Abstract: JavaScript libraries are widely used and evolve rapidly. When adapting client code to non-backwards compatible changes in libraries, a major challenge is how to locate affected API uses in client code, which is currently a difficult manual task. In this paper we address this challenge by introducing a simple pattern language for expressing API access points and a pattern-matching tool based on lightweight static analysis. Experimental evaluation on 15 popular npm packages shows that typical breaking changes are easy to express as patterns. Running the static analysis on 265 clients of these packages shows that it is accurate and efficient: it reveals usages of breaking APIs with only 14