# app.rb without Ruby logger library (not working, get error Error “undefined local variable or method `logger'")

require 'sinatra'

get '/' do
  logger.info "COUCOU"
  'Hello world!'
end

# app.rb with Ruby logger library (working)

require 'sinatra'
require 'logger'

get '/' do
  logger = Logger.new STDERR
  logger.info "COUCOU"
  'Hello world!'
end