Istanbul (nyc)

Istanbul instruments ES5 and ES2015+ JavaScript code with line counters, so that it can be tracked how well unit tests exercise the codebase. nyc is its command-line client, which wraps test commands to collect coverage.

nyc works with most JavaScript testing frameworks, such as tap, mocha and AVA, and also instruments subprocesses. Coverage of statements, branches, functions and lines is shown in the terminal or in an HTML report highlighting the uncovered code, and other reporters produce formats such as lcov or Cobertura for CI tools. Source maps allow transpiled code, for example TypeScript, to be reported against the original sources. The Istanbul libraries are also used by other test runners, such as Jest, to measure coverage.

Istanbul and nyc are free and open source under the ISC licence.