prefer-toBeUndefined.md

Prefer toBeUndefined

This rule recommends using toBeUndefined instead of the more generic matcher toBe(undefined).

Rule details

This rule forces a codebase to be consistent when expecting values to be undefined in unit tests.

Options

always

The "always" option (default) prefers toBeUndefined(). Select this option if you'd like developers to use a matcher that tests specifically for undefined.

Examples of incorrect code for the "always" option:

expect(value).toBe(undefined);
expect(value).toBe(undefined, 'with an explanation');

Examples of correct code for the "always" option:

expect(value).toBeUndefined();
expect(value).toBeUndefined('with an explanation');

never

The "never" option prefers toBe(undefined). Select this option if you'd like developers to use the toBe matcher consistently, whether testing for undefined or not.

Examples of incorrect code for the "never" option:

expect(value).toBeUndefined();
expect(value).toBeUndefined('with an explanation');

Examples of correct code for the "never" option:

expect(value).toBe(undefined);
expect(value).toBe(undefined, 'with an explanation');