TypeScript friendly assert

#assert.ts

/**
 * Writes an error message to the console if the assertion is false.
 *
 * If the assertion is true, nothing happens.
 *
 * If your code only runs in Node.js, prefer `import { strict as assert } from 'node:assert'`
 * because it throws.
 *
 * https://gist.github.com/tkrotoff/1a216f376cb4fba5bc7d8b5109c3a32e
 * https://devblogs.microsoft.com/typescript/announcing-typescript-3-7/#assertion-functions
 */
export function assert(_condition: boolean, _message?: string): asserts _condition {
  // eslint-disable-next-line no-console, prefer-rest-params
  console.assert(...arguments);
}

assert.test.ts

import { assert } from './assert';

test('TypeScript "asserts condition"', () => {
  const foobar = (): number | string => 'foobar';

  const str = foobar();
  assert(typeof str === 'string');

  // Without assert(), TypeScript error: "Property 'includes' does not exist on type 'number'"
  str.includes('foo');
});

test('console output', () => {
  expect.assertions(4);

  const consoleMock = jest.spyOn(console, 'assert').mockImplementation();

  assert(false);

  // "Unreachable code detected" for TypeScript

  expect(consoleMock).toHaveBeenCalledTimes(1);
  // Tests that it's not "false, undefined"
  expect(consoleMock).toHaveBeenCalledWith(false);

  assert(false, 'a message');
  expect(consoleMock).toHaveBeenCalledTimes(2);
  expect(consoleMock).toHaveBeenCalledWith(false, 'a message');

  consoleMock.mockRestore();
});