Promises and async/await have become popular mechanisms for implementing asynchronous computations in JavaScript, but despite their popularity, programmers have difficulty using them. This paper identifies 8 anti-patterns in promise-based JavaScript code that are prevalent across popular JavaScript repositories. We present a light-weight static analysis for automatically detecting these anti-patterns. This analysis is embedded in an interactive visualization tool that additionally relies on dynamic analysis to visualize promise lifetimes and instances of anti-patterns executed at run time. By enabling the user to navigate between promises in the visualization and the source code fragments that they originate from, problems and optimization opportunities can be identified.

We implement this approach in a tool called DrAsync, and found 2.5k static instances of anti-patterns in 20 popular JavaScript repositories. Upon examination of a subset of these, we found that the majority of problematic code reported by DrAsync could be eliminated through refactoring. Further investigation revealed that, in a few cases, the elimination of anti-patterns reduced the time needed to execute the refactored code fragments. Moreover, DrAsync’s visualization of promise lifetimes and relationships provided additional insight into the execution behavior of asynchronous programs and helped identify further optimization opportunities.