Uglifier issue: Extra configuration is required for Uglifier 3.x to support ES6 in production mode (it is supported in development mode) #2755

[improvethings]

Minimal Mistakes version: 4.21.0
Ruby gem or remote theme version: 2.7.6.2
Jekyll version: 3.8.7
uglifier 4.2.0
Operating system: Debian 10 (Buster) on Linux 5.4.0-0.bpo.4-amd64 x86_64

Publishing website to Vercel from github.

Description

Downstream issue for reference

Steps

$ JEKYLL_ENV=production bundle exec jekyll serve

Output

...
Incremental build: disabled. Enable with --incremental
Generating...
Jekyll Feed: Generating feed for posts
jekyll 3.8.7 | Error: Unexpected token: keyword (const). To use ES6 syntax, harmony mode must be enabled with Uglifier.new(:harmony => true).
=> const fs = require("fs");
2 const pkg = require("./package.json");
3 const filename = "assets/js/main.min.js";
4 const script = fs.readFileSync(filename);
5 const padStart = str => ("0" + str).slice(-2);
6 const dateObj = new Date();
7 const date = ${dateObj.getFullYear()}-${padStart( 8 dateObj.getMonth() + 1 9 )}-${padStart(dateObj.getDate())};

Expected

Same output at when I run $ bundle exec jekyll serve

(brings up production environment on local webserver)

[Yankie]

I'll save you some time :)
Add following in your _config.yml:

jekyll-minifier:
  uglifier_args:
    harmony: true