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,
    },
  }),

Source

UglifyJS does not support es6. const is an es6 declaration, so it throws an error.

What is weird is that the package you use does not transpile its files to es5 to be used anywhere.

If you want to still use UglifyJS (to re-use the configuration for example) use the ES6+ compatible version, uglify-es. (Warning: uglify-es is now abandoned.)

And as Ser mentionned, you should now use terser-webpack-plugin.

I had the same issue and the gulp plugin gulp-uglify-es resolved the problem.

I think it's the simpliest decision.

Just install:

npm i gulp-uglify-es --save-dev

after that in your code change only this line

const uglify = require('gulp-uglify');

to this:

const uglify = require('gulp-uglify-es').default;

N.B. property .default is crucial otherwise you'll have an error that uglify is not a function.

As mentioned above and as being part of ES6 const operator can only be processed by more modern es6 gulp plugin "gulp-uglify-es"

The rest of your code no need to be changed.

Best regards!

I just had this issue with a Gulp project I refactored and for some reason I was having trouble with the official Terser Gulp plugin. This one (gulp-terser) worked with no issues.