Change babel-core to peerDependency for compatibility with Babel 7

[bradchristensen]

Summary

This makes babel-jest compatible with whichever version of Babel is used in a given project, since babel-core@6 is not necessarily compatible with plugins made for Babel 7.

Note that this is a breaking change (albeit low impact).

Fixes #4055 (babel-jest is incompatible with plugins made for Babel 7).

Test plan

In a project using [email protected] and the following Babel packages:

"babel-cli": "^7.0.0-alpha.15",
"babel-core": "^7.0.0-alpha.15",
"babel-eslint": "^8.0.0-alpha.15",
"babel-loader": "^7.1.1",
"babel-plugin-dynamic-import-node": "^1.0.2",
"babel-plugin-syntax-dynamic-import": "^7.0.0-alpha.15",
"babel-plugin-transform-class-properties": "^7.0.0-alpha.15",
"babel-plugin-transform-object-rest-spread": "^7.0.0-alpha.15",

Install babel-jest using:

npm install babel-jest --save-dev

I run tests using a .jestrc.js config file (however the extra config can be omitted for this example):

jest --config .jestrc.js

The command fails with errors such as the following (indicating that JSX has not been transformed by Babel):

I published this fork of babel-jest as an npm package and installed it in the project as follows (public so that anyone can reproduce this):

npm uninstall babel-jest --save-dev
npm install @ubiquios/babel-jest --save-dev

Then modified the jest transform configuration to use it (instead of babel-jest which is default):

.jestrc.js

module.exports = {
    ...

    transform: {
        '^.+\\.jsx?$': '@ubiquios/babel-jest',
    },
};

Then run the tests again using jest --config .jestrc.js and see that all is well:

Note that although this is technically a breaking change, in general users should already have babel-core installed with their project, so I would expect impact to be minimal.

Also note that I've run the unit tests on my own (Windows) machine and they all pass, including at least some that fail in the CI runs. From what I can see, these tests seem to have been failing in CI already prior to this commit.

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[cpojer]

Seems like this at least breaks Circle

[bradchristensen]

Ah I think I see why this is a problem now. The tests worked on my machine because I had misguidedly deleted the node_modules folder from the babel-jest package, which caused it to rely on the packages installed at the root. I assume that removing babel-core@7 as a devDependency would fix the issue for the other tests in this repository that rely on 6, since they all reference the babel-jest directory where 7 is installed, containing the devDependency that conflicts with Babel installed elsewhere. My goal with this was to at least prove that Babel 7 worked but since there are no tests specific to babel-jest, this doesn't make sense anyway 😅

In any case though, do you agree with the general idea of changing this to a peerDependency?

[cpojer]

Yeah, I think that's reasonable.

[hzoo]

We've done this in babel-loader for a while https://github.com/babel/babel-loader/blob/f0bbb68ad484b5211926dafb57f25fd418d06dc4/package.json#L18

Might need to do the same if we release beta/rc 😛

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