browserslist and sync completion issues with gulp 4

I am trying to debug an error in my GULP file. I cannot seem to work out what exactly I should change. The error is as following:

Replace Autoprefixer browsers option to Browserslist config. Use browserslist key in package.json or .browserslistrc file.

Using browsers option cause some error. Browserslist config can be used for Babel, Autoprefixer, postcss-normalize and other tools.

If you really need to use option, rename it to overrideBrowserslist.

Learn more at: https://github.com/browserslist/browserslist#readme https://twitter.com/browserslist

[12:20:01] The following tasks did not complete: default, compile-scss [12:20:01] Did you forget to signal async completion?

var gulp = require('gulp');

// Include plugins
var path = require('path');
var sass = require('gulp-sass');
var postcss = require('gulp-postcss');
var autoprefixer = require('autoprefixer');
var concat = require('gulp-concat');
var uglify = require('gulp-uglify');
var cleanCSS = require('gulp-clean-css');
var folders = require('gulp-folders');
var imagemin = require('gulp-imagemin');

// Config
var inputFolders =  'src'
var outputFolders = 'public'
var scssInput =     'styles';
var scssOutput =    'styles'
var jsInput =       'scripts';
var jsOutput =      'scripts';
var mediaInput =    'images';
var mediaOutput =   'images';
var backupImageFolder = 'backup-image';

// compile scss into css
gulp.task('compile-scss', folders(inputFolders, function (folder) {
  var processors = [
    autoprefixer({browsers: ['last 2 versions', 'IE 10', 'Safari 8']})
  ];
  return gulp.src(inputFolders + '/' + folder + '/' + scssInput  + '/*.scss')
    .pipe(sass().on('error', sass.logError))
    .pipe(postcss(processors))
    .pipe(cleanCSS({compatibility:'*'}))
    .pipe(gulp.dest(outputFolders + '/' + folder + '/' + scssOutput));
}));

// concatenate and minify js
gulp.task('scripts', folders(inputFolders, function(folder) {
  return gulp.src(inputFolders + '/' + folder + '/' + jsInput + '/*.js')
    // .pipe(concat('scripts.js'))
    .pipe(uglify())
    .pipe(gulp.dest(outputFolders + '/' + folder + '/' + jsOutput));
}));

// deploy any media and losslessly optimise if possible
gulp.task('media', folders(inputFolders, function(folder) {
  return gulp.src([inputFolders + '/' + folder + '/' + mediaInput + '/*', inputFolders + '/' + folder + '/' + mediaInput + '/**/*'])
    .pipe(imagemin())
    .pipe(gulp.dest(outputFolders + '/' + folder + '/' + mediaOutput));
}));

// copy across html
gulp.task('html', folders(inputFolders, function(folder) {
  return gulp.src(inputFolders + '/' + folder + '/*.html')
    .pipe(gulp.dest(outputFolders + '/' + folder))
}));

// copy across back-up image for the banner
gulp.task('copy-backup-image', folders(inputFolders, function(folder) {
  return gulp.src(inputFolders + '/' + folder + '/' + backupImageFolder + '/*' )
  .pipe(gulp.dest(outputFolders + '/' + folder + '/' + backupImageFolder))
}));

// Watch files for changes, run affected tasks
gulp.task('watch', function() {
  gulp.watch([inputFolders + '/*/' + scssInput + '/*.scss'], gulp.series('compile-scss'));
  gulp.watch([inputFolders + '/*/' + scssInput + '/**/*.scss'], gulp.series('compile-scss'));
  gulp.watch([inputFolders + '/*/' + jsInput + '/*.js'], gulp.series('scripts'));
  gulp.watch([inputFolders + '/*/' + mediaInput + '/*'], gulp.series('media'));
  gulp.watch([inputFolders + '/*/' + mediaInput + '/**/*'], gulp.series('media'));
  gulp.watch([inputFolders + '/*/*.html'],  gulp.series('html'));
  gulp.watch([inputFolders + '/*/' + backupImageFolder + '/*'],  gulp.series('copy-backup-image'));
});

// Default task
gulp.task('default', 
gulp.series(gulp.parallel('compile-scss', 'scripts', 'media', 'html', 'copy-backup-image'), 'watch'));

Make a .browserslistrc in your working root or workspace directory with something like this in it:

# Browsers that we support

last 2 versions
IE 10
Safari 8

And then change this code:

var processors = [ autoprefixer({browsers: ['last 2 versions', 'IE 10', 'Safari 8']}) ];

to simply:

var processors = [ autoprefixer() ];

That should get rid of the complaint about the browsers option. Then run your default task to see if that fixed your 'compile-scss' failed to complete error message and report back.

---

Now for the async signal problem. That gulp-folders plugin has not been updated for 5 years so I suggest a different approach.

const glob = require('glob');

    // this gets an array of matching folders
const folders = glob.sync(inputFolders');

gulp.task('compile-scss', () => {

  let stream;

  // work on each folder separately
  folders.forEach(function (folder) {

    stream = gulp.src( inputFolders + '/' + folder + '/' + scssInput  + '/*.scss' )

     // do your other stuff

      .pipe(gulp.dest( outputFolders + '/' + folder + '/' + scssOutput ));
  });
  return stream;
});