Parcel does not transpile ES6+ codes in node_modules

[jeongtae]

🐛 bug report

🎛 Configuration (.babelrc, package.json, cli command)

{
  "name": "parcel-test",
  "version": "1.0.0",
  "browserslist": "> 1%, ie 11, not dead",
  "scripts": {
    "build": "parcel build ./src/index.js --no-optimize"
  },
  "devDependencies": {
    "parcel": "^2.0.1",
  },
  "dependencies": {
    "color-convert": "^2.0.1",
    "jsesc": "^3.0.2"
  }
}

🤔 Expected Behavior

All ES6 codes of the color-convert module must be transpiled to ES5 through the @parcel/transformer-js module. because the IE 11 browser is specified in the browserslist property in the package.json file as you can see above.

😯 Current Behavior

The ES6 specific statements and syntax(e.g. for..of statement of color-convert, arrow function syntax of jsesc) are not being transpiled to legacy ES5. So a SyntaxError is thrown on IE 11.

💁 Possible Solution

Use [email protected] which is deprecated. Parcel v1 seems can handle the package.json#engines property of the color-convert module to make decision to execute transpilation or not. When i edit the value of the engines#node property of color-convert module to more older version like >=0.0.1 then the module is not being transpiled to ES5 by Parcel v1.

🔦 Context

I tried to use Babel but there was no effect. And I read the source code of @parcel/transformer-js, I discovered targets value is not assigned when the target asset is not a source(assets.isSource). I'm not sure, but I think Parcel v2 does not transpile modern codes in external dependencies(in node_module directory) intentionally. Is it correct?

💻 Code Sample

src/index.js

import Convert from "color-convert";
console.log(Convert.rgb.hsl(140, 200, 100));

node_modules/color-convert/conversions.js:10

for (const key of Object.keys(cssKeywords)) {
	reverseKeywords[cssKeywords[key]] = key;
}

dist/index.js:758

for (const key of Object.keys($330224f6db11e147$exports))$d64509da21885dbc$var$reverseKeywords[$330224f6db11e147$exports[key]] = key;

🌍 Your Environment

Software	Version(s)
Parcel	2.0.1
Node	16.9.1
npm/Yarn	Yarn 1.22.11
Operating System	macOS Big Sur 11.5.2 (Apple Silicon)

[mischnic]

This is indeed getting worse as more packages move to newer syntax

Duplicate of #1655

I'm not sure, but I think Parcel v2 does not transpile modern codes in external dependencies(in node_module directory) intentionally. Is it correct?

Correct