TouchDevelop represents a radically new programming environment that enables users to develop mobile applications directly on mobile devices. TouchDevelop has successfully drawn a huge number of end users, who have published thousands of TouchDevelop scripts online. To enhance end-user programming on mobile devices, we conduct the first comprehensive field study of 17322 TouchDevelop scripts and 4275 users. Our study consists of an overall study on the characteristics of scripts (e.g., structural features, code reuse) and users (e.g., expertise), and a longitudinal study on how they evolve over time. Our study results show important characteristics of scripts such as dense external method calls, high code reuse ratio, and also reveal interesting evolution patterns of users. The findings and implications in our study provide valuable guidelines for improving tool support or services for end users and increasing the popularity of end-user programming on mobile devices.