gulpjs - "Did you forget to signal async completion?" after switching from uglifyjs to terser

Question

I know there are a lot of questions on this already, but I'm posting because things were working totally fine until I changed from UglifyJS to Terser as I wanted to be able to compress ES6 code.

I'm returning the streams and using done() where relevant as well, yet I am getting this error when running gulp on the command line?

Here is the relevant section where I am using Terser:

var terser_options = {
    compress: {
        keep_fnames: true
    },
    mangle: false,
    keep_classnames: true,
    keep_fnames: true
};

gulp.task('uglify', gulp.series('scripts', function() {

    let stream;

    for (const bundle of bundleFolders) {

        // Get just the last directory of 'js/dev/bootstrap', 'js/dev/lib`, etc.
        let thisBundle = path.basename(bundle);

        if (thisBundle === 'bootstrap') {

            stream = merge2(

                gulp.src(bootstrap_files)
                    .pipe(terser(terser_options))
                    .pipe(concat(thisBundle + '.min.js'))
                    .pipe(gulp.dest('./js'))

            );

        } else {

            stream = gulp.src(bundle + "**/*.js")
                .pipe(gulpif('!**/*.min.js', terser(terser_options)))
                .pipe(concat(thisBundle + '.min.js'))
                .pipe(gulp.dest('./js'));

        }

    }

    return stream;

}));

Here is the majority of the code:

var bootstrap_files = [
    // We have to set the bootstrap lines separately as some need to go before others
    'js/dev/bootstrap/alert.js',
    'js/dev/bootstrap/collapse.js',
    'js/dev/bootstrap/tooltip.js',
    'js/dev/bootstrap/popover.js',
    'js/dev/bootstrap/tab.js',
    'js/dev/bootstrap/transition.js'
];

const bundleFolders = glob.sync('js/dev/*/'); // returns an array of folders

var terser_options = {
    compress: {
        keep_fnames: true
    },
    mangle: false,
    keep_classnames: true,
    keep_fnames: true
};

gulp.task('scripts', () => {

    let stream;

    for (const bundle of bundleFolders) {

        // Get just the last directory of 'js/dev/bootstrap', 'js/dev/lib`, etc.
        let thisBundle = path.basename(bundle);

        if (thisBundle === 'bootstrap') {

            stream = merge2(

                gulp.src(bootstrap_files)
                    .pipe(sourcemaps.init())
                    .pipe(concat(thisBundle + '.js'))
                    .pipe(sourcemaps.write('../maps'))
                    .pipe(gulp.dest('./js'))

            );

        } else {

            stream = gulp.src(bundle + "**/*.js")
                .pipe(sourcemaps.init())
                .pipe(concat(thisBundle + '.js'))
                .pipe(sourcemaps.write('../maps'))
                .pipe(gulp.dest('./js'));

        }

    }

    return stream;

});

gulp.task('uglify', gulp.series('scripts', function() {

    let stream;

    for (const bundle of bundleFolders) {

        // Get just the last directory of 'js/dev/bootstrap', 'js/dev/lib`, etc.
        let thisBundle = path.basename(bundle);

        if (thisBundle === 'bootstrap') {

            stream = merge2(

                gulp.src(bootstrap_files)
                    .pipe(terser(terser_options))
                    .pipe(concat(thisBundle + '.min.js'))
                    .pipe(gulp.dest('./js'))

            );

        } else {

            stream = gulp.src(bundle + "**/*.js")
                .pipe(gulpif('!**/*.min.js', terser(terser_options)))
                .pipe(concat(thisBundle + '.min.js'))
                .pipe(gulp.dest('./js'));

        }

    }

    return stream;

}));

// create a task that ensures the `uglify` task is complete before
// reloading browsers
gulp.task('js-watch', gulp.series('uglify', function (done) {
    browserSync.reload();
    done();
}));

/* Creates the standard version */

gulp.task('styles', function() {
    return gulp.src('scss/**/*.scss')
        .pipe(sourcemaps.init())
            .pipe(sass().on('error', sass.logError))
        .pipe(sourcemaps.write('../maps'))
        .pipe(gulp.dest('./css/'))
        .pipe(browserSync.stream());
});

/* Creates the minified version */

gulp.task('css-minify', gulp.series('styles', function() {
    return gulp.src('scss/**/*.scss')
        .pipe(sourcemaps.init())
            .pipe(sass({
                outputStyle: 'compact' // Options: nested, expanded, compact, compressed
            }).on('error', sass.logError))
            .pipe(postcss([
                autoprefixer({
                    cascade: false
                }),
            ]))
            .pipe(cleanCSS({
                advanced: false,
                aggressiveMerging: false
            }))
            .pipe(rename({suffix: '.min'}))
        .pipe(sourcemaps.write('../maps'))
        .pipe(gulp.dest('./css/'));
}));

gulp.task('browser-sync', function(done) {
    browserSync.init({
        open: 'external',
        proxy: site_url,
        host: site_url,
        // port: 5000,
        browser: "chrome",
    });
    done();
});

gulp.task('watch', gulp.series('browser-sync', function() {
    gulp.watch('scss/**/*.scss', gulp.series('css-minify'));
    gulp.watch('js/dev/**/*.js', gulp.series('js-watch'));
}));

gulp.task('default', gulp.series('js-watch', 'css-minify'));

Edit: I did do a npm install when I added terser as well, so it may have updated gulp, but I have been using 4.* for a while now, specifically this is the version line from my package.json: ^4.0.2

Edit 2: Seems 4.0.2 still is the latest version and hasn't been updated in at least a year, so I would still be on the same version of Gulp.

Edit3: I just updated the gulp.js file on another project to use terser and it seemed to work fine. The gulp.js files are exactly the same.

What is the issue here?

Answer 1

Ok, strange one but figured out the issue.

Unsure why it happened, but it seems it was having issues and giving this error because my js/dev directory was empty.

Not sure how the files got deleted, but fortunately I was able to restore them, but at the end of the day it wasn't a very helpful error message.

If you're getting this error and it doesn't appear to be for the reasons stated by the error message, check your source directory/files actually exist.

Now it works fine.