Support static replacement from transformers

[101arrowz]

🙋 feature request

Support custom replacement of build-time constants.

🤔 Expected Behavior

It is possible to run custom replacements on JavaScript from a transformer.

😯 Current Behavior

It is not possible to run custom replacements on JavaScript from a transformer.

💁 Possible Solution

Make it possible to run custom replacements on JavaScript from a transformer.

To be more serious: right now, only the JS transformer has the ability to run said replacements, so I think adding a meta property for custom replacement keys and values should make this possible.

🔦 Context

Support Vue's runtime globals from the Vue transformer as mentioned in this comment.

[mischnic]

Support Vue's runtime globals from the Vue transformer as mentioned in this comment.

Asking once again: in which files should this replacement be done?

[101arrowz]

Just in Vue's internal files, nowhere else. Sorry for not clarifying.

[mischnic]

so I think adding a meta property for custom replacement keys and values should make this possible.

These would be a kind of global configuration. The Vue transformer would certainly not be able to set them...

One possibility would be setting them similar to these config options: https://v2.parceljs.org/features/node-emulation/#%F0%9F%94%A7-disabling-these-features. I think this would be fine for Vue's usecase?

[101arrowz]

The reason those don't work is that the globals are not defined on process.env. I was thinking something along the lines of reading the Vue entry file with the Vue transformer, returning it as a dependency with a meta property replacements: { __VUE_OPTIONS_API__: true } and forwarding the meta property to child dependencies from the JS transformer. Would that be too hacky?

[mischnic]

The reason those don't work is that the globals are not defined on process.env

I was thinking of:

{
  "name": "my-project",
  "dependencies": {
    ...
  },
  "@parcel/transformer-js": {
    "define": {
      "__VUE_OPTIONS_API__": false,
	}
  }
}

forwarding the meta property to child dependencies from the JS transformer. Would that be too hacky?

Propagating the meta properties unconditionally to every child recursively? That's not an option

[mischnic]

@parcel/transformer-js

We also want to prevent feature creep on JS transformer though. The alternative would be a specific transformer for this (which could also reuse the previous ASTs).

[mischnic]

I just ran into another usecase of replacing globals.
Maybe allowing transformers to set asset.meta.jsGlobals = {"__XYZ__": true}; would make sense (and then the JS transformer would apply that).

[devongovett]

What types of values would be supported? Just JSON stringifyable types?