How to minify ES6 functions with gulp-uglify?

You can leverage gulp-babel as such...

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

gulp.task('minify', () => {
  return gulp.src('src/**/*.js')
    .pipe(babel({
      presets: ['es2015']
    }))
    .pipe(uglify())
    // [...]
});

This will transpile your es6 early in the pipeline and churn out as widely supported "plain" javascript by the time you minify.

May be important to note - as pointed out in comments - the core babel compiler ships as a peer dependency in this plugin. In case the core lib is not being pulled down via another dep in your repo, ensure this is installed on your end.

Looking at the peer dependency in gulp-babel the author is specifying @babel/core (7.x). Though, the slightly older babel-core (6.x) will work as well. My guess is the author (who is the same for both projects) is in the midsts of reorganizing their module naming. Either way, both npm installation endpoints point to https://github.com/babel/babel/tree/master/packages/babel-core, so you'll be fine with either of the following...

npm install babel-core --save-dev

or

npm install @babel/core --save-dev

The accepted answer doesn't really answer how to minify straight es6. If you want to minify es6 without transpiling, gulp-uglify v3.0.0 makes that possible:

Update March 2019

Using my original answer, you definitely want to replace the uglify-es package with terser, as it seems uglify-es is no longer being maintained.

Original answer, still works:

1.) First, upgrade your gulp-uglify package to > 3.0.0 If you're using yarn and want to update to the latest version:

yarn upgrade gulp-uglify --latest

2.) Now you can use uglify-es, the "es6 version" of uglify, as so:

const uglifyes = require('uglify-es');
const composer = require('gulp-uglify/composer');
const uglify = composer(uglifyes, console);

gulp.task('compress', function () {
    return gulp.src('src/*.js')
    .pipe(uglify())
    .pipe(gulp.dest('dist'))
});

For more info: https://www.npmjs.com/package/gulp-uglify

You actually can uglify ES6 code without transpilation. Instead of gulp-uglify plugin, use gulp-uglifyes plugin.

var gulp = require('gulp');
var rename = require('gulp-rename'); 
var uglify = require('gulp-uglifyes');
var plumber = require('gulp-plumber');
var plumberNotifier = require('gulp-plumber-notifier');
var sourcemaps = require('gulp-sourcemaps');
var runSequence = require('run-sequence').use(gulp);

gulp.task('minjs', function () {
  return gulp.src(['/dist/**/*.js', '!/dist/**/*.min.js'])
    .pipe(plumberNotifier())
    .pipe(sourcemaps.init())
    .pipe(uglify({ 
       mangle: false, 
       ecma: 6 
    }))
    .pipe(rename(function (path) {
      path.extname = '.min.js';
    }))
    .pipe(sourcemaps.write('.'))
    .pipe(gulp.dest('/dist'));
});

gulp-uglify:

For ES6 and newer.

1. install: npm install --save-dev gulp-uglify
2. install: npm install --save-dev gulp-babel @babel/core @babel/preset-env

Usage:

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

gulp.task('script', () => {
    return gulp.src('src/**/*.js')
        .pipe(babel({
            presets: ['@babel/env']
        }))
        .pipe(uglify())
        .pipe(gulp.dest('src/dist'))
});