gulpfile.js

var gulp = require('gulp');

var gutil = require('gulp-util');
var babel = require('gulp-babel');
var cache = require('gulp-cached');
var remember = require('gulp-remember');
var plumber = require('gulp-plumber');

var source = ["src/**/*.js"]

gulp.task('babel', function() {
	return gulp.src(source)
		.pipe(plumber())
		.pipe(cache("babel"))
		.pipe(babel({presets: ["es2015"]}).on('error', gutil.log)).on('data', gutil.log)
		.pipe(remember("babel"))
		.pipe(gulp.dest("lib/"));
});

gulp.task('watch', function () {
	gulp.watch(source, ['babel']);
});

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