Programming language tools help software developers to understand a program and to recognize possible pitfalls. Used with the right knowledge, they can be instrumented to achieve better software quality. However, creating language tools that integrate well into the development environment and workflow is challenging. This thesis utilizes a user-centered design process to identify the needs of professional developers through in-depth interviews, address those needs through a concept, and finally implement and evaluate the concept. Taking 'scope' as an exemplary source of misconceptions in programming, a “Scope Inspector” plug-in for the Atom IDE—targeting experienced JavaScript developers in the open source community—is implemented.