gulpfile.babel.js

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

function compile(watch) {

    var props = {
        debug: true,
        entries: './src/index.js'
    };

    var browserifyTransform = browserify(props).transform("babelify",
        {presets: ["es2015", "react"]},
        {plugins: "transform-regenerator"});
    var bundler = watch ? watchify(browserifyTransform): browserifyTransform;

    function rebundle() {
        console.log('-> bundling...');
        bundler.bundle()
            .on('error', function(err) { console.error(err); this.emit('end'); })
            .pipe(source('build.js'))
            .pipe(buffer())
            .pipe(sourcemaps.init({ loadMaps: true }))
            .pipe(sourcemaps.write('./'))
            .pipe(gulp.dest('./build'));
    }

    if (watch) {
        bundler.on('update', function() {
            rebundle();
        });
    }

    rebundle();
}

function watch() {
    return compile(true);
}

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

gulp.task('default', ['watch']);