ES6 syntax, harmony mode must be enabled with Uglifier.new(:harmony => true

Question

I am facing this problem

Uglifier::Error: Unexpected token: keyword (const). To use ES6 syntax, harmony mode must be enabled with Uglifier.new(:harmony => true). while deploying the project through capistrano on production.

I followed this solution

https://github.com/lautis/uglifier/issues/127#issuecomment-352224986

which suggests

replacing

config.assets.js_compressor = :uglifier

with

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

but even after doing that I am still facing the same error. I dont understand what went wrong. I am using uglifier (4.1.20) version

Answer 1

Just leaving the answer here too:

In config/environments/production.rb replace

config.assets.js_compressor = :uglifier

with

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

Answer 2

On Rails 6.1, I managed to solve this issue by:

1. Removing the uglifier gem from Gemfile.
2. Removing the config.assets.js_compressor entry in config/environments/production.rb and config/environments/development.rb.

Ref.: https://www.mintbit.com/blog/rails-5-6-upgrade-es6-uglifier-bug

Answer 3

Uglifier only works with ES5. If you need to compress ES6, ruby-terser is a better option.