Why does SWC output "unknown" first before any code?

[peterlindstrom123]

I'm calling swc compile with this config:

{
  "jsc": {
    "target": "es2018",
    "externalHelpers": false,
    "parser": {
        "syntax": "ecmascript",
        "jsx": true
      },
    "transform": {
      "react": {
        "pragma": "CE",
        "pragmaFrag": "React.Fragment",
        "useBuiltins": true
      }
    }
  },
  "isModule": false
}

Then the first thing SWC outputs is a line containing only unknown, which is obviously not valid JavaScript.

How do I prevent this behaviour? I'm not using Node for anything btw.

I've got my own little tool for concatenating JavaScript files into bigger files, and then I'm running SWC on the bigger files - I'm not using any kind of Modules, just plain JavaScript.

Please see attached screenshot.

[kdy1]

I don't think it's injected by SWC. If it is, please provide a minimal reproduction.

[peterlindstrom123]

OK, have a JS file with this content:

const lala = {};

Run:

cat test.js | swc compile

(Where test.js has that content)

It results in this output:

unknown
var lala = {};

Should we file a bug report?

[kdy1]

Yeap, with a minimal reproduction (github repo)

[peterlindstrom123]

Why would a repo be needed when the reproduction is there, and record-breakingly minimal? 😅

[kdy1]

Because reproducing is a complex issue than you think