How do I use babel's `useBuiltIns: 'usage'` option on the vendors bundle?

Question

Since I need to support also IE11, I need to transpile also node_modules.

This is the babel config I use on the node_modules:

presets: [
  ['@babel/preset-env', { modules: false, useBuiltIns: 'usage' }],
],

I use the useBuiltIns options because it was giving an error Symbol is not defined, the polyfill was needed.

However this configuration breaks at compile time, supposedly because it injects some imports in the code, here is the error:

Basically it's not liking the module.exports. So how do I use useBuiltIns in the vendors bundle?

For now I solved by always requiring the babel polyfill in the index.html, however this is not ideal.

Answer 1

Babel by default assumes that files it processes are ES modules (using import and export). If you are running Babel on things in node_modules (which are likely CommonJS modules), you'll need to either tell Babel to process all node_modules as scripts, or tell Babel to guess the type based on the presence of import and export. Guessing is easiest, so you can add

sourceType: "unambiguous"

and also tell Babel not to run the usage transform on core-js itself with

ignore: [
  /\/core-js/,
],

because otherwise the usage transform will actually be inserting references to core-js into itself causing dependency cycles.

So in your top-level Babel configuration, you'd do e.g.

{
  ignore: [
    /\/core-js/,
  ],
  sourceType: "unambiguous",
  presets: [
    ['@babel/preset-env', { modules: false, useBuiltIns: 'usage' }],
  ],
}

If you wanted to be extra specific about it, you could also do

{
  ignore: [
    /\/core-js/,
  ],
  presets: [
    ['@babel/preset-env', { modules: false, useBuiltIns: 'usage' }],
  ],
  overrides: [{
    test: "./node_modules",
    sourceType: "unambiguous",
  }],
}

to only set the flag for files inside node_modules, but there is likely not much to gain by doing that.

As for why this fixes that error, the issue is that, if Babel thinks something is an ES module, it will insert import statements. If you insert import statements into a file that also uses CommonJS things like module.exports, it means the file would now be using both module systems in the same file, which is a big issue and causes the errors you are seeing.