How do you debug Jest Tests?

Question

I can't find any information on debugging my unit tests written with Jest.

How do you debug Jest Tests ?

Answer 1

You do not need Chrome for Jest tests.

The simplest solution I found is to use VS Code JavaScript Debug Terminal .

And it works with Typescript and Nrvl.Nx work-spaces out of the box.

1. Open the command palette and start Debug: JavaScript Debug Terminal :

2. Run tests in that terminal in a Watch mode npm test --watch .
3. Set a break-point in your file.
4. Make any change in a file you want to debug and save.
5. watch will run Jest tests against modified files.

When you want to narrow down the number of files run by the --watch, press p in the terminal and enter a pattern, which is just a part of the file name you want to test and hit [Enter]

To limit it to a single test in a file - focus it with f, so change it(...) to fit(...)

Answer 2

You can use Chrome DevTools to debug Jest tests.

First , start Node debugger in your project:

node --inspect-brk <path to jest.js> --runInBand <path to your test file>

Examples:

• If you install Jest locally (Linux example):

  node --inspect-brk ./node_modules/jest/bin/jest.js --runInBand mymodule/test.js

• If you install Jest globally (Windows example):

  node --inspect-brk "C:\\Program Files\\nodejs\\node_modules\\jest\\bin\\jest.js" --runInBand mymodule\\test.js

Then , you can open the Google Chrome browser, and type in the address bar:

chrome://inspect

Now click the inspect link under "Remote Target" to open Chrome DevTools.

Note that you probably need to add the source code folder to the workspace in chrome-devtools, so as to be able to set breakpoints.

Now you can press F8 to start debugging.

[FYI] :

• My Node version: v10.11.0
• My Jest version: 23.6.0
• My Google Chrome version: 71.0.3578.98

---

[Update] Regarding the step of adding the source code folder to the workspace in chrome-devtools (as asked by Sam), it looks like below:

And then you can open your script files and set the breakpoints:

You find the simple demo project on my GitHub repo .

Answer 3

I wrote a blog post on 7 ways to debug Jest tests in terminal . Here's the fastest way to do it from it.

You can install ndb , an improved debugging experience for Node.js, enabled by Chrome DevTools, with

npm install -g ndb

Then you can run:

ndb npm run test

which will open the DevTools for you and run the tests.

Or you can just put in one command with:

npx ndb npm run test

and you're good to go. But do check out the blog post where I go into details of different ways to debug Jest tests.

Answer 4

This is my basic config for debugging Jest in VSCode, add to your launch configurations within settings.json

"launch" : {
  "configurations": [
    {
      "type": "node",
      "request": "launch",
      "name": "Jest",
      "program": "${workspaceRoot}/node_modules/jest/bin/jest.js",
      "args": [
          "-i"
      ],
       "skipFiles": [
        "<node_internals>/**/*.js", "node_modules",
      ]
    }
  ],
},

Answer 5

As of 2021, check Debugging in VS Code from Jest docs. Add the configurations below to your .vscode/launch.json file.

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Debug Jest Tests",
      "type": "node",
      "request": "launch",
      "runtimeArgs": [
        "--inspect-brk",
        "${workspaceRoot}/node_modules/jest/bin/jest.js",
        "--runInBand"
      ],
      "console": "integratedTerminal",
      "internalConsoleOptions": "neverOpen",
      "port": 9229
    }
  ]
}

Answer 6

For anyone seeking an answer, you debug jest with node-inspector. However this is a very slow process (2-5 minutes to debug a single test), and wasn't very useful in actual usage.

I haven't found a better way to debug Jest, however I have seen a lot of people doing the extra work to make jasmine or mocha work with react components. So that may be a more viable option until node-inspector is actually usable.

Answer 7

Run

node --debug-brk node_modules/.bin/jest

in your project directory. It should start the node process paused an listening (usually at port 5858) for a debugger. Visual Studio Code is an example of a nice graphical debugger for node.js that can be used to connect to the listening node process.

Answer 8

@Yuci answer was perfect for me.

I'm just precising it in case of you're using jest inside local Docker container. You need to precise --inspect-brk=0.0.0.0:9229

node --inspect-brk=0.0.0.0:9229 <path to jest.js> --runInBand <path to your test file>

Answer 9

Adding jest configuration would work only if there is only one project. However, if there are multiple projects within its own folder (ie rootfolder/project1, rootfolder/project2), running "jest --watchAll --runInBand --coverage" with breakpoint either in the test file or the file under test would be good option

Answer 10

Here's a one-liner you can add to your projects to use debugger; in your Jest test files:

1. add this script to package.json :

  "test:debug": "open -a \"Google Chrome\" chrome://inspect && node --nolazy --inspect-brk ./node_modules/jest/bin/jest.js --runInBand --colors --verbose",

2. run the script within your project:

yarn test:debug insert/path/to/file

3. Now you can use debugger; within your test files to trigger line-of-code breakpoints in Google Chrome! 🐛 🔫

The script has two parts. First, it opens Chrome — to: chrome://inspect — and then it will start a Node process (where we can use debugger) that runs a jest file (from node modules) on a specific test file. As a result, you must have Jest installed in your project dependencies!