Gulpfile.js

const gulp = require('gulp');
const babel = require('gulp-babel');
const mocha = require('gulp-mocha');
const del = require('del');

gulp.task('clean', function () {
    return del('lib');
});

gulp.task('lint', function () {
    const eslint = require('gulp-eslint');

    return gulp
        .src(['src/**/*.js', 'test/**/*.js', 'Gulpfile.js'])
        .pipe(eslint())
        .pipe(eslint.format())
        .pipe(eslint.failAfterError());
});

gulp.task(
    'build',
    gulp.series('clean', 'lint', function () {
        return gulp.src('src/**/*.js').pipe(babel()).pipe(gulp.dest('lib'));
    }),
);

gulp.task(
    'test',
    gulp.series('build', function () {
        return gulp.src('test/**.js').pipe(
            mocha({
                ui:       'bdd',
                reporter: 'spec',
                timeout:  typeof v8debug === 'undefined' ? 2000 : Infinity, // NOTE: disable timeouts in debug
            }),
        );
    }),
);