This is my webpack.backend.mix.js code, im using node 6

gistfile1.txt

let mix = require('laravel-mix');
const webpack = require('webpack');

/*
 |--------------------------------------------------------------------------
 | Mix Back Panel Asset Management
 |--------------------------------------------------------------------------
 |
 | Mix provides a clean, fluent API for defining some Webpack build steps
 | for your Laravel application. By default, we are compiling the Sass
 | file for the application as well as bundling up all the JS files.
 |
 */

mix.webpackConfig({
    plugins: [
        new webpack.ContextReplacementPlugin(/moment[\/\\]locale/,/select2[\/\\]dist[\/\\]js[\/\\]i18n/)
    ]
});

mix.js('resources/assets/backend/js/app.js', 'public/js')
    .js([
        'resources/assets/backend/js/site-dashboard.js',
    ], 'public/js/site-dashboard.js')
    .js([
        'resources/assets/backend/js/dashboard.js',
    ], 'public/js/dashboard.js')
   .js([
        'resources/assets/backend/js/adminlte.js',
        'resources/assets/backend/js/theme_settings.js',
        'resources/assets/backend/js/bootstrap-datetimepicker.min.js',
        'resources/assets/backend/js/bootstrap-toggle.min.js',
    ], 'public/js/theme.js')
    .extract([
        'jquery', 'bootstrap', 'icheck', 'jquery-validation', 'jquery-slimscroll', 'fastclick',
        'datatables.net', 'datatables.net-bs', 'datatables.net-responsive-bs'
    ])
    .sass('resources/assets/backend/sass/app.scss', 'public/css')
    .sass('resources/assets/backend/sass/print.scss', 'public/css')
    .sass('resources/assets/backend/sass/vendor.scss', 'public/css')
    .sass('resources/assets/backend/sass/colorpicker.scss', 'public/css')
    .sass('resources/assets/backend/sass/report.scss', 'public/css')
    .js([
        'resources/assets/backend/js/colorpicker.js',
    ], 'public/js/colorpicker.js')
    .styles([
        'resources/assets/backend/css/AdminLTE.css',
        'resources/assets/backend/css/_all-skins.css',
        'resources/assets/backend/css/bootstrap3-wysihtml5.min.css',
        'resources/assets/backend/css/bootstrap-datetimepicker.min.css',
        'resources/assets/backend/css/bootstrap-toggle.min.css'
    ], 'public/css/theme.css')
    .styles('resources/assets/backend/css/site-dashboard.css', 'public/css/site-dashboard.css')
    .styles('resources/assets/backend/css/pace.css', 'public/css/pace.css')
    .autoload({
        jquery: ['$', 'window.jQuery', 'jQuery', 'jquery'],
        moment: ['window.moment', 'moment'],
    });


mix.options({
    processCssUrls: true,
    // uglify: {},
    purifyCss: false,
    // purifyCss: {},
    clearConsole: false
});


// copy non processing files to public path
mix.copy('resources/assets/backend/js/bootstrap3-wysihtml5.all.min.js', 'public/js/editor.js');
mix.copy('resources/assets/backend/js/pace.js', 'public/js/pace.js');
mix.copyDirectory('resources/assets/backend/images', 'public/images');



if (mix.inProduction()) {
    mix.version();
    mix.sourceMaps();
}

mix.browserSync({
    proxy: 'l5.school.test'
});

This sis the Error Message when i run
npm run backend-prod

@ backend-prod /home/330475.cloudwaysapps.com/vkuasgyehy/public_html
npm run backend-production

@ backend-production /home/330475.cloudwaysapps.com/vkuasgyehy/public_html
cross-env NODE_ENV=production node_modules/webpack/bin/webpack.js --no-progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js --env.mixfile=webpack.backend.mix.js

/home/330475.cloudwaysapps.com/vkuasgyehy/public_html/webpack.backend.mix.js:25
]
^

