feat(examples): add Jest example

[purkhusid]

This PR adds an example of how you might create a Jest rule

Unfortunately Jest does not handle symlinks very well so we have to add some patches so that it behaves correctly(See: jasongwartz/bazel_rules_nodejs_contrib#4 (comment))

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What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
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[purkhusid]

Note that this is not working yet since I can't get the autogenerated rule to resolve babel-loader

[alexeagle]

@Globegitter I'm following the threads of your work on this.

@purkhusid if the problem is really that jest-cli doesn't have a dependency declared in its package.json on something it wants to require(), this can be solved now with an attribute of yarn_install that adds missing dependency edges to Bazel's graph:

dynamic_deps = {
        "jest-cli": "@npm//@babel/preset-env",
    },

but this doesn't seem to change anything for me. Let's skip that for now and unroll the macro one step.

nodejs_test(
      entry_point = "@npm//:node_modules/jest-cli/bin/jest.js",
      install_source_map_support = False,
      data = ["@npm//jest-cli:jest-cli", "@npm//:node_modules"] + kwargs.pop("data", []),
      **kwargs
    )

note the @npm//:node_modules there which just says that the jest binary should include all of node_modules as deps.

Even then, the failure still seems like a syntax error, not a module resolution problem:

    /usr/local/google/home/alexeagle/.cache/bazel/_bazel_alexeagle/34f8c1c4c845d6e8057f92b20b01a493/sandbox/linux-sandbox/17/execroot/examples_jest/bazel-out/k8-fastbuild/bin/test.sh.runfiles/examples_jest/index2.test.js:1
    ({"Object.<anonymous>":function(module,exports,require,__dirname,__filename,global,jest){import indexfile from './index.js';
                                                                                                    ^^^^^^^^^

    SyntaxError: Unexpected identifier

      at ScriptTransformer._transformAndBuildScript (../npm/node_modules/jest-runner/node_modules/@jest/transform/build/ScriptTransformer.js:537:17)

[purkhusid]

@alexeagle babel.config.js was missing from deps so now I'm getting the expected error:

Cannot find module '@babel/preset-env' from '/private/var/tmp/_bazel_danielpurkhus/039f5a9ccff2ae48767831e8b1a82438/sandbox/darwin-sandbox/73/execroot/examples_jest/bazel-out/darwin-fastbuild/bin/test.sh.runfiles/examples_jest'

I believe that it's acutally babel that is complaining about not finding the package so this might be more complicated than I had imagined. So we actually have a dependency chain that looks like this:

jest-cli  -> babel-jest(Loaded in the jest.config.js) -> @babel/preset-env(loaded in the babel.config.js)

The runfiles folder structure looks like this:

- examples_jest
    - <source files>
- npm
    - node_modules
        - <modules>   

Is there any reason for that we can not symlink npm/node_modules into the root of the examples_jest workspace? That way I think most tools should work since they usually traverse the folder structure upwards until they find a package.json or a node_modules directory.

[purkhusid]

@alexeagle @Globegitter If there are any ideas on how we might be able to recreate the traditional Node folder structure within the runfiles then I could take a stab at it. Bazel is still kind of new to me so I don't know all the available tools.

[alexeagle]

I fixed the node_modules layout in bc848f9 - the Jest example is passing for me now.

We should take out some of the patches and put them into the new bazel_require_script.js whose job is to patch the nodejs environment before calling the entry script.

[alexeagle]

@purkhusid could you sign the CLA so I can merge this if green?

[purkhusid]

@googlebot I signed it!

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[purkhusid]

@alexeagle You want the missing fs functions before you merge or just add them afterwards?

[alexeagle]

Cool, let's get this in, next step is for @soldair to delete the patches by making the bazel_require_script do the right thing

[marcus-sa]

Doesn't work for me.
At first I thought it'd be a flaky test, but it keeps running into module resolution issues.
It can't really be wrong configuration, since it happens in this repo as well.