UglifyJS throws unexpected token: keyword (const) with node_modules

A small project I started make use a node module (installed via npm) that declares const variables. Running and testing this project is well, but browserify fails when UglifyJS is executed.

Unexpected token: keyword (const)

Here is a generic Gulp file that I have successfully been using for a few other past projects without this issue (i.e. without that particular node module).


'use strict';

const browserify = require('browserify');
const gulp = require('gulp');
const source = require('vinyl-source-stream');
const derequire = require('gulp-derequire');
const buffer = require('vinyl-buffer');
const uglify = require('gulp-uglify');
const sourcemaps = require('gulp-sourcemaps');
const gutil = require('gulp-util');
const path = require('path');
const pkg = require('./package');
const upperCamelCase = require('uppercamelcase');

const SRC_PATH = path.dirname(pkg.main);
const DIST_PATH = path.dirname(pkg.browser);

const INPUT_FILE = path.basename(pkg.main);
const OUTPUT_FILE = path.basename(pkg.browser);

const MODULE_NAME = upperCamelCase(;

gulp.task('default', () => {
  // set up the browserify instance on a task basis
  var b = browserify({
    entries: INPUT_FILE,
    basedir: SRC_PATH,
    transform: ['babelify'],
    standalone: MODULE_NAME,
    debug: true

  return b.bundle()
    .pipe(sourcemaps.init({loadMaps: true}))
    .on('error', gutil.log)

I have tried fixing this by replace all const to var in that npm-installed module, and everything is fine. So I do not understand the failure.

What's wrong with const? Unless someone uses IE10, all major browsers support this syntax.

Is there a way to fix this without requiring a change to that node module?


I have temporarily (or permanently) replaced UglifyJS with Butternut and seem to work.


  • 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,