Causal inference of server- and client-side code smells in web apps evolution

Abstract Context Code smells (CS) are symptoms of poor design and implementation choices that may lead to increased defect incidence, decreased code comprehension, and longer times to release. Web applications and systems are seldom studied, probably due to the heterogeneity of platforms (server and client-side) and languages, and to study web code smells, we need to consider CS covering that diversity. Furthermore, the literature provides little evidence for the claim that CS are a symptom of poor design, leading to future problems in web apps. Objective To study the quantitative evolution and inner relationship of CS in web apps on the server- and client-sides, and their impact on maintainability and app time-to-release (TTR). Method We collected and analyzed 18 server-side, and 12 client-side code smells, aka web smells, from consecutive official releases of 12 PHP typical web apps, i.e., with server- and client-code in the same code base, summing 811 releases. Additionally, we collected metrics, maintenance issues, reported bugs, and release dates. We used several methodologies to devise causality relationships among the considered irregular time series, such as Granger-causality and Information Transfer Entropy(TE) with CS from previous one to four releases (lag 1 to 4). Results The CS typically evolve the same way inside their group and its possible to analyze them as groups. The CS group trends are: Server, slowly decreasing; Client-side embed, decreasing and JavaScript,increasing. Studying the relationship between CS groups we found that the "lack of code quality", measured with CS density proxies, propagates from client code to server code and JavaScript in half of the applications. We found causality relationships between CS and issues. We also found causality from CS groups to bugs in Lag 1, decreasing in the subsequent lags. The values are 15% (lag1), 10% (lag2), and then decrease. The group of client-side embed CS still impacts up to 3 releases before. In group analysis, server-side CS and JavaScript contribute more to bugs. There are causality relationships from individual CS to TTR on lag 1, decreasing on lag 2, and from all CS groups to TTR in lag1, decreasing in the other lags, except for client CS. Conclusions There is statistical inference between CS groups. There is also evidence of statistical inference from the CS to web applications’ issues, bugs, and TTR. Client and server-side CS contribute globally to the quality of web applications, this contribution is low, but significant. Depending on the outcome variable (issues, bugs, time-to-release), the contribution quantity from CS is between 10% and 20%.