Uh oh, something went wrong? Use this guide to resolve issues with Jest.

Tests are Failing and You Don't Know Why

Try using the debugging support built into Node.

Place a debugger; statement in any of your tests, and then, in your project's directory, run:

node --inspect-brk node_modules/.bin/jest --runInBand [any other arguments here]
or on Windows
node --inspect-brk ./node_modules/jest/bin/jest.js --runInBand [any other arguments here]

This will run Jest in a Node process that an external debugger can connect to. Note that the process will pause until the debugger has connected to it.

To debug in Google Chrome (or any Chromium-based browser), simply open your browser and go to chrome://inspect and click on "Open Dedicated DevTools for Node", which will give you a list of available node instances you can connect to. Simply click on the address displayed in the terminal (usually something like localhost:9229) after running the above command, and you will be able to debug Jest using Chrome's DevTools.

The Chrome Developer Tools will be displayed, and a breakpoint will be set at the first line of the Jest CLI script (this is done simply to give you time to open the developer tools and to prevent Jest from executing before you have time to do so). Click the button that looks like a "play" button in the upper right hand side of the screen to continue execution. When Jest executes the test that contains the debugger statement, execution will pause and you can examine the current scope and call stack.

Note: the --runInBand cli option makes sure Jest runs test in the same process rather than spawning processes for individual tests. Normally Jest parallelizes test runs across processes but it is hard to debug many processes at the same time.

Debugging in VS Code

There are multiple ways to debug Jest tests with Visual Studio Code's built in debugger.

To attach the built-in debugger, run your tests as aforementioned:

node --inspect-brk node_modules/.bin/jest --runInBand [any other arguments here]
or on Windows
node --inspect-brk ./node_modules/jest/bin/jest.js --runInBand [any other arguments here]

Then attach VS Code's debugger using the following launch.json config:

{
  "version": "0.2.0",
  "configurations": [
    {
      "type": "node",
      "request": "attach",
      "name": "Attach",
      "port": 9229
    }
  ]
}

To automatically launch and attach to a process running your tests, use the following configuration:

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

or the following for Windows:

{
  "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"
    }
  ]
}

If you are using Facebook's create-react-app, you can debug your Jest tests with the following configuration:

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Debug CRA Tests",
      "type": "node",
      "request": "launch",
      "runtimeExecutable": "${workspaceRoot}/node_modules/.bin/react-scripts",
      "args": ["test", "--runInBand", "--no-cache", "--env=jsdom"],
      "cwd": "${workspaceRoot}",
      "protocol": "inspector",
      "console": "integratedTerminal",
      "internalConsoleOptions": "neverOpen"
    }
  ]
}

More information on Node debugging can be found here.

Debugging in WebStorm

The easiest way to debug Jest tests in WebStorm is using Jest run/debug configuration. It will launch tests and automatically attach debugger.

In the WebStorm menu Run select Edit Configurations.... Then click + and select Jest. Optionally specify the Jest configuration file, additional options, and environment variables. Save the configuration, put the breakpoints in the code, then click the green debug icon to start debugging.

If you are using Facebook's create-react-app, in the Jest run/debug configuration specify the path to the react-scripts package in the Jest package field and add --env=jsdom to the Jest options field.

Caching Issues

The transform script was changed or babel was updated and the changes aren't being recognized by Jest?

Retry with --no-cache. Jest caches transformed module files to speed up test execution. If you are using your own custom transformer, consider adding a getCacheKey function to it: getCacheKey in Relay.

Unresolved Promises

If a promise doesn't resolve at all, this error might be thrown:

- Error: Timeout - Async callback was not invoked within timeout specified by jasmine.DEFAULT_TIMEOUT_INTERVAL.`

Most commonly this is being caused by conflicting Promise implementations. Consider replacing the global promise implementation with your own, for example global.Promise = require.requireActual('promise'); and/or consolidate the used Promise libraries to a single one.

If your test is long running, you may want to consider to increase the timeout by calling jest.setTimeout

jest.setTimeout(10000); // 10 second timeout

Watchman Issues

Try running Jest with --no-watchman or set the watchman configuration option to false.

Also see watchman troubleshooting.

Tests are Extremely Slow on Docker and/or Continuous Integration (CI) server.

While Jest is most of the time extremely fast on modern multi-core computers with fast SSDs, it may be slow on certain setups as our users have discovered.

Based on the findings, one way to mitigate this issue and improve the speed by up to 50% is to run tests sequentially.

In order to do this you can run tests in the same thread using --runInBand:

# Using Jest CLI
jest --runInBand

# Using npm test (e.g. with create-react-app)
npm test -- --runInBand

Another alternative to expediting test execution time on Continuous Integration Servers such as Travis-CI is to set the max worker pool to ~4. Specifically on Travis-CI, this can reduce test execution time in half. Note: The Travis CI free plan available for open source projects only includes 2 CPU cores.

# Using Jest CLI
jest --maxWorkers=4

# Using npm test (e.g. with create-react-app)
npm test -- --maxWorkers=4

Tests are slow when leveraging automocking

Whether via automock: true in config or lots of jest.mock('my-module') calls in tests, automocking has a performance cost that can add up in large projects. The more dependencies a module has, the more work Jest has to do to mock it. Something that can offset this performance cost significantly is adding a code transformer that moves import or require calls from the top of a module, where they are always executed, down into the body of the module, where they are usually not executed. This can lower the number of modules Jest has to load when running your tests by a considerable amount.

To transform import statements, there is babel-plugin-transform-inline-imports-commonjs, and to transform require statements, there is Facebook's inline-requires babel plugin, which is part of the babel-preset-fbjs package.

I'm using npm3 and my node_modules aren't properly loading.

Upgrade jest-cli to 0.9.0 or above.

I'm using babel and my unmocked imports aren't working?

Upgrade jest-cli to 0.9.0 or above.

Explanation:

jest.dontMock('foo');

import foo from './foo';

In ES6, import statements get hoisted before all other

const foo = require('foo');
jest.dontMock('foo'); // Oops!

In Jest 0.9.0, a new API jest.unmock was introduced. Together with a plugin for babel, this will now work properly when using babel-jest:

jest.unmock('./foo'); // Use unmock!

import foo from './foo';

// foo is not mocked!

See the [Getting Started]GettingStarted.md#using-babel) guide on how to enable babel support.

I upgraded to Jest 0.9.0 and my tests are now failing?

Jest is now using Jasmine 2 by default. It should be easy to upgrade using the Jasmine upgrade guide.

If you would like to continue using Jasmine 1, set the testRunner config option to jasmine1 or pass --testRunner=jasmine1 as a command line option.

Compatibility issues

Jest takes advantage of new features added to Node 4. We recommend that you upgrade to the latest stable release of Node. The minimum supported version is v4.0.0. Versions 0.x.x are not supported.

coveragePathIgnorePatterns seems to not have any effect.

Make sure you are not using the babel-plugin-istanbul plugin. Jest wraps Istanbul, and therefore also tells Istanbul what files to instrument with coverage collection. When using babel-plugin-istanbul, every file that is processed by Babel will have coverage collection code, hence it is not being ignored by coveragePathIgnorePatterns.

Still unresolved?

See Help.