yarn build failed for Javascript setup with error "Unrecognized Token"

[hemantbaxi]

Bug Report

With perspective master branch, it builds fine in python setup but failing to build in Javascript setup with following error:

Steps to Reproduce:

1. Clone the master branch from github
2. select Javascript setup
3. yarn build ... I did get some other errors before getting to this which were because of missing dependencies and got fixed after I installed them.

Expected Result:

Build to work fine.

Actual Result:

$ babel src/js --source-maps --out-dir dist/esm
SyntaxError: src/js/api/client.js: Unexpected token (61:83)
  59 |                 this.send(msg);
  60 | 
> 61 |                 if ((msg.cmd === "table" || msg.cmd === "view") && !this._features?.wait_for_response && resolve) {
     |                                                                                    ^
  62 |                     resolve();
  63 |                 }
  64 |             });
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
ERROR: "build:babel:esm" exited with 1.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
lerna ERR! yarn build exited 1 in '@finos/perspective'
lerna ERR! yarn build exited 1 in '@finos/perspective'

After doing google search, it seems this is babel optional-chaining feature which webpack is not recognizing. I checked
I have @babel/preset-env plugin. These are the versions of babel and webpack on my system

axihemant@hb-finos:~/finos/perspective$ npm list webpack
/home/baxihemant/finos/perspective
└── webpack@4.46.0

baxihemant@hb-finos:~/finos/perspective$ npm list babel
/home/baxihemant/finos/perspective
└── babel@6.23.0

baxihemant@hb-finos:~/finos/perspective$ npm list babel-cli
/home/baxihemant/finos/perspective
└── babel-cli@6.26.0

baxihemant@hb-finos:~/finos/perspective$ npm list @babel/preset-env
/home/baxihemant/finos/perspective
└── @babel/preset-env@7.14.2

Environment:

master branch
OS : Ubuntu 20.10
npm -v
6.14.8
Virtualized environment

Thanks,
Hemant