Example for harmony in readme is very dangerous about uglifier HOT 4 CLOSED

   ActionView::Template::Error:
   
     <--- Last few GCs --->
   
     [367:0x2642080]    53048 ms: Mark-sweep 1419.2 (1478.7) -> 1419.1 (1447.7) MB, 1132.9 / 0.0 ms  (+ 0.0 ms in 0 steps since start of marking, biggest step 0.0 ms, walltime since start of marking 1133 ms) last resort GC in old space requested
     [367:0x2642080]    53780 ms: Mark-sweep 1419.1 (1447.7) -> 1419.1 (1447.7) MB, 731.1 / 0.0 ms  last resort GC in old space requested
   
   
     <--- JS stacktrace --->
   
     ==== JS stack trace =========================================
   
     Security context: 0x25bb8425879 <JSObject>
         2: optimize [/tmp/execjs20190921-165-1yrzpkhjs:3798] [bytecode=0x3e88a7812099 offset=37](this=0x2bb5d4f0c2e1 <AST_Call map = 0x36f4f3cdbee1>,compressor=0x72f30c82c59 <Compressor map = 0x36f4f3cc62a9>)
         3: before [/tmp/execjs20190921-165-1yrzpkhjs:3798] [bytecode=0x223f4c6266b1 offset=64](this=0x72f30c82c59 <Compressor map = 0x36f4f3cc62a9>,node=0x2bb5d4f0c2e1 <AST_Call map = 0x36f4f3cdbee1...
   
     FATAL ERROR: CALL_AND_RETRY_LAST Allocation failed - JavaScript heap out of memory
      1: node::Abort() [nodejs]
      2: 0x8cbf4c [nodejs]
      3: v8::Utils::ReportOOMFailure(char const*, bool) [nodejs]

rake aborted!
ExecJS::ProgramError: Unexpected token: operator (>) (line: 165515, col: 37, pos: 6800637)

Error
    at new JS_Parse_Error (/tmp/execjs20190920-13904-1230of4js:3572:11870)
    at js_error (/tmp/execjs20190920-13904-1230of4js:3572:12089)
    at croak (/tmp/execjs20190920-13904-1230of4js:3572:20898)
    at token_error (/tmp/execjs20190920-13904-1230of4js:3572:21035)
    at unexpected (/tmp/execjs20190920-13904-1230of4js:3572:21123)
    at expr_atom (/tmp/execjs20190920-13904-1230of4js:3572:29660)
    at maybe_unary (/tmp/execjs20190920-13904-1230of4js:3573:143)
    at expr_ops (/tmp/execjs20190920-13904-1230of4js:3573:901)
    at maybe_conditional (/tmp/execjs20190920-13904-1230of4js:3573:993)
    at maybe_assign (/tmp/execjs20190920-13904-1230of4js:3573:1436)
new JS_Parse_Error ((execjs):3572:11870)

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

bundle show uglifier
/home/circleci/project/vendor/cache/ruby/2.5.0/gems/uglifier-4.2.0

  Failure/Error: = stylesheet_link_tag 'application'

     ActionView::Template::Error:

       <--- Last few GCs --->

       [397:0x4172080]    58497 ms: Mark-sweep 1419.8 (1480.2) -> 1419.8 (1449.2) MB, 1126.9 / 0.0 ms  (+ 0.0 ms in 0 steps since start of marking, biggest step 0.0 ms, walltime since start of marking 1127 ms) last resort GC in old space requested
       [397:0x4172080]    59603 ms: Mark-sweep 1419.8 (1449.2) -> 1419.8 (1449.2) MB, 1106.2 / 0.0 ms  last resort GC in old space requested


       <--- JS stacktrace --->

       ==== JS stack trace =========================================

       Security context: 0x16200f125879 <JSObject>
           2: /* anonymous */ [0x3b87c8b822d1 <undefined>:1] [bytecode=0x22658bbc7789 offset=0](this=0x393e3eb0c2f1 <JSGlobal Object>)
           5: builtin exit frame: new Function(aka Function)(this=0x3b87c8b822d1 <undefined>,0x36791dfde7c1 <String[118]: switch(str){case "(":case "[":case "+":case "*":case "/":case "-":case ",":case ".":case "`":return true}return false;>,0x16200f1416e9 <String[3]: str>)

        ...

       FATAL ERROR: CALL_AND_RETRY_LAST Allocation failed - JavaScript heap out of memory
        1: node::Abort() [nodejs]
        2: 0x8cbf4c [nodejs]
        3: v8::Utils::ReportOOMFailure(char const*, bool) [nodejs]
        4: v8::internal::V8::FatalProcessOutOfMemory(char const*, bool) [nodejs]
        5: v8::internal::Factory::NewCode(v8::internal::CodeDesc const&, unsigned int, v8::internal::Handle<v8::internal::Object>, bool, int) [nodejs]
        6: v8::internal::CodeGenerator::MakeCodeEpilogue(v8::internal::TurboAssembler*, v8::internal::EhFrameWriter*, v8::internal::CompilationInfo*, v8::internal::Handle<v8::internal::Object>) [nodejs]
        7: v8::internal::compiler::CodeGenerator::FinalizeCode() [nodejs]
        8: v8::internal::compiler::PipelineImpl::FinalizeCode() [nodejs]
        9: v8::internal::compiler::PipelineCompilationJob::FinalizeJobImpl() [nodejs]
       10: v8::internal::Compiler::FinalizeCompilationJob(v8::internal::CompilationJob*) [nodejs]
       11: v8::internal::OptimizingCompileDispatcher::InstallOptimizedFunctions() [nodejs]
       12: v8::internal::StackGuard::HandleInterrupts() [nodejs]
       13: v8::internal::Runtime_StackGuard(int, v8::internal::Object**, v8::internal::Isolate*) [nodejs]
       14: 0x51233a042fd
     # (execjs):1
     # ./vendor/cache/ruby/2.5.0/gems/execjs-2.7.0/lib/execjs/external_runtime.rb:219:in `exec_runtime'
     # ./vendor/cache/ruby/2.5.0/gems/execjs-2.7.0/lib/execjs/external_runtime.rb:39:in `exec'
     # ./vendor/cache/ruby/2.5.0/gems/execjs-2.7.0/lib/execjs/external_runtime.rb:21:in `eval'
     # ./vendor/cache/ruby/2.5.0/gems/execjs-2.7.0/lib/execjs/external_runtime.rb:46:in `call'
     # ./vendor/cache/ruby/2.5.0/gems/uglifier-4.2.0/lib/uglifier.rb:221:in `run_uglifyjs'
     # ./vendor/cache/ruby/2.5.0/gems/uglifier-4.2.0/lib/uglifier.rb:166:in `compile'
     # ./vendor/cache/ruby/2.5.0/gems/sprockets-3.7.1/lib/sprockets/compressing.rb:65:in `block in js_compressor='
     # ./vendor/cache/ruby/2.5.0/gems/sprockets-3.7.1/lib/sprockets/legacy_proc_processor.rb:31:in `call'

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