How to solve this minification error on Gulp?

UglifyJS does not currently support EcmaScript 6 structures like classes.

You'll probably need to run your JavaScript through a transpiler step first, or find a minifier that knows what to do with ES6 code.

Update 2017-06-17

The branch of UglifyJS that is designed to work with ES6 is now published as uglify-es on npm.

Update 2018-09-10

terser is the new uglify-es, uglify-es is no longer maintained.

If using gulp both npmjs gulp-uglify-es and npmjs gulp-terser packages support terser.

npm install gulp-terser --save-dev

const gulp = require('gulp');
const terser = require('gulp-terser');
 
function es(){
  return gulp.src('./src/index.js')
    .pipe(terser())
    .pipe(gulp.dest('./build'))
}

gulp.task('default', es);

If you run into this problem and you have in fact a transpiler step like Babel, make sure that you include the proper Babel preset in you .babelrc file. Otherwise Babel will simply leave your code as is.

E.g.

{
  "presets": ["es2015"]
}