gulpfile.js

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

gulp.task('eslint', function () {
    return gulp.src(['src/**/*.js'])
        .pipe(eslint())
        .pipe(eslint.format())
        .pipe(eslint.failAfterError());
});

gulp.task('build', function () {
    return gulp.src('src/**/*.js')
        .pipe(babel({
            presets: [
                ["@babel/preset-env", {
                    "targets": {
                        "browsers": ["last 2 versions"],
                        "node": "8.11.3"
                    }
                }]
            ],
            plugins: [
                "@babel/plugin-transform-runtime"
            ]
        }))
        .pipe(gulp.dest('lib/'))
})

gulp.task('watch', function () {
    gulp.watch(['src/**/*.js'], ['eslint']);
})

gulp.task('default', gulp.parallel('eslint'));
gulp.task('dev', gulp.parallel('eslint', 'watch'));