  Web apps are built by using a combination of HTML, CSS, and JavaScript. While
building modern web apps, it is common practice to make use of third-party
libraries and frameworks, as to improve developers' productivity and code
quality. Alongside these benefits, the adoption of such libraries results in
the introduction of JavaScript dead code, i.e., code implementing unused
functionalities. The costs for downloading and parsing dead code can negatively
contribute to the loading time and resource usage of web apps. The goal of our
study is two-fold. First, we present Lacuna, an approach for automatically
detecting and eliminating JavaScript dead code from web apps. The proposed
approach supports both static and dynamic analyses, it is extensible and can be
applied to any JavaScript code base, without imposing constraints on the coding
style or on the use of specific JavaScript constructs. Secondly, by leveraging
Lacuna we conduct an experiment to empirically evaluate the run-time overhead
of JavaScript dead code in terms of energy consumption, performance, network
usage, and resource usage in the context of mobile web apps. We applied Lacuna
four times on 30 mobile web apps independently developed by third-party
developers, each time eliminating dead code according to a different
optimization level provided by Lacuna. Afterward, each different version of the
web app is executed on an Android device, while collecting measures to assess
the potential run-time overhead caused by dead code. Experimental results,
among others, highlight that the removal of JavaScript dead code has a positive
impact on the loading time of mobile web apps, while significantly reducing the
number of bytes transferred over the network.
