gulpfile.js

const gulp = require('gulp'),
    terser = require('gulp-terser'),
    typescript = require('gulp-typescript'),
    rename = require('gulp-rename'),
    replace = require('gulp-replace');

const transpile = (target, module) => {
    return (
        gulp
            .src('src/*.ts')

            // First, we transpile back to JS.
            .pipe(
                typescript({
                    target: target,
                    module: module
                })
            )

            // Next, uglify it.
            .pipe(
                terser({
                    output: {
                        comments: '/^!/'
                    }
                })
            )
    );
};

/** Save plugin to be used without UMD pattern or ES6 module. */
const js = cb => {
    return transpile('es5', 'es6')
        .pipe(rename('dotdotdot.js'))
        .pipe(replace('export default Dotdotdot;', ''))
        .pipe(gulp.dest('dist'));
};

/** Save plugin to be used as an ES6 module. */
const jsES6 = cb => {
    return transpile('es6', 'es6')
        .pipe(rename('dotdotdot.es6.js'))
        .pipe(gulp.dest('dist'));
};

/** Save plugin to be used with bundlers that support the pkg.module definition. */
const jsESM = cb => {
    return transpile('es5', 'es6')
        .pipe(rename('dotdotdot.esm.js'))
        .pipe(gulp.dest('dist'));
};

/** Save plugin to be used with UMD pattern. */
const jsUMD = cb => {
    return transpile('es5', 'umd')
        .pipe(rename('dotdotdot.umd.js'))
        .pipe(gulp.dest('dist'));
};

exports.default = gulp.parallel(js, jsES6, jsESM, jsUMD);

// Watch task 'gulp watch': Starts a watch on JS tasks
const watch = cb => {
    gulp.watch('src/*.ts', gulp.parallel(js, jsES6, jsUMD));
    cb();
};
exports.watch = watch;