Bugs in SCRATCH programs can spoil the fun and inhibit learning success. Many common bugs are the result of recurring patterns of bad code. In this paper we present a collection of common code patterns that typically hint at bugs in SCRATCH programs, and the LitterBox tool which can automatically detect them. We empirically evaluate how frequently these patterns occur, and how severe their consequences usually are. While fixing bugs inevitably is part of learning, the possibility to identify the bugs automatically provides the potential to support learners.

Block-based programming languages like Scratch support learners by providing high-level constructs that hide details and by preventing syntactically incorrect programs. Questions nevertheless frequently arise: Is this program satisfying the given task? Why is my program not working? To support learners and educators, automated program analysis is needed for answering such questions. While adapting existing analyses to process blocks instead of textual statements is straightforward, the domain of programs controlled by block-based languages like Scratch is very different from traditional programs: In Scratch multiple actors, represented as highly concurrent programs, interact on a graphical stage, controlled by user inputs, and while the block-based program statements look playful, they hide complex mathematical operations that determine visual aspects and movement. Analyzing such programs is further hampered by the absence of clearly defined semantics, often resulting from ad-hoc decisions made by the implementers of the programming environment. To enable program analysis, we define the semantics of Scratch using an intermediate language. Based on this intermediate language, we implement the Bastet program analysis framework for Scratch programs, using concepts from abstract interpretation and software model checking. Like Scratch, Bastet is based on Web technologies, written in TypeScript, and can be executed using NodeJS or even directly in a browser. Evaluation on 279 programs written by children suggests that Bastet offers a practical solution for analysis of Scratch programs, thus enabling applications such as automated hint generation, automated evaluation of learner progress, or automated grading.