How to debug unit tests in VSCode

Create or modify launch.json in VSCode by opening the Run and Debug view, selecting your environment (e.g., Python, Node.js), and configuring it for your test framework (e.g., pytest, Jest). 2. Set breakpoints in your test file, select the debug configuration, and start debugging with F5 to pause at breakpoints, inspect variables, and step through code. 3. Debug specific tests by targeting individual test functions in args for Python (e.g., "tests/test_models.py::test_create_user") or using test.only in Jest to run only the focused test. 4. Use the VSCode Testing sidebar to view and debug tests directly if your framework is detected and proper extensions (e.g., Python extension, Jest Runner) are installed, avoiding manual launch.json setup in many cases. Common tips include setting "justMyCode": false, ensuring correct test file naming (e.g., test_.py, .test.js), using --runInBand for Jest, and inserting debugger; statements in JavaScript for manual pause points, ensuring breakpoints are hit and tests are discovered properly.

Debugging unit tests in VSCode is straightforward once you set up the environment correctly. Here's how to do it effectively, especially for common languages like Python, JavaScript/TypeScript, or C#.

✅ 1. Set up a debug configuration for your tests

VSCode uses launch.json to define debugging configurations. You need to create or modify this file to run and debug your unit tests.

Steps:

• Open the Run and Debug view (Ctrl Shift D or Cmd Shift D).
• Click "create a launch.json file" if you don’t have one.
• Choose your environment (e.g., Python, Node.js, .NET).

Example: Python with unittest or pytest

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Debug Python Unit Test",
      "type": "python",
      "request": "launch",
      "program": "${workspaceFolder}/-m",
      "args": [
        "pytest",
        "tests/test_example.py::test_something",
        "-v"
      ],
      "console": "integratedTerminal",
      "justMyCode": false
    }
  ]
}

Tip: Replace pytest with unittest if using the built-in module, and adjust the path and test name accordingly.

Example: Node.js with Jest

{
  "name": "Debug Jest Test",
  "type": "node",
  "request": "launch",
  "runtimeExecutable": "npm",
  "runtimeArgs": ["run", "test:debug"],
  "port": 9229,
  "console": "integratedTerminal",
  "skipFiles": ["<node_internals>/**"]
}

You’d also need to add a script in package.json:

"scripts": {
  "test:debug": "node --inspect-brk node_modules/.bin/jest --runInBand"
}

✅ 2. Add breakpoints and start debugging

• Open the test file you want to debug.
• Click the left margin (next to line numbers) to set a breakpoint.
• Go to the Run and Debug panel.
• Select your test configuration from the dropdown.
• Click the green "Run" button (or press F5).

The test will run under the debugger and pause at your breakpoint.

You can then:

• Inspect variables in the VARIABLES pane
• Step over/into/through code (F10, F11, Shift F11)
• Use the Debug Console to evaluate expressions

✅ 3. Debug specific tests (avoid running all tests)

To save time, only debug the test you're working on.

Python (pytest):

Use the full path to the test function:

"args": [
  "tests/test_models.py::test_create_user",
  "-v"
]

JavaScript (Jest):

Use .only in your test:

test.only('should return true', () => {
  // only this test runs
});

Then start the Jest debug session.

✅ 4. Use the Testing sidebar (VSCode's built-in test explorer)

VSCode has a Testing sidebar (left panel, triangle icon) that shows all discovered tests.

• Make sure your test framework is properly detected (e.g., pytest, Jest).
• Hover over a test and click the bug icon to debug it directly.
• This skips needing to manually configure launch.json in many cases (especially for Jest or Python with proper extensions).

Ensure you have the right extensions installed:

• Python extension (for Python)
• Jest Runner or Jest Integration (for Jest)
• C# Dev Kit or similar (for .NET)

Common Issues & Tips

• Breakpoints not hit?

  • Make sure the test file is actually being executed.
  • Disable justMyCode in launch.json: "justMyCode": false
  • For Jest, use --runInBand to prevent parallelization.

• Test not discovered?

  • Check test file naming (e.g., test_*.py or *.test.js)
  • Run test discovery manually (look for a "Discover" button in Testing panel)

• Use console.log debugger
  In JS, insert debugger; statements — they’ll pause execution when debugging is active.

Basically, it comes down to: set up launch.json, pick the right runtime, target your test, and use breakpoints. Once configured, debugging unit tests in VSCode is fast and intuitive.

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