In today’s web applications asynchronous requests to remote services using callbacks or futures are omnipresent. The continuation of such a non-blocking task is represented as a callback function that will later be called with the result of the request. This style of programming where the remainder of a computation is captured in a continuation function is called continuation-passing style (CPS). This style of programming can quickly lead to a phenomenon called “call- back hell”, which has a negative impact on the maintain- ability of applications that employ this style. Several alter- natives to callbacks are therefore gaining traction within the web domain. For example, there are a number of frameworks that rely on automatically transforming sequential style code into the continuation-passing style. However, these frame- works often employ a conservative approach in which each function call is transformed into CPS. This conservative approach can sequentialise requests that could otherwise be run in parallel. So-called delimited continuations can remedy, but require special marks that have to be manually inserted in the code for marking the beginning and end of the continuation. In this paper we propose an alternative strategy in which we apply a delimited CPS transformation that operates on a Program Dependence Graph instead to find the limits of each continuation.We implement this strategy in JavaScript and demonstrate its applicability to various web programming scenarios.

Whereas "responsive" web applications already offered a more desktop-like experience, there is an increasing user demand for "rich" web applications (RIAs) that offer collaborative and even off-line functionality. Realizing these qualities requires distributing previously centralized application logic and state vertically from the server to a client tier (e.g., for desktop-like and off-line client functionality), and horizontally between instances of the same tier (e.g., for collaborative client functionality and for scaling of resource-starved services). Both bring about the essential complexity of distributing application assets and maintaining their consistency, along with the accidental complexity of reconciling a myriad of heterogenous tier-specific technology. Tierless programming languages enable developing web applications as a single artefact that is automatically split in tier-specific code - resulting in a development process akin to that of a desktop application. This relieves developers of distribution and consistency concerns, as well as the need to align different tier-specific technologies. However, programmers still have to adopt a new and perhaps esoteric language. We therefore advocate developing tierless programs in a general-purpose language instead. In this poster, we introduce our approach to tierless programming in JavaScript. We expand upon our previous work by identifying development challenges arising from this approach that could be resolved through tool support.