import.meta: (Stage 3)

[babel-bot]

Original issue submitted by @hzoo in babel/babel#5832

Original issue submitted by @hzoo in babel/babylon#539

import.meta for stage 2 (Domenic Denicola) (@domenic)

Info

Proposed at TC39 Meeting: May 2017
Spec Repo: https://github.com/tc39/proposal-import-meta
Spec Text: https://tc39.github.io/proposal-import-meta/
Moved to Stage 3 at the Sept 2017 meeting

Example

(async () => {
  const response = await fetch(new URL("../hamsters.jpg", import.meta.url));
  const blob = await response.blob();

  const size = import.meta.scriptElement.dataset.size || 300;

  const image = new Image();
  image.src = URL.createObjectURL(blob);
  image.width = image.height = size;

  document.body.appendChild(image);
})();

Parsing

• Done in : Add plugin for import.meta proposal babylon#544

Transform

• Not sure, maybe webpack should handle? Or in node?

[jkrems]

Any preferences for what this should desugar to in a babel world? A possible default implementation would just create an empty Object.create(null) object per module. I assume that anything else would be making too many assumptions (outside of webpack doing its own thing).

[ljharb]

I believe ModuleRecords should be frozen as well as being null objects.

[jkrems]

The import.meta object is an arbitrary object associated with a module, not the module record itself. The only thing the spec requires is that it was created as an ordinary object with null-prototype. The host may choose to freeze it but it's not required.

[jkrems]

See also "Will this be locked down": https://github.com/tc39/proposal-import-meta#will-this-object-be-locked-down-in-any-way

[jamiebuilds]

Since it's left up to the implementation, my opinion is that Babel should freeze it until either the spec is finalized or we discover a reason it can't be for Babel's purposes. We'll see what other implementations want to do and what the committee finally accepts.

[domenic]

The spec is finalized; it won't be locked down.

[justinfagnani]

Regarding the transform:

Not sure, maybe webpack should handle? Or in node?

We're in need of a simple transform to replace import.meta with an object that has a url property based on the file's location on disk. When we have something I can ping this issue again to see if it'd be useful enough for other people to contribute.