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Marcell Cruz
Marcell Cruz

Posted on • Updated on

How to debug failing tests in jest

TL;DR

node inspect ./node_modules/jest/bin/jest.js --t 'test description text' --runInBand
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Quick tip for debugging failing tests with jest
If you already know why would you want to run tests with the debugger you can skip the next paragraph

Why run a test with a debugger

If you run a test with the debugger you can use the debugger keyword inside your test or code to see what's happening and why the test is failing, if you don't know how to use the debugger you can read this article debugging node applicatoins

How to run a single test with the debugger

In order to debug a specific test you going to need to

  • run only the failing test
  • run the test in the same node process
  • run jest with the node debugger

So let's go over each step

Running only the failing test

To run only one test you going to need two things

  • run only one test file
  • use the keyword only to skip all other tests in that file

So let's say you have a test case with the description 'this is the falling test', to run only that test case you need to run jest with the -t option

jest -t 'this is the failing test'
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now you're running only the test case that you need to debug

// a lot of other tests above
...
test.only("this is the failing test :(", () => {
....
...
// a lot of other tests below
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Run the test in the same process

Jest run tests in parallel to make the test suit faster, but this makes debugging impossible since the test could be running in another spawn process, in order to run the test with just one process you have to use the option --runInBand

jest -t 'failling test' --runInBand
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Run jest with the node debugger

Now for the final part you just need to run jest with the debugger, since you can pass the debugger option after calling the program executable you going to need to find the executable installed in your process and run it manually with node, the good part is that it's always in the same place, assuming in are in the root folder of your project it should be in ./node_modules/jest/bin/jest.js
now the whole command

node inspect ./node_modules/jest/bin/jest.js -t 'failling test' --runInBand
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and that's about it.

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