Development nature matters: An empirical study of code clones in JavaScript applications

Code cloning is one of the active research areas in the software engineering community. Specifically, researchers have conducted numerous empirical studies on code cloning and reported that 7 % to 23 % of the code in a typical software system has been cloned. However, there was less awareness of code clones in dynamically-typed languages and most studies are limited to statically-typed languages such as Java, C, and C++. In addition, most previous studies did not consider different application domains such as standalone projects or web applications. As a result, very little is known about clones in dynamically-typed languages, such as JavaScript, in different application domains. In this paper, we report a large-scale clone detection experiment in a dynamically-typed programming language, JavaScript, for different application domains: web pages and standalone projects. Our experimental results showed that unlike JavaScript standalone projects, JavaScript web applications have 95 % of inter-file clones and 91–97 % of widely scattered clones. We observed that web application developers created clones intentionally and such clones may not be as risky as claimed in previous studies. Understanding the risks of cloning in web applications requires further studies, as cloning may be due to either good or bad intentions. Also, we identified unique development practices such as including browser-dependent or device-specific code in code clones of JavaScript web applications. This indicates that features of programming languages and technologies affect how developers duplicate code.