Uh oh, something went wrong? Use this guide to resolve issues with Jest.
Try using the debugging support built into Node.
debugger; statement in any of your tests, and then, in your project's directory, run:
node --debug-brk ./node_modules/.bin/jest --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.
For example, to connect the Node Inspector debugger to the paused process, you would first install it (if you don't have it installed already):
npm install --global node-inspector
Then simply run it:
This will output a link that you can open in Chrome. After opening that link, 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.
--runInBandcli 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.
More information on Node debugging can be found here.
The transform script was changed or babel was updated and the changes aren't being recognized by Jest?
--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.
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 specified in
jasmine.DEFAULT_TIMEOUT_INTERVAL = 10000; // 10 second timeout
Try running Jest with
--no-watchman or set the
watchman configuration option to
Also see watchman troubleshooting.
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
# Using Jest CLI jest --runInBand # Using npm test (e.g. with create-react-app) npm test -- --runInBand
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
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.
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
0.9.0 or above.
0.9.0 or above.
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
jest.unmock('foo'); // Use unmock! import foo from './foo'; // foo is not mocked!
See the Getting Started guide on how to enable babel support.
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.
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
0.x.x are not supported.
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Licensed under the BSD License.