Making JavaScript Better by Making It Even Slower

On mobile devices, such as smart phones and tablets, client-side JavaScript is a significant contributor to power consumption, and thus battery lifetime. We claim that this is partially due to JavaScript interpretation running faster than is necessary to maintain a satisfactory user experience, and we propose that JavaScript implementations include a user-configurable throttle. To evaluate our claim we developed a web proxy system, named JSSlow, that reduces power consumption by transcoding client-side JavaScript and injecting "sleep" invocations. This can be done safely, even given JavaScript's single-threaded nature, through the use of continuation passing, and the proxy model requires neither server nor client-side changes. Using JSSlow we studied the 120 most popular sites and found that the technique could reduce power consumption by an average of 5% on Android phones. We also considered buggy code (52% reduction) and advertising (10% reduction). To evaluate the system's impact on the user experience, we conducted a user study consisting of interactive tasks the user carried out on. The perceived performance impact varies by user and site, with the variation being highest on the most interactive sites, such as games. This argues for making the throttle user-configurable in some cases.