What's the right way to enable the node debugger with mocha's --debug-brk switch?

Question

I have some debugger statements in my module under test and want to run mocha with --debug-brk set and hit my breakpoint so that I can inspect the state of my module. Unfortunately, whenever I run mocha with this option, I end up with a blank cursor on the next line. I can enter text, but there's nothing that appears to be processing my commands (it certainly doesn't look like the node debugger):

$ mocha --debug-brk tests.js -R spec debugger listening on port 5858 [BLANK CURSOR] 

Am I doing something wrong with how I'm launching mocha?

Answer 1

Using a recent version of nodejs (>=v6.3.0) and mocha (>=3.1.0), you can use V8 inspector integration.

V8 Inspector integration allows attaching Chrome DevTools to Node.js instances for debugging and profiling

Usage

--inspect activates V8 inspector integration, and --debug-brk adds a breakpoint at the beginning. Since nodejs v7.6.0 and mocha v3.3.0, you can use the --inspect-brk shorthand for --inspect --debug-brk

$ mocha --debug-brk --inspect Debugger listening on port 9229. Warning: This is an experimental feature and could change at any time. To start debugging, open the following URL in Chrome: chrome-devtools://devtools/remote/serve_file/@62cd277117e6f8ec53e31b1be58290a6f7ab42ef/inspector.html?experiments=true&v8only=true&ws=localhost:9229/node 

With npm scripts

If you have a npm test script that uses mocha, you can pass the options from npm to your mocha script by using the end of option delimiter --:

$ npm test -- --inspect --debug-brk

Chrome tip

Instead of copy-pasting the url each time, go to chrome://inspect and click the appropriate link in the "Remote target" section.