As developers debug, they often have to seek the origins of wrong values they see in their debugger. This search must be performed backwards in time since the code causing the wrong value is executed before the wrong value appears. Therefore, locating the origin of wrong values with breakpoint-or log-based debuggers demands persistence and significant experience.Querypoint, is a Firefox plugin that enhances the popular Firebug JavaScript debugger with a new, practical feature called lastChange. lastChange automatically locates the last point at which a variable or an object property has been changed. Starting from a program suspended on a breakpoint, the lastChange algorithm applies queries to the live program during re-execution, recording the call stack and limited program state each time the property value changes. When the program halts again on the breakpoint, it shows the call stack and program state at the last change point. To evaluate the usability and effectiveness of Querypoint we studied four experienced JavaScript developers applying the tool to two test cases.

JavaScript developers create programs by calling functions and they use functions to construct objects. JavaScript development tools need to report to developers about those functions and constructors, for example in debugger callstacks and in object representations. However, most functions are anonymous: developers need not to specify names for functions. Based on our analysis of ten large, widely used JavaScript projects, less than 7% of JavaScript functions are named by developers. After studying examples from these JavaScript projects, we propose Static FunctionObject Consumption, a principled, automated approach based on local source code analysis for providing names to nameless JavaScript functions. We applied our approach to 90000 anonymous functions that appeared in the analyzed JavaScript projects. The approach is successful in naming more than 99% (91% are unique within their file) of anonymous functions while the average length of function names is kept less than 37 characters.

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