catch-unknown: Utility functions for writing type-safe JavaScript catch blocks

While exceptions thrown in JavaScript are usually objects of class Error, they can actually be values of any type. This is particularly relevant in TypeScript, which in version 4.4 starting defaulting catch variables to unknown type (instead of any). This small library provides two functions to make writing type-safe catch blocks easier: isError returns whether a value conforms to the Error interface, and asError will convert any value to an object conforming to Error if necessary:

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

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

This library has no runtime dependencies, compiles to ES6 for wide compatibility, and has a package size of 2.9 kB.

Installation

npm install catch-unknown

Usage

Typical usage might look something like this:

import { asError } from 'catch-unknown';

try {
  // stuff
} catch (err) {
  logger.warn(`Stuff failed due to ${asError(err).message}`);
  throw err;
}

Examples

Hopefully you never see a non-Error thrown, but if you do, nothing else will break:

import { asError, isError } from 'catch-unknown';

try {
  throw new Error('Something is wrong');
} catch (err) {
  console.log(isError(err)); // true
  console.log(asError(err)); // Error: Something is wrong
}

try {
  throw { message: 'An odd thing to throw' };
} catch (err) {
  console.log(isError(err)); // false
  console.log(asError(err)); // Object: An odd thing to throw
}

try {
  throw { x: 12, y: 5 };
} catch (err) {
  console.log(isError(err)); // false
  console.log(asError(err)); // Object: {"x":12,"y":5}
}

try {
  throw new Date(0);
} catch (err) {
  console.log(isError(err)); // false
  console.log(asError(err)); // Date: Thu Jan 01 1970 00:00:00 GMT+0000 (Coordinated Universal Time)
}

try {
  throw 42;
} catch (err) {
  console.log(isError(err)); // false
  console.log(asError(err)); // number: 42
}

License

catch-unknown is available under the ISC license.