Scala.js: Default to ECMAScript 2015

[davesmith00000]

The official default for Scala.js is ECMAScript 2015, but Mill currently defaults to ES 5.1, here:

def useECMAScript2015: T[Boolean] = false

https://github.com/com-lihaoyi/mill/blob/main/scalajslib/src/ScalaJSModule.scala#L200

Mill should default to ES 2015 instead. The workaround is to add this line to your build.sc file:

override def useECMAScript2015 = T(true)

[lefou]

Can you provide a link to that claim?

I only found this:

ES modules and Node.js

Node.js needs explicit signaling that a module is an ECMAScript module (the default is CommonJS).

from https://www.scala-js.org/doc/project/module.html, 2022-01-11

[lolgab]

@lefou ESModule is an orthogonal setting to the ESVersion.
Scala.js recently (1.6.0) deprecated the useECMAScript2015 for a esVersion setting that can have multiple values (ES5, ES2015, ES2020).
Mill should probably do the same.

[lefou]

Yeah. Makes sense. We might infer the default from the selected ScalaJS version then? If that is not possible, we might better provide no default at all.

[davesmith00000]

Fair challenge, all I can only offer is that Scala.js's author suggested I raise this issue (to be fair he says for 1.x and above), which I realise is an appeal to authority but...

I discovered this was a problem for me when I noticed that code compiled in sbt was working as expected, but identical code compiled through Mill resulted in runtime errors.

Currently all you've got is a boolean flag, so being consistent with sbt is easy enough, just set it to true as in the PR - however that is a bit of a magic value. Inferring the right version based on Scala.js version will be a marginally more involved change, but I'd also be ok with it being a setting the user had to provide to avoid ambiguity.

[lolgab]

I think that for now we can change the default as you did.
Changing for a single version of ScalaJS is not possible for how Mill works. But I don't think it is a problem to change the default in 0.6 I don't think anybody still uses 0.6.

In the future we could change mill to have esVersion and deprecate the option.

[lefou]

Changing for a single version of ScalaJS is not possible for how Mill works.

Why? We can just depend on scalaJS target and if the version is high enough (> 1.6 or even > 1.x) we can set it to ECMAScript2015.

In the future we could change mill to have esVersion and deprecate the option.

We should do this now. The change is simple and we are still before a 0.10 release, so for adding and deprecating API this is the best time.