SyntaxError: Unexpected token ]
at NativeCompileCache._moduleCompile (/home/330475.cloudwaysapps.com/vkuasgyehy/public_html/node_modules/v8-compile-cache/v8-compile-cache.js:240:18)
at Module._compile (/home/330475.cloudwaysapps.com/vkuasgyehy/public_html/node_modules/v8-compile-cache/v8-compile-cache.js:186:36)
at Object.Module._extensions..js (module.js:586:10)
at Module.load (module.js:494:32)
at tryModuleLoad (module.js:453:12)
at Function.Module._load (module.js:445:3)
at Module.require (module.js:504:17)
at require (/home/330475.cloudwaysapps.com/vkuasgyehy/public_html/node_modules/v8-compile-cache/v8-compile-cache.js:161:20)
at Object. (/home/330475.cloudwaysapps.com/vkuasgyehy/public_html/node_modules/laravel-mix/setup/webpack.config.js:12:1)
at Module._compile (/home/330475.cloudwaysapps.com/vkuasgyehy/public_html/node_modules/v8-compile-cache/v8-compile-cache.js:192:30)
at Object.Module._extensions..js (module.js:586:10)
at Module.load (module.js:494:32)
at tryModuleLoad (module.js:453:12)
at Function.Module._load (module.js:445:3)
at Module.require (module.js:504:17)
at require (/home/330475.cloudwaysapps.com/vkuasgyehy/public_html/node_modules/v8-compile-cache/v8-compile-cache.js:161:20)
at WEBPACK_OPTIONS (/home/330475.cloudwaysapps.com/vkuasgyehy/public_html/node_modules/webpack-cli/bin/utils/convert-argv.js:114:13)
at requireConfig (/home/330475.cloudwaysapps.com/vkuasgyehy/public_html/node_modules/webpack-cli/bin/utils/convert-argv.js:116:6)
at /home/330475.cloudwaysapps.com/vkuasgyehy/public_html/node_modules/webpack-cli/bin/utils/convert-argv.js:123:17
at Array.forEach (native)
at module.exports (/home/330475.cloudwaysapps.com/vkuasgyehy/public_html/node_modules/webpack-cli/bin/utils/convert-argv.js:121:15)
at yargs.parse.err (/home/330475.cloudwaysapps.com/vkuasgyehy/public_html/node_modules/webpack-cli/bin/cli.js:71:45)
at Object.parse (/home/330475.cloudwaysapps.com/vkuasgyehy/public_html/node_modules/webpack-cli/node_modules/yargs/yargs.js:567:18)
at /home/330475.cloudwaysapps.com/vkuasgyehy/public_html/node_modules/webpack-cli/bin/cli.js:49:8
at Object. (/home/330475.cloudwaysapps.com/vkuasgyehy/public_html/node_modules/webpack-cli/bin/cli.js:366:3)
at Module._compile (module.js:577:32)
at Object.Module._extensions..js (module.js:586:10)
at Module.load (module.js:494:32)
at tryModuleLoad (module.js:453:12)
at Function.Module._load (module.js:445:3)
at Module.require (module.js:504:17)
at require (internal/module.js:20:19)
at Object. (/home/330475.cloudwaysapps.com/vkuasgyehy/public_html/node_modules/webpack/bin/webpack.js:156:2)
at Module._compile (module.js:577:32)
at Object.Module._extensions..js (module.js:586:10)
at Module.load (module.js:494:32)

npm ERR! Linux 4.9.0-11-amd64
npm ERR! argv "/usr/bin/node" "/usr/bin/npm" "run" "backend-production"
npm ERR! node v6.17.1
npm ERR! npm v3.10.10
npm ERR! code ELIFECYCLE
npm ERR! @ backend-production: cross-env NODE_ENV=production node_modules/webpack/bin/webpack.js --no-progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js --env.mixfile=webpack.backend.mix.js
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the @ backend-production script 'cross-env NODE_ENV=production node_modules/webpack/bin/webpack.js --no-progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js --env.mixfile=webpack.backend.mix.js'.
npm ERR! Make sure you have the latest version of node.js and npm installed.
npm ERR! If you do, this is most likely a problem with the package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR! cross-env NODE_ENV=production node_modules/webpack/bin/webpack.js --no-progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js --env.mixfile=webpack.backend.mix.js
npm ERR! You can get information on how to open an issue for this project with:
npm ERR! npm bugs
npm ERR! Or if that isn't available, you can get their info via:
npm ERR! npm owner ls
npm ERR! There is likely additional logging output above.