[Heroku] How to solve when an error is displayed on git push heroku master

I got some errors when deploying Heroku I hope it helps the solution.

Development environment

ruby 2.5.7 Rails 5.2.4.3 OS: macOS Catalina

flow

1 Check the error in the terminal 2 Log in to my page on heroku 3 Examine the error location 4 Clear the error

Log in to heroku's My Page

https://dashboard.heroku.com/apps Please log in here. At first, a confirmation screen will appear, but you can accept it as it is. (Please translate and check just in case)

After logging in, there is a link for the created app name, so click it and There is "Latest activity" on the right side, If you see a deployment error below it, The X mark is in red.

If you jump to the "View build log" next to it, you will find a detailed error.

The error that actually occurred is described below, so I hope you can refer to it.

Precompiling assets failed.

Check for errors in the terminal

Terminal

remote:  !
remote:  !     Precompiling assets failed.
remote:  !
remote:  !     Push rejected, failed to compile Ruby app.

Log in to heroku's My Page and check for errors

It's in the middle of the log, so look for the Error character.

heroku/ViewBuildLog

...

Uglifier::Error: Unexpected token: name (mapping). To use ES6 syntax, harmony mode must be enabled with Uglifier.new(:harmony => true).
       --
       
        ...
       
        18254 
        18255 
        18256 
        18257 
        18258 function initMap(){
           =>   let mapping
       
...

Clear the error

In this case, the description of let uses ES6 syntax and an error occurred. The solution is ①

config/environments/production.rb

config.assets.js_compressor = :uglifier
↓
config.assets.js_compressor = :Uglifier.new(:harmony => true)

②

app/aseets/application.js

function initMap(){
  let mapping
↓
function initMap(){
  var mapping

In my case, I tried ①, but I couldn't solve the error, so I solved it in ②.

Summary

Of course, most errors can be resolved by searching for the wording of the error as it is. However, if the situation is different for each individual, it may not be possible to deal with it with the same solution. I think it is important to know the various solutions. I hope you find it helpful.

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