Gulpfile.js

var gulp = require('gulp');
var sass = require('gulp-sass');
var rename = require('gulp-rename');
var babel = require('babelify');
var browserify = require('browserify');
var source = require('vinyl-source-stream');
var watchify = require('watchify');

gulp.task('styles', function() {
    return gulp.src('index.scss')
        .pipe(sass())
        .pipe(rename('app.css'))
        .pipe(gulp.dest('public'));
});

gulp.task('assets', function() {
    return gulp
        .src('assets/*')
        .pipe(gulp.dest('public'));

});



function compile(watch) {
    var bundle = watchify(browserify('./src/index.js', { debug: true }));

    function rebundle() {
        bundle
            .transform(babel)
            .bundle()
            .on('error', function(error) { console.log(error);
                this.emit('end') })
            .pipe(source('index.js'))
            .pipe(rename('app.js'))
            .pipe(gulp.dest('public'));
    }

    if (watch) {
        bundle.on('update', function() {
            console.log('--> Bundling...');
            rebundle();
        });
    }
    rebundle();
}
/*
gulp.task('scripts', function() {
    return browserify('./src/index.js')
        .transform(babel)
        .bundle()
        .pipe(source('index.js'))
        .pipe(rename('app.js'))
        .pipe(gulp.dest('public'));
})
*/
gulp.task('build', function() {
    return compile();
});

gulp.task('watch', function() {
    return compile(true);
});

gulp.task('default', gulp.parallel('styles', 'assets', 'build'))