broccoli-babel-transpiler

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Babel accepts plugins as (among other things) either a string denoting a module to require, or a tuple containing that string and optionally some configuration for that plugin. It will also accept a singleton array with just the module path as a sort of degenerate version of the second case, i.e.

plugins: [
  '/path/to/some/plugin.js',
  ['/path/to/some/plugin.js', { config: true }],
  ['/path/to/some/plugin.js']
]

For the third entry, the parallelization logic interprets that plugin as ['/path/to/some/plugin.js', undefined], and since undefined isn't JSON-serializable, the plugin is considered unparallelizable. That's easy enough to work around, but the error message we got with throwUnlessParallelizable turned on looked like:

[broccoli-persistent-filter:Babel > [Babel: ember-data]: Babel: ember-data] was configured to `throwUnlessParallelizable` and was unable to parallelize a plugin.
plugins:
1: name: unknown, location: unknown

If the relevant plugin path had been included in that output, it would have helped with tracking down the source of the problem.

@dfreeman it sounds like the best solution would be for us to simply ignore or custom serialize undefined here right?

Yep, I think treating ['foo-bar'] as equivalent to bare 'foo-bar' would solve the specific error I was seeing here. 👍