Even though a lot of effort has been invested in analyzing client-side web applications during the past decade, the existing tools often fail to deal with the complexity of modern JavaScript applications. However, from an attacker point of view, the client side of such web applications can reveal invaluable information about the server side. In this paper, first we study the existing tools and enumerate the most crucial features a security-aware client-side analysis should be supporting. Next, we propose GELATO to detect vulnerabilities in modern client-side JavaScript applications that are built upon complex libraries and frameworks. In particular, we take the first step in closing the gap between state-aware crawling and client-side security analysis by proposing a feedback-driven security-aware guided crawler that is able to analyze complex frameworks automatically, and increase the coverage of security-sensitive parts of the program efficiently. Moreover, we propose a new lightweight client-side taint analysis that outperforms the start-of-the-art tools, requires no modification to browsers, and reports non-trivial taint flows on modern JavaScript applications.