Error: expected file.path to export a function or instance - babeljs

Question

I creating a generator plugin named generate-swap-project for the generate generator tool.
Let's first see:
https://github.com/generate: A new command line tool and developer framework for scaffolding out GitHub projects. A more powerful, composable, functional alternative to Yeoman.
https://github.com/generate/generate: A new command line tool and developer framework for scaffolding out GitHub projects. Generate offers the robustness and configurability of Yeoman, the expressiveness and simplicity of Slush, and more powerful flow control and composability than either.
https://github.com/rbecheras/generate-swap-project A generator for SWAP projects (the generator I'm working on)
https://github.com/rbecheras/generate-swap-project/tree/1-convert-to-standard-code-style: A generator for SWAP projects (the readme on master branch)
https://github.com/rbecheras/generate-swap-project/tree/1-convert-to-standard-code-style: (the readme on problematic branch)
The master branch works fine and tests pass on travis AND on local dev host.
Tests are run by the following yarn script (on master):
$ yarn test
However in the PR #2 Resolve #1 "Convert to ESNext/StandardJS code style", I made the following changes:
converted ES5 codebase to ESNext
moved codebase to src/lib and src/tests
added a generator.js file a t root level that just requires ./dist/lib/generator.js
added yarn scripts: clear + lint + build + test (cf. Readme section about the build pipeline)
Ref. to the branch 1-convert-to-standard-code-style
Tests are run by the following yarn script (on branch 1-convert-to-standard-code-style):
$ yarn travis
or (its alias)
$ yarn pipeline
that is task sequence for yarn clear && yarn lint && yarn build && yarn test
But one test fails (1 test fails for 14 test pass) with a strange error: Error: expected file.path to export a function or instance:
$ yarn pipeline
yarn run v1.3.2
$ yarn clear && yarn lint && yarn build && yarn test
$ yarn rimraf build dist
$ /home/remi/d/dev/generate-swap-project/node_modules/.bin/rimraf build dist
$ yarn lint:esnext
$ yarn standard src/*.js src/**/*.js
$ /home/remi/d/dev/generate-swap-project/node_modules/.bin/standard 'src/*.js' src/lib/generator.js src/tests/plugin.js src/tests/test.js
$ yarn build:lib && yarn build:tests && yarn build:docs
$ yarn babel src/lib/ --out-dir dist/lib --source-maps
$ /home/remi/d/dev/generate-swap-project/node_modules/.bin/babel src/lib/ --out-dir dist/lib --source-maps
src/lib/generator.js -> dist/lib/generator.js
$ yarn babel src/tests/ --out-dir dist/tests --source-maps
$ /home/remi/d/dev/generate-swap-project/node_modules/.bin/babel src/tests/ --out-dir dist/tests --source-maps
src/tests/plugin.js -> dist/tests/plugin.js
src/tests/test.js -> dist/tests/test.js
$ echo WARNING: No documentation build available yet
WARNING: No documentation build available yet
$ yarn mocha dist/tests/**/*.js
$ /home/remi/d/dev/generate-swap-project/node_modules/.bin/mocha 'dist/tests/**/*.js'
generate-swap-project
plugin
✓ should add tasks to the instance (80ms)
✓ should only register the plugin once
generate-swap-project
tasks
✓ should extend tasks onto the instance
✓ should run the `default` task with .build (186ms)
✓ should run the `default` task with .generate
swap-project (CLI)
X => 1) should run the default task using the `generate-swap-project` name (global install)
✓ should run the default task using the `swap-project` generator alias (local generator.js)
swap-project (API)
✓ should run the default task on the generator
✓ should run the `swap-project` task
✓ should run the `default` task when defined explicitly
sub-generator
✓ should work as a sub-generator
✓ should run the `default` task by default
✓ should run the `generator:default` task when defined explicitly
✓ should run the `generator:swap-project` task
✓ should work with nested sub-generators
✓ should run tasks as a sub-generator
15 passing (1s)
1 failing
1) generate-swap-project
swap-project (CLI)
should run the default task using the `generate-swap-project` name (global install):
Error: expected file.path to export a function or instance
at Env.get (node_modules/base-env/lib/file.js:256:11)
at Env.invoke (node_modules/base-env/lib/env.js:117:19)
at Generate.<anonymous> (node_modules/base-generators/lib/generator.js:62:30)
at Generate.getGenerator (node_modules/base-generators/index.js:157:22)
at Object.exports.process (node_modules/base-generators/lib/tasks.js:127:24)
at node_modules/base-generators/lib/tasks.js:86:23
at Array.reduce (<anonymous>)
at exports.parse (node_modules/base-generators/lib/tasks.js:85:15)
at Generate.generate (node_modules/base-generators/index.js:490:21)
at Generate.generate (node_modules/base-generators/index.js:463:23)
at Context.<anonymous> (dist/tests/test.js:87:11)
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
Here is the travis config
{
"sudo": false,
"language": "node_js",
"node_js": "lts/carbon",
"script": [
"npm link",
"npm install --global generate .",
"yarn travis"
],
"group": "stable",
"dist": "trusty",
"os": "linux"
}
And here is:
the travis failling build for the PR #2
and the log for the job running on Node LTS/Carbon (v8.y.z)
I can't understand why that test (the one that runs the generator in CLI mode) fails after ESNext convertion and babel transpilation.
What's wrong with my PR ?

Answer 1

That bug is indeed related to the consequences of the babel transpilation.
The file generator.js at the root level directory use CommonJS module loading, that's required by the direct call by the global generate binary.
Here is the code of ./generator.js:
module.exports = require('./dist/lib/generator')
But:
since the module ./dist/lib/generator.js is the result of a babel transpilation of ./src/lib/generator.js, that exports the generator as a default anonymous function,
and since the module ./generator.js is a CJS module and use require()
then the required module is not the exported generator function but an object which contains a property default; and the value of this property is the generator function.
Thus, the correct way to implement ./generator.js is as following:
module.exports = require('./dist/lib/generator').default
That's all!
Here is the PR that fix the issue, and here is the passing travis build for that PR.