throw new Error("only one instance of babel/polyfill is allowed");

[fundon]

babel/polyfill problem

dirs tree:

├── server.js
├── node_modules
│   └── trek

Both server.js and node_modules/trek use ES6+ syntax.

server.js

import Trek from 'trek';

var app = new Trek(__dirname);

app.get('/', function* (next) {
  this.body = 'Hello Trek.js!';
});

app.on('error', function (err, context) {
  app.logger.error(err);
});

app.run();

index.js of trek module

import Trek from './lib/trek';

export default Trek;

run the server.js

$ babel-node server.js

output:

/Users/fundon/dev/trekjs/trek-cli/trekapp/node_modules/babel/lib/babel/polyfill.js:4
  throw new Error("only one instance of babel/polyfill is allowed");
        ^
Error: only one instance of babel/polyfill is allowed
    at Object.<anonymous> (/Users/fundon/dev/trekjs/trek-cli/trekapp/node_modules/babel/lib/babel/polyfill.js:4:9)
....

How to solve it?

[sebmck]

The error message is pretty clear, only one instance of the polyfill should be used at all times. You have two modules with different versions of babel polluting globals with polyfills which leads to weird bugs. Either have the modules that are using babel/polyfill set to the same version so it resolves to the same module or compile the code with the runtime transformer.

[fundon]

When running babel-node cmd will ignore the packages in the node_modules directory.
https://github.com/babel/babel/blob/master/src/babel/api/register/node.js#L34

So I hacked it in the server.js file, https://github.com/trekjs/trek-cli/blob/master/templates/app/esnext.js based on https://github.com/postcss/postcss/blob/master/index.js.

var escape = function (str) {
    return str.replace(/[\[\]\/{}()*+?.\\^$|-]/g, "\\$&");
};

var regexp = ['lib', 'test', 'node_modules/god'].map(function (i) {
    return '^' + escape(path.join(__dirname, i) + path.sep);
}).join('|');

require('babel/register')({
    only:   new RegExp('(' + regexp + ')'),
    ignore: [ 'node_modules' ],
    loose: 'all'
});

And the shouldIgnore function
https://github.com/babel/babel/blob/master/src/babel/api/register/node.js#L75-L77

Maybe this will be better:

var shouldIgnore = function shouldIgnore(filename) {
  if (onlyRegex && onlyRegex.test(filename)) {
    return false;
  }
  return ignoreRegex && ignoreRegex.test(filename);
};

@sebmck Do you need I submit a PR for this?
I know this very small change.

[fundon]

I think Babel should not ignore all node_modules path.
Maybe check a identifier in package.json and transform the module automagically that will be great.

[hax]

This issue should be reopen. It's impossible to control all the deps (and the deps of a dep) to use same version (and even same versions still throw error) or use runtime transformer.

So I believe it's the duty of babel itself to solve the problem.

[loganfsmyth]

@hax Dependencies should not be loading the polyfill at all. If they rely on global ES6 features, then they need to document that and the root application should ensure that a polyfill is loaded, or the deps should use the runtime transform to load a non-global version of the polyfill.

[hax]

@loganfsmyth I assume all you said is correct , then:

1. The document of babel should include the caveat for the library writers
2. Runtime transformer should be in the default list not optional, and
3. Ask the major libraries which use babel to recompile and republish their packages.

[ruslansavenok]

If you use react.js on client and server, you might have babel/polyfill already loaded in node.js but not browser.

This is how i solved it:

// In node.js env, polyfill might be already loaded (from any npm package),
// that's why we do this check.
if (!global._babelPolyfill) {
  require('babel-core/polyfill')
}

[stan-ros]

@ruslansavenok thx, works for me.

[hax]

@fundon I agree with you that babel should support compiling module by checking the fields in package.json (maybe "babel": {"main": "src/index.js"})

[fundon]

@hax thx.
haha, You can submit a PR for this.

[jameslk]

So what are we supposed to do with Babel 6? It looks like the runtime is no longer used. I'm trying to write a library using ES6 and I do not want to pollute the global namespace with polyfills.

[loganfsmyth]

@jameslk Please see our README:

For questions and support please visit the Slack community or StackOverflow.

[jameslk]

@loganfsmyth Thanks. I figured it out anyway. It looks like babel-runtime has been moved to babel-plugin-transform-runtime and this needs to be added to the list of plugins to use it. Would of helped if that was documented somewhere.