Characterizing refactoring graphs in Java and JavaScript projects

Refactoring is an essential activity during software evolution. Frequently, practitioners rely on such transformations to improve source code maintainability and quality. As a consequence, this process may produce new source code entities or change the structure of existing ones. Sometimes, the transformations are atomic, i.e., performed in a single commit. In other cases, they generate sequences of modifications performed over time. To study and reason about refactorings over time, we rely on refactoring graphs. Using this abstraction, we provide quantitative and qualitative investigation on 20 popular open-source Java and JavaScript-based projects. After eliminating trivial graphs, we characterize a large sample of 1,525 refactoring graphs, providing quantitative data on their size, commits, age, refactoring composition, ownership, operations over time, and refactoring graph patterns. Besides, we contact the authors of subgraphs describing large refactoring operations to understand the reasons behind their operations. We conclude by discussing applications and implications of refactoring graphs, for example, to improve code comprehension, detect refactoring patterns, and support software evolution studies.