Assert the current page is, or is not, the route configured as the site's front page. Core says so itself in drupalSettings.path.isFront, whatever the theme; where a page carries no settings, the path-frontpage body class of core's themes (the "front page" named selector) and then the URL decide.
Deliberately does not accept the one-word "frontpage": webship-js's own "should be on the frontpage" already claims that phrasing (it compares URLs), and a second definition for it would make every scenario using it ambiguous.
Examples
Then I should be on the front page
And we should be on the front page
Then should be on the front page
And I should land on the front page
Then we should not be on the front page
Pattern
Then(/^(I |we )*should( not)* (?:be on|land on) the front page$/, …)
Part of @webship-js/drupal-cms-steps (github.com/webship/drupal-cms-steps), a step pack for Webship-js 2.0.x.