In order to have a better failure message, toHaveLength() should be used upon asserting expections on object's length property.
This rule triggers a warning if toBe() is used to assert object's length property.
expect(files.length).toBe(1);This rule is enabled by default.
The following pattern is considered warning:
expect(files.length).toBe(1);The following pattern is not warning:
expect(files).toHaveLength(1);