An alternative to `assert.IfError(value)` for Jest users

Explore Node's assert module documentation specifically focused on the assert.ifError function.

When used, assert.ifError will throw an error if the provided value is not undefined or null. This comes in handy during testing, especially when examining the error argument within callbacks. The stack trace will display all frames corresponding to the error passed to ifError(), along with any new frames introduced by ifError() itself.

assert.ifError(null);
// OK
assert.ifError(0);
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 0
assert.ifError('error');
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 'error'
assert.ifError(new Error());
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: Error

Curious about Jest's equivalent of asser.IfError? Let's dive into it!

javascripttypescriptjestjs

Answer №1

Alright, I've got it all figured out now.

Solution 1: Use the expect.toContain matcher.

expect([undefined, null]).toContain(value);

Solution 2: Utilize the jest.extended module with the toBeNil method.

Installation process

yarn add -D jest-extended

Whenever you need to verify a value is either null or undefined, use .toBeNil.

test('Ensure value is null or undefined', () => {
  expect(null).toBeNil();
  expect(undefined).toBeNil();
  expect(true).not.toBeNil();
});

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