Istanbul is a JavaScript code coverage tool and instrumentation engine that measures the execution of statements, lines, functions, and branches. It functions as a test coverage analysis tool capable of monitoring code across unit, functional, and browser tests to identify untested areas of a codebase.
The project distinguishes itself through a transparent instrumentation engine that uses module loader hooks to inject tracking code without requiring manual source modifications. It supports distributed test reporting by aggregating fragmented coverage data from multiple concurrent processes into a single unified dataset.
The tool provides comprehensive coverage analysis, including the ability to enforce minimum coverage thresholds and exclude specific code blocks or files from metrics using source comments and filtering patterns. Execution data is exported through a multi-format report generation system that produces summaries in HTML, LCOV, JSON, and console output.
