Locality of Behaviour (LoB) — Summary

Carson Gross formulates Locality of Behaviour as a prescriptive software design principle: the behaviour of a unit of code should be as obvious as possible by looking only at that unit of code. It operationalizes Richard Gabriel's observation that "the primary feature for easy maintenance is locality."

The essay contrasts two AJAX implementations: an htmx button (<button hx-get="/clicked">Click Me</button>) where behaviour is visible on the element itself, versus a jQuery approach where behaviour is spread across separate files with a selector ($("#d1").on("click", ...)) — what Gross calls "spooky action at a distance."

A critical distinction runs through the argument: surfacing behaviour is not inlining implementation. Declaring that a button issues a GET request is different from inlining how that GET request is executed — just as calling a function is different from copy-pasting its body. Framework developers carry extra responsibility to make LoB "as easy and as conceptually clean as possible."

LoB inevitably conflicts with other principles, especially DRY (Don't Repeat Yourself) and Separation of Concerns. Gross treats these as subjective tradeoffs rather than absolutes: behaviour moved to a parent element a few lines away is a mild LoB violation; behaviour in a separate file is a severe one. The rise of inline styles in recent years is offered as evidence that Separation of Concerns is already losing ground.

The conclusion frames LoB as a subjective principle that must be balanced against other concerns and system constraints, but one that — pursued as far as practical — increases "software maintainability, quality and sustainability."