Lately I’ve been using a lot Jest as a unit testing runner. It’s great because it allows me to do:

  • snapshot testing which makes updating unit tests a breeze
  • runs tests in parallel so it’s super fast
  • has a nice command line app allowing me to filter the tests i want to run
  • built-in code coverage reports

In general i can manage debugging failing tests using just the built in test report in Jest and occasionaly sprinkle some console.log statements for the expressions I want to inspect.

But there are situations in which a full fledged debugging environment is needed in order to figure out what happens with my Jest tests.

If you are a fan of VSCode , like I’m, you can set it up very simple:

  • Open up launch.json file ( CTRL + SHIFT+P -> type launch.json )
  • Create a new configuration
"configurations": [
        {
            "name": "Jest", // This is the configuration name you will see in debug sidebar
            "type": "node",
            "request": "launch",
            "port": 5858,
            "address": "localhost",
            "stopOnEntry": false,
            "runtimeExecutable": null,
            "env": {
                "NODE_ENV": "production" // You can setup here any env vars you 
            },
            "runtimeArgs": [
                "--debug-brk",
                "./node_modules/.bin/jest", // Path to Jest
                "-i"
            ],
            "cwd": "${workspaceRoot}"
        }
    ]

Please not that this config will work if you’re using Node 6.x.

For Node 7.x , 8.x you will need to use --inspect-brk instead of debug-brk .

Also , if you transpile your unit tests, eg using Babel or TypeScript you will need to ensure VSCode finds the source maps for the transformation, otherwise it will not react to breakpoints.

You can do that by defining "sourceMaps": true to the above configuration.

Even if it doesn’t, and you can’t setup source maps, you can still use a debugger statement in code to trigger the breakpoint you want and then step throught as usual.

To run the debugger just hit F5 . Enjoy!