Undefined createTransformer function in jest-preprocess.js when following Gatsby unit testing instructions

[amcgregor101]

Description

When following the Gatsby unit testing instructions, all tests fail to run because the createTransformer function in the object returned by require("babel-jest") is undefined.

Steps to reproduce

gatsby new my-gatsby-project https://github.com/gatsbyjs/gatsby-starter-default
cd my-gatsby-project/
npm install --save-dev jest babel-jest react-test-renderer babel-preset-gatsby identity-obj-proxy
[copy various files listed in unit testing instructions]
[add a basic test case to src/__tests__/header.js (see below)]
npm test

I used the following test case in my reproduction:

test("test", () => {
	expect(true).toBe(true)
})

Link to a reproduction: https://github.com/amcgregor101/gatsby-testing-bug

Expected result

The test passes.

Actual result

I get the following error:

TypeError: require(...).createTransformer is not a function
    at Object.<anonymous> (/home/***/my-gatsby-project/jest-preprocess.js:4:40)
    at Module._compile (internal/modules/cjs/loader.js:956:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:973:10)
    at Module.load (internal/modules/cjs/loader.js:812:32)
    at Function.Module._load (internal/modules/cjs/loader.js:724:14)
    at Module.require (internal/modules/cjs/loader.js:849:19)
    at require (internal/modules/cjs/helpers.js:74:18)
    at requireOrImportModule (/home/***/my-gatsby-project/node_modules/jest-util/build/requireOrImportModule.js:51:28)
    at /home/***/my-gatsby-project/node_modules/@jest/transform/build/ScriptTransformer.js:381:73
    at Array.map (<anonymous>)

Environment

$ gatsby info --clipboard

  System:
    OS: Linux 4.19 Debian GNU/Linux 10 (buster) 10 (buster)
    CPU: (8) x64 Intel(R) Core(TM) i7-7700 CPU @ 3.60GHz
    Shell: 5.0.3 - /bin/bash
  Binaries:
    Node: 12.13.0 - /usr/local/bin/node
    Yarn: 1.22.10 - ~/.npm-global/bin/yarn
    npm: 7.14.0 - ~/.npm-global/bin/npm
  Languages:
    Python: 2.7.16 - /usr/bin/python
  Browsers:
    Chrome: 91.0.4472.77
    Firefox: 88.0.1
  npmPackages:
    gatsby: ^3.4.1 => 3.4.1
    gatsby-plugin-gatsby-cloud: ^2.4.1 => 2.4.1
    gatsby-plugin-image: ^1.4.0 => 1.4.0
    gatsby-plugin-manifest: ^3.4.0 => 3.4.0
    gatsby-plugin-offline: ^4.4.0 => 4.4.0
    gatsby-plugin-react-helmet: ^4.4.0 => 4.4.0
    gatsby-plugin-sharp: ^3.4.1 => 3.4.1
    gatsby-source-filesystem: ^3.4.0 => 3.4.0
    gatsby-transformer-sharp: ^3.4.0 => 3.4.0
  npmGlobalPackages:
    gatsby-cli: 3.6.0

[LekoArts]

The documentation was written with jest/babel-jest v26 in mind, and just three days ago jest/babel-jest v27 was released. I'm pretty sure that's the reason why it's not working anymore as in repositories of mine with this exact setup (but v26) it still works.

If anyone can comb through the breaking changes https://github.com/facebook/jest/blob/master/CHANGELOG.md#2700 and see what needs to be changed: Would be highly appreciated!

[sj-rai]

@LekoArts I went through the changelog for jest v.27.0.0 and found that one of the changes included the function to be exported as default, so the code snippet in this documentation would throw an error.

I have created a PR to update the docs, #31649 . But since createTransformer is used in other places with the project, they may need to be updated if Jest is updated to v27.0.0 or above.