private class fields don't work with compatibility-mode=extended

[mstoykov]

Brief summary

private class fields are supported by goja but not by the internal babel. Which means that if babel needs to be used (at this point only for ESM support) it will give you an error on the private field

class Value {
  #internal_private_value;

  constructor(value) {
    this.#internal_private_value = value;
  }

  get value() { return this.#internal_private_value; }
}

const test_object = new Value(5);
console.log(test_object.value);
export default function() { }

will error out with

: Unexpected character '#' (2:2)
  1 | class Value {
> 2 |   #internal_private_value;
    |   ^
  3 |
  4 |   constructor(value) {
  5 |     this.#internal_private_value = value; at <internal/k6/compiler/lib/babel.min.js>:2:28536(100)

But if you use exports.default = function () {} at the end it will work as normal

k6 version

v0.42.0

OS

all

Docker version and image (if applicable)

No response

Steps to reproduce the problem

Run

class Value {
  #internal_private_value;

  constructor(value) {
    this.#internal_private_value = value;
  }

  get value() { return this.#internal_private_value; }
}

const test_object = new Value(5);
console.log(test_object.value);
export default function() { }

Expected behaviour

It works.

Actual behaviour

babel gives you an error

From this community forum post

[mstoykov]

The fix for this as with other cases where babel gives error but goja will handle the code is to not use babel which at this point is again only used for ESM support.

This hopefully will be picked up in the next release(s) as the major blockage of lack of async/await support is gone.