must-be-err

Error type assertion for TypeScript 4.4+ unknown errors in catch clause.

Installation

npm i must-be-err

Usage

import mustBeErr from 'must-be-err';

try {
  // ...
} catch (err) {
  // `err` is unknown in TypeScript 4.4+
  mustBeErr(err); // It throws if `err` is not an `Error`.
  // `err` is now an `Error`.
  console.error(err.message);
}

const notAnError = 123;
mustBeErr(notAnError);
// Error: The argument `err` is not an `Error`, got 123.

Source

export default function mustBeErr(err: unknown): asserts err is Error {
  if (err instanceof Error === false) {
    throw new Error(`The argument \`err\` is not an \`Error\`, got "${err}".`);
  }
}