Developers building web applications constantly face challenges, particularly in working with complex APIs. In response, developers often turn to Stack Overflow, offering a window into the programming barriers developers face. We examined 301 posts on Stack Overflow related to front-end web development and systematically characterized the challenges present in these posts. We found that most challenges reflected not a request for new code or an explanation of an error message but a request about how a specific code snippet might be edited to make its behavior as desired. Many challenges also reflected an underlying need to gather information about how specific code idioms are implemented within a framework or library. We identified 28 barriers developers face in front-end web development. Our findings suggest opportunities for facilitating more effective interactions with complex APIs through new types of programming content and tools that better address barriers in working with code idioms.