UglifyJS throws unexpected token: keyword (const) with node_modules

As ChrisR mentionned, UglifyJS does not support ES6 at all.

You need to use terser-webpack-plugin for ES6 (webpack@5 will use this plugin for uglification)

npm install terser-webpack-plugin --save-dev

Then define in your plugins array

const TerserPlugin = require('terser-webpack-plugin')

  new TerserPlugin({
    parallel: true,
    terserOptions: {
      ecma: 6,
    },
  }),

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