You can debug Karma by attaching the debugger to a Chrome instance. You'd want to set your launch.json config to something like this:

{
    "version": "0.2.0",
    "configurations": [
        {
            "type": "chrome",
            "request": "attach",
            "name": "Attach Karma Chrome",
            "address": "localhost",
            "port": 9333,
            "pathMapping": {
                "/": "${workspaceRoot}/",
                "/base/": "${workspaceRoot}/"
            }
        }
    ]
}

But you also need to adjust your karma.conf.js config, so that it launches Chrome instance with dev tools listening to 9333 port, like so:

browsers: [
  'ChromeDebugging'
],

customLaunchers: {
  ChromeDebugging: {
    base: 'Chrome',
    flags: [ '--remote-debugging-port=9333' ]
  }
},

With such setup you can just run your karma server (with captured browser), and then start debugging in visual studio.

If you'd like to find more details I made a tutorial on debugging Karma with Visual Studio Code.

Update for Angular version 9

The source files have been moved but you can still debug this way if you do the following steps

• In devtools, select the sources tab
• Press CTRL + P
• Type in the name of the file you want to debug

Valid for versions below 9

The other answers are completely valid answers but having been using Angular for around 18 months now I tend to do it in the browser - chrome tools!

Run ng test then f12 and find the spec file via the webpack context. Add a breakpoint(s) and refresh and it will hit said breakpoints. As per screenshot

I finally came across a suggestion in an article by Micha Pierzchala that was linked on https://jestjs.io/docs/testing-frameworks#angular. The article recommends IDE integrations, with the one for VS Code being vscode-jest. This is exactly what I was looking for and provides a fantastic debugging experience for your TypeScript code.

I didn't have to change anything about my jest configuration. I simply added this extension to my VS Code workspace.

Edit: I found vscode-jest to be quite buggy, thus I switched to vscode-jest-runner. With this extension, you initiate the test from a right-click-context menu. It has the same result in making the debugging experience much more pleasant for your TypeScript code.

In karma.conf.js I updated added debug option in your version.

customLaunchers: {
   Chrome_with_debugging: {
     base: 'Chrome',
     flags: ['--remote-debugging-port=9222'],
     debug: true
}}

launch.json Add below snippet as launch configuration,

{
    "name": "Debug tests",
    "type": "chrome",
    "request": "attach",
    "port": 9222,
    "sourceMaps": true,
    "webRoot": "${workspaceRoot}"
}

Then triggered the tests using below command,

ng test --browsers Chrome_with_debugging

Use Visual Studio Code debug option "Debug tests" to get attached to UT. With this I am able to debug unit tests using breakpoints in "Visual Studio Code + Debugger for Chrome extension".

Tested with Angular 5 / CLI 1.5.5

1. Install the Chrome Debugger Extension
2. Create the launch.json (inside .vscode folder)
3. Use my launch.json (see below)
4. Create the tasks.json (inside .vscode folder)
5. Use my tasks.json (see below)
6. Press CTRL+SHIFT+B
7. Press F5

launch.json for angular/cli >= 1.3

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Launch Chrome",
      "type": "chrome",
      "request": "launch",
      "url": "http://localhost:4200/#",
      "webRoot": "${workspaceFolder}"
    },
    {
      "name": "Attach Chrome",
      "type": "chrome",
      "request": "attach",
      "url": "http://localhost:4200/#",
      "webRoot": "${workspaceFolder}"
    },
    {
      "name": "Launch Chrome (Test)",
      "type": "chrome",
      "request": "launch",
      "url": "http://localhost:9876/debug.html",
      "webRoot": "${workspaceFolder}"
    },
    {
      "name": "Launch Chrome (E2E)",
      "type": "node",
      "request": "launch",
      "program": "${workspaceFolder}/node_modules/protractor/bin/protractor",
      "protocol": "inspector",
      "args": ["${workspaceFolder}/protractor.conf.js"]
    }
  ]
}

tasks.json for angular/cli >= 1.3

{
    "version": "2.0.0",
    "tasks": [
      {
        "identifier": "ng serve",
        "type": "npm",
        "script": "start",
        "problemMatcher": [],
        "group": {
          "kind": "build",
          "isDefault": true
        }
      },
      {
        "identifier": "ng test",
        "type": "npm",
        "script": "test",
        "problemMatcher": [],
        "group": {
          "kind": "test",
          "isDefault": true
        }
      }
    ]
  }

Tested with Angular 2.4.8

1. Install the Chrome Debugger Extension
2. Create the launch.json
3. Use my launch.json (see below)
4. Start the NG Live Development Server (ng serve)
5. Press F5

launch.json for angular/cli >= 1.0.0-beta.32

{
  "version": "0.2.0",
  "configurations": [
    {
      "type": "chrome",
      "request": "launch",
      "name": "Launch Chrome",
      "url": "http://localhost:4200",
      "webRoot": "${workspaceFolder}",
      "sourceMaps": true,
      "userDataDir": "${workspaceFolder}/.vscode/chrome",
      "runtimeArgs": [
        "--disable-session-crashed-bubble"
      ]
    },
    {
      "name": "Attach Chrome",
      "type": "chrome",
      "request": "attach",
      "url": "http://localhost:4200",
      "port": 9222,
      "webRoot": "${workspaceFolder}",
      "sourceMaps": true
    }
  ]
}

launch.json for angular/cli < 1.0.0-beta.32

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Lunch Chrome",
      "type": "chrome",
      "request": "launch",
      "url": "http://localhost:4200",
      "webRoot": "${workspaceFolder}/src/app",
      "sourceMaps": true,
      "sourceMapPathOverrides": {
        "webpack:///./~/*": "${workspaceFolder}/node_modules/*",
        "webpack:///./src/*": "${workspaceFolder}/src/*"
      },
      "userDataDir": "${workspaceFolder}/.vscode/chrome",
      "runtimeArgs": [
        "--disable-session-crashed-bubble"
      ]
    },
    {
      "name": "Attach Chrome",
      "type": "chrome",
      "request": "attach",
      "url": "http://localhost:4200",
      "port": 9222,
      "webRoot": "${workspaceFolder}/src/app",
      "sourceMaps": true,
      "sourceMapPathOverrides": {
        "webpack:///./~/*": "${workspaceFolder}/node_modules/*",
        "webpack:///./src/*": "${workspaceFolder}/src/*"
      }
    }
  ]
}