Bug Report

Current Behavior

This is a very confusing bug, and I want to say first that I’m not sure where the bug is (in Babel or elsewhere), so I am hoping for feedback in order to refine this bug report and possibly move it. (Is it more likely a Babel bug, a Webpack bug, a minifier bug?)

The problem: Our project uses https://www.npmjs.com/package/clipboard, which is published to NPM in transpiled form. As such, it contains the _typeof helper:

 var _typeof = typeof Symbol === "function" && typeof Symbol.iterator === "symbol" ? function (obj) {
        return typeof obj;
    } : function (obj) {
        return obj && typeof Symbol === "function" && obj.constructor === Symbol && obj !== Symbol.prototype ? "symbol" : typeof obj;
    };

After transformation via Webpack (4.17.1), Babel (6.26.3) uglifyjs-webpack-plugin or (currently) terser-webpack-plugin (1.0.2, terser 3.8.2), it seems that the typeof Symbol.iterator part has been transformed a second time, and resulted in a name collision. The minified code, reformatted for legibility:

r = "function" == typeof Symbol && "symbol" === r(Symbol.iterator) ?
	function (e) {
		return void 0 === e ? "undefined" : r(e)
	} :
	function (e) {
		return e && "function" == typeof Symbol && e.constructor === Symbol && e !== Symbol.prototype ?
			"symbol" :
			void 0 === e ? "undefined" : r(e)
	}

Since both typeof instances were minified, presumably via the _typeof helper, to the same name (here: r), the expression fails to evaluate because the r in r(Symbol.iterator) isn’t initialized yet.

Input Code

Unfortunately, I have been unable to reproduce this in a minimal repro case. Importing the same library (clipboard) in a minimal sample and minifying it with the same options did not reproduce the behaviour.

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

{
  "presets": ["babel-preset-env", {
    "browsers": [
	">5%",
	"IE >= 11",
	"last 2 Chrome versions",
	"last 2 ChromeAndroid versions",
	"last 2 Edge versions",
	"last 2 Firefox versions",
	"last 2 iOS versions"
    ]
  }],
  "plugins": [
	"babel-plugin-lodash",
	"babel-plugin-syntax-dynamic-import",
	"babel-plugin-transform-decorators-legacy",
	"babel-plugin-transform-class-properties",
	"babel-plugin-transform-flow-comments",
	"babel-plugin-transform-object-rest-spread",
	"babel-plugin-transform-react-jsx"
  ]
}

Environment

• Babel version(s): 6.26.3
• Node/npm version: Node 8.11.1/npm 6.4.0
• OS: Ubuntu 18.04
• Monorepo: No
• How you are using Babel: babel-loader/Webpack