Can't use dynamic imports with swc

[petercoulton]

Search Terms

• swc dynamic imports
• swc 'regenerator-runtime'

Expected Behavior

Dynamic imports should continue to work after switching to the swc transpiler.

Actual Behavior

Throws Error: Cannot find module 'regenerator-runtime

Steps to reproduce the problem

const example = async () => {
    const other = await import('./other')
    console.log(other)
}

example().then(() => console.log(`done`))

See TypeStrong/ts-node-repros#20 for full example.

Minimal reproduction

TypeStrong/ts-node-repros#20

Specifications

• ts-node version: v10.2.1
• node version: v16.8.0
• TypeScript version: 4.4.3
• tsconfig.json, if you're using one:

{
  "extends": "ts-node/node16/tsconfig.json",

  "ts-node": {
    "transpileOnly": true,
    "transpiler": "ts-node/transpilers/swc-experimental"
  },

  "include": ["**/*.ts"],
  "exclude": ["node_modules"]
}

• Operating system and version: macOS 11.6 Big Sur
• If Windows, are you using WSL or WSL2?: no

[cspotcode]

Does swc's transpiled output have a runtime dependency on regenerator-runtime?

[petercoulton]

@cspotcode My application doesn't use regenerator-runtime directly, and the code runs correctly with ts-node without enabling swc, but it seems that swc expects regenerator-runtime to be available.

[cspotcode]

That would fall into the same category as tslib and @swc/helpers. Sometimes changing compiler options necessitates that additional dependencies be available.

Notably, even if we declared them as peerDependencies, they would still need to be declared in your package.json since they are being imported/required by your code as opposed to by ts-node. I believe certain flavors of yarn enforce this.

We don't want to declare them as peerDependencies because then they get eagerly installed by npm even in projects that don't need them.

We will accept a PR adding this to our docs, probably belongs on this page where we mention @swc/helpers

https://typestrong.org/ts-node/docs/transpilers#bundled-swc-integration

[petercoulton]

Okay, that makes sense. Adding regenerator-runtime as a dev dependency seems to work. I've created a PR #1509 that updates the transpilers docs page with the additional dependency.

[cspotcode]

Thanks. It also occurred to me, we can detect when an import fails and log a helpful warning. For example, when a file compiled by ts-node attempts to require('regenerator-runtime') and fails, we can detect this and log a helpful warning suggesting that the user should install regenerator-runtime. I've added a note about this to #1504