JavaScript is widely used for client-side web application and increasingly for mobile applications. Unlike for C, C++ and Java, there are few tools for analysis and testing of JavaScript applications. I will present a simple yet powerful framework, called Jalangi, for writing portable heavy-weight dynamic analyses. Our implementation is applicable to real-world JavaScript programs running on multiple platforms. We have implemented concolic testing, an analysis to detect likely type inconsistencies, a memory profiler, an analysis to detect JIT-unfriendly code, and a dynamic linter in Jalangi. I will describe our experience on implementing and applying these analyses to real-world JavaScript programs and benchmarks.