Heroku Precompiling assets failed. が解決できない

やりたいこと

rails アプリをherokuでデプロイしたい
##エラー
error: failed to push some refs to 'https://git.heroku.com/cryptic-everglades-97405.git'
でデプロイされない

ターミナル
Uglifier::Error: Unexpected token: keyword (const). To use ES6 syntax, harmony mode must be enabled with Uglifier.new(:harmony => true).
省略
remote:         2299 
remote:         2300 
remote:         2301 
remote:         2302 
remote:           => const nav = document.getElementsByClassName("nav-js")[0];
remote:         2304 const button = document.getElementsByClassName("js-nav-button")[0];
remote:         2305 
remote:         2306 button.addEventListener("click", () => {
remote:         2307   nav.classList.toggle("active");
remote:         2308 
remote:         2309   if (nav.classList.contains("active")) {
remote:         2310     button.classList.add("active");
remote:         2311   } else {
remote:        ==
省略
remote:  !
remote:  !     Precompiling assets failed.
remote:  !
省略
To https://git.heroku.com/cryptic-everglades-97405.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/cryptic-everglades-97405.git'

=> const nav = document.getElementsByClassName("nav-js")[0];
が赤くなっている

application.js

// This is a manifest file that'll be compiled into application.js, which will include all the files
// listed below.
//
// Any JavaScript/Coffee file within this directory, lib/assets/javascripts, or any plugin's
// vendor/assets/javascripts directory can be referenced here using a relative path.
//
// It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
// compiled file. JavaScript code in this file should be added after the last require_* statement.
//
// Read Sprockets README (https://github.com/rails/sprockets#sprockets-directives) for details
// about supported directives.
//
//= require rails-ujs
//= require activestorage
//= require_tree .

const nav = document.getElementsByClassName("nav-js")[0];
const button = document.getElementsByClassName("js-nav-button")[0];

button.addEventListener("click", () => {
  nav.classList.toggle("active");

  if (nav.classList.contains("active")) {
    button.classList.add("active");
  } else {
    button.classList.remove("active");
  }
});

config/environments/production.rb

 config.public_file_server.enabled = ENV['RAILS_SERVE_STATIC_FILES'].present?

  # Compress JavaScripts and CSS.
  config.assets.js_compressor = Uglifier.new(harmony: true)
  # config.assets.css_compressor = :sass

  # Do not fallback to assets pipeline if a precompiled asset is missed.
  config.assets.compile = true

##　試したこと
Uglifier.new(harmony: true)と記述したが、エラーが発生し、なにが間違っているのかがわからない