I've made .route chainable, as you suggest ... but making .navigate chainable doesn't make any sense. There's absolutely no reason why you'd ever go to one route, and then immediately to another without the user taking any action.

@jashkenas I have chained other calls onto .navigate calls before. Obviously not subsequent .navigate calls, but to other local methods that I wanted to run after triggering a route, such as updating an auxiliary history tracker (part of an undo stack) and clearing a cache of some description. These are only the two that come to mind, but even if it's only vaguely useful, where's the harm in including it?

Because it's a bizarre return value for a side-effect-ful function after which there's no reason to chain. If you're already extending your Router with extra functions that you do want to chain, feel free to override navigate and make it chainable at the same time.