how to uglify javascript classes?

Question

I'm writing basic javascript using the classes syntax and trying to uglify it with gulp, but getting this error:

events.js:72
throw er; // Unhandled 'error' event

I simply used the example code from mozilla's website:

class Polygon {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }
}

And nothing special about my gulp file - works perfectly for any non-class js:

// Include gulp
var gulp = require('gulp');

// Include Our Plugins
var jshint = require('gulp-jshint');
var concat = require('gulp-concat');
var uglify = require('gulp-uglify');
var rename = require('gulp-rename');

// Lint Task
gulp.task('lint', function() {
    return gulp.src('js/*.js')
        .pipe(jshint())
        .pipe(jshint.reporter('default'));
});

// Concatenate & Minify JS
gulp.task('scripts', function() {
    return gulp.src('js/*.js')
        .pipe(concat('all.js'))
        .pipe(gulp.dest('dist/js'))
        .pipe(rename('all.min.js'))
        .pipe(uglify())
        .pipe(gulp.dest('dist/js'));
});

// Watch Files For Changes
gulp.task('watch', function() {
    gulp.watch('js/*.js', ['lint', 'scripts']);
    gulp.watch('scss/*.scss', ['sass']);
});

// Default Task
gulp.task('default', ['lint', 'scripts', 'watch']);

Answer 1

At the time, I resolved this with a workaround by transpiling to ECMA 5 with Babel.

Later I discovered it was not a problem with my Node environment as the comments led me to believe, but in fact it was because the default uglify branch used in gulp-uglify doesn't support ECMA 6 yet.

The correct solution was to instead use the harmony branch of UglifyJS2 or to use the npm wrapper uglify-js-harmony.