Support for type only imports & exports

[chrstntdd]

Hello there! This project is fantastic and I use it heavily. Thank you for all your hard work!

I have a project using the latest stable TypeScript version (3.8.3) and tried to use Type-Only Imports and Export but was unable to compile with sucrase - throwing the following error.

SyntaxError: Error transforming src/index.ts: Unexpected token (8:13)

// src/index.ts
// ...
import type { TerserOpts } from "./types"
// ...

Are there any plans to support this feature in sucrase? If so, is there any way I can help contribute? Would love to have sucrase support this.
Cheers!

[alangpierce]

Hi @chrstntdd , thanks for the kind words. 😄

Yep, I've been following along on type-only imports, and certainly I want Sucrase to support them. I was hoping that they'd eventually get rid of the need to detect imported identifiers as type-only or not (which causes quite a bit of complexity and slowness in Sucrase), but looks like statements like import {a, b} from 'foo'; still need to have a or b removed based on usage in the file.

From a code standpoint, getting this working will involve these steps:

1. To implement parsing, port all recent babel-parser changes, similar to Port babel-parser changes from 2019-09-06 to 2019-12-22 #487 . That should naturally pick up parsing the type token.
2. Change CJSImportProcessor and ESMImportProcessor to remove import type lines. I think this actually is already working because flow has the same syntax, so this just needs some TS tests I think.
3. Maybe change CJSImportProcessor and ESMImportProcessor to optionally no longer remove type-only import lines, equivalent to importsNotUsedAsValues=preserve in TS and onlyRemoveTypeImports in Babel.

I'm actually unsure how to approach that last point, or whether to do anything at all, and I'd love feedback. I think my plan for my own projects is to use importsNotUsedAsValues=error everywhere so that I need to be explicit, and that means that the Sucrase behavior doesn't matter because I never intend to have type-only imports without type. Do you think you'd want a Sucrase flag to mimic importsNotUsedAsValues=preserve (keeping type-only import statements as side-effect-only imports)? Certainly if TS makes that the default, Sucrase will follow suit, but in general I'd prefer to keep Sucrase configuration minimal.

I think it probably makes sense for me to do step 1 since it's a bit hard to do without a lot of project context. But it would be great to get your feedback and if you could help test the feature when it's ready. At the moment, I don't have any codebases using that syntax that I can test with, so I worry that I may miss some detail.

[alangpierce]

Sorry for the delay! I just released this as Sucrase 3.14.0. Let me know if you see any issues!