescript 3.7 you can make an assertion type guard

export function assertIsError(error: unknown): asserts error is Error {
    // if you have nodejs assert: 
    // assert(error instanceof Error);
    // otherwise
    if (!(error instanceof Error)) {
        throw error
    }
}

} catch (err) {
  assertIsError(err);
  // err is now typed as Error
  return { message: `Things exploded (${err.message})` };
}

Visit https://devblogs.microsoft.com/typescript/announcing-typescript-4-4/

" Using unknown in Catch Variables Users running with the --strict flag may see new errors around catch variables being unknown, especially if the existing code assumes only Error values have been caught. This often results in error messages such as:

Property 'message' does not exist on type 'unknown'. Property 'name' does not exist on type 'unknown'. Property 'stack' does not exist on type 'unknown'.

To get around this, you can specifically add runtime checks to ensure that the thrown type matches your expected type. Otherwise, you can just use a type assertion,

add an explicit : any to your catch variable, or turn off --useUnknownInCatchVariables."

you can simply add in tsconfig.json:

"compilerOptions": { "useUnknownInCatchVariables": false }

or you can use like:

catch (e: any) {console.log(e.message)}

TL;DR: I made a tiny library to make catching unknown safely easy:

import { asError } from 'catch-unknown';

try {
  throw new Error('foo');
} catch (err) {
  return {
    message: `Things exploded (${asError(err).message})`,
  };
}

According to Typescript's lib.es5.d.ts file, an Error is any object with string properties called name and message and optionally stack.

interface Error {
    name: string;
    message: string;
    stack?: string;
}

In practice, errors are generally created using the Error constructor (which is required for instanceof Error to be true):

interface ErrorConstructor {
    new(message?: string): Error;
    (message?: string): Error;
    readonly prototype: Error;
}

declare var Error: ErrorConstructor;

The catch-unknown library provides two simple functions:

export declare function isError(err: unknown): err is Error;
export declare function asError(err: unknown): Error;

isError is a typical type guard, which checks for the correct property types, as opposed to using instanceof (for maximum compatibility). asError is essentially isError(err) ? err : { name: typeof err, message: String(err) }, plus handling of a few special cases. 99.99% of the time, values you catch are already conform to Error, so all that happens is a quick type check.

TypeScript 4.0 introduced the ability to declare the type of a catch clause variable... so long as you type it as unknown:

TypeScript 4.0 now lets you specify the type of catch clause variables as unknown instead. unknown is safer than any because it reminds us that we need to perform some sorts of type-checks before operating on our values.

We don't have the ability to give caught errors arbitrary types; we still need to use type guards to examine the caught value at runtime:

try {
  throw new Error('foo');
} catch (err: unknown) {
  if (err instanceof Error) {
    return {
      message: `Things exploded (${err.message})`,
    };
  }
}

The catch parameter type can only be any or unknown. I think in your case it's considered as any.

Is this working in your environment?

try {
  throw new Error('foo');
} catch (err: unknown) {
  const { message } = err as Error; // I removed : { message: string } because it should be infered
  return {
    message: `Things exploded (${message})`,
  };
}