Trying to run tests with Jest, I got the following error: Requires Babel "^7.0.0-0", but was loaded with "6.26.3"...

Looking at the stack trace (in full at the end of this post), however, there were only mentions of @babel/core and babel-core .

That though, turns out to have been the problem and it’s a known problem.

The Jest Team actually addresses this specific problem in their Getting Started documentation for using Babel .

In a big yellow attention box, they write:

Note: If you are using Babel version 7 then you need to install babel-jest, babel-core@^7.0.0-bridge.0 and @babel/core with the following command:

yarn add --dev babel-jest babel-core@^7.0.0-bridge.0 @babel/core regenerator-runtime

You will need to use babel.config.js in order to transpile node_modules. See https://babeljs.io/docs/en/next/config-files for more information.

You can also see the example in the Jest repository: https://github.com/facebook/jest/tree/master/examples/babel-7

Another reason to start with the official docs. They often hold the answers!

Why The Error Happens

Okay, so the Jest team is aware of the problem, but what is actually happening and what is babel-core@^7.0.0-bridge.0 ?

When the babel team upgraded to v7 , they changed the syntax for their updates from babel-core ( v6 ) to @babel/core ( v7 ).

This meant that every package that used babel as a peer dependency would be forced to make a breaking change to update along with Babel.

The Babel team created the problem with their name change. They also created a fix. That’s what babel-core@^7.0.0-bridge.0 does.

Instead of forcing all package maintainers that used Babel as a peer dependency to update their package, the bridge allows them to keep using Babel without pushing a breaking change.

Stack Trace

FAIL  jestTests/app.test.js
  ● Test suite failed to run
    Requires Babel "^7.0.0-0", but was loaded with "6.26.3". If you are sure you have a compatible version of @ba
bel/core, it is likely that something in your build process is loading the wrong version. Inspect the stack trace
 of this error to look for the first entry that doesn't mention "@babel/core" or "babel-core" to see what is call
ing Babel. (While processing preset: "/Users/Stephen/Documents/_coding/matilda/node_modules/@babel/preset-env/lib
/index.js")
 at throwVersionError (node_modules/@babel/helper-plugin-utils/lib/index.js:65:11)
      at Object.assertVersion (node_modules/@babel/helper-plugin-utils/lib/index.js:13:11)
      at _default (node_modules/@babel/preset-env/lib/index.js:154:7)
      at node_modules/@babel/helper-plugin-utils/lib/index.js:19:12
      at node_modules/babel-core/lib/transformation/file/options/option-manager.js:317:46
          at Array.map (<anonymous>)
      at OptionManager.resolvePresets (node_modules/babel-core/lib/transformation/file/options/option-manager.js:275:20)