The main goal of my research is the verification, specification, and testing of JavaScript applications with a particular focus on client-side Web applications that interact with the DOM API. I am primarily interested in the enforcement of security properties (such as secure information flow) and the automated verification of functional correctness properties of critical JavaScript code. To achieve these goals, I make use of program analyses and instrumentation techniques well established for static languages, advancing them to the setting of a highly complex, dynamic language, such as JavaScript. My research goes from theory to practice delivering industrial-strength tools that enable JavaScript programmers to better test, verify, and understand their code. As JavaScript is the key element of the internet of today, these tools can significantly impact the quality of current web applications, potentially improving the experience of users on a global scale.