toStrictEqual not only checks that two objects contain the same data but also that they have the same structure. It is common to expect objects to not only have identical values but also to have identical keys. A stricter equality will catch cases where two objects do not have identical keys.

This rule triggers a warning if toEqual() is used to assert equality.

The following pattern is considered warning:

expect({ a: 'a', b: undefined }).toEqual({ a: 'a' }) // true

The following pattern is not warning:

expect({ a: 'a', b: undefined }).toStrictEqual({ a: 'a' }) // false