How can I safely access caught Error properties in TypeScript?

Issue

The Situation

I am using TypeScript, and have a try / catch block. The caught error is typed (Error). I would like to use the error's message attribute.

I am using eslint with the @typescript-eslint/no-unsafe-assignment rule enabled.

The Code

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

The Problem

When I run my linter, I receive the following:

  4:9  error  Unsafe assignment of an any value  @typescript-eslint/no-unsafe-assignment

This is confusing to me, since the error is typed (Error).

The Question

How can I catch an error in TypeScript and access the Error's properties?

Solution

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})`,
    };
  }
}

Answered By - Jason Owen