Webpack build is broken

We are now seeing d is not a function in our babel/webpack-built app.

The error happens in loggers.js, but that could be triggered by an issue with the spread operator.

_Originally posted by @jlowcs in https://github.com/transloadit/uppy/issues/3328#issuecomment-990141390_

[jlowcs]

Alright, we actually decided to turn loose off on our app, and it did the trick. Thanks for your help @aduh95!

[jlowcs]

Context

For more context, in the case of our app, we are transpiling (loose: true) uppy’s code with babel, since uppy contains ES2015+ code. We are also minimizing it.

Diagnostic

Source of the exception

Investigating using the dev tools, I can confirm that the issue is indeed coming from the following line from the bundle:

const steps = [..._classPrivateFieldLooseBase(this, _preProcessors)[_preProcessors], ..._classPrivateFieldLooseBase(this, _uploaders)[_uploaders], ..._classPrivateFieldLooseBase(this, _postProcessors)[_postProcessors]];

The minimized transpiled code for that line looks like this:

s = [].concat(l(this, k)[k], l(this, R)[R], l(this, I)[I])

Later, the code does:

d = s[u]
// ...
d(c.fileIDs, t);

But, l(this, k)[k] is apparently a Set, therefore d is a Set, and therefore not a function.

Ouch, that code is hard to read

For better readability, I tried compiling without minification:

steps = [].concat(_classPrivateFieldLooseBase(this, _preProcessors)[_preProcessors], _classPrivateFieldLooseBase(this, _uploaders)[_uploaders], _classPrivateFieldLooseBase(this, _postProcessors)[_postProcessors])
// ...
fn = steps[step];
// ...
return fn(updatedUpload.fileIDs, uploadID);

Basically, babel replaces the spread operators with the use of [].concat(), which makes sense. But concat doesn’t spread iterables (which Set is).

Explanation of the issue

That all comes from the fact that [].concat(new Set(), new Set(), new Set()) returns an array of Set, not an array of values from those Set.

That’s why the Array.from() solution was suggested, as this seems to work much better:

What about the legacy bundle?

I actually thing the issue is also present in the legacy bundle, since it is also using Set and the same [].concat() approach:

Possible solutions

1. go back to using Array.from (and figure out why it was not working for netdown)
2. replace the 3 Set with simple arrays
3. ?