jekyll 3.8.7 | Error: Unexpected token: keyword (const). To use ES6 syntax, harmony mode must be enabled with Uglifier.new(:harmony => true).

[improvethings]

$ bundle exec jekyll serve # works fine, the following does not:

$ JEKYLL_ENV=production bundle exec jekyll serve
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())};