How to migrate gulp.watch from gulp 3.x to gulp 4.x

I am currently working on migrating gulp from 3.x to 4.x I am struck while converting gulp. Below is the gulp.watch code

function isOnlyChange(event) {
    return event.type === 'changed';
  }

  function watch(){
    gulp.watch([path.join(conf.paths.src, '/*.html')], ['inject-reload']);

     gulp.watch([
      path.join(conf.paths.src, '/app/**/*.css'),
      path.join(conf.paths.src, '/app/**/*.scss')
    ], function (event) {
      if (isOnlyChange(event)) {
        gulp.start('styles-reload');
      } else {
        gulp.start('inject-reload');
      }
    });

    gulp.watch([path.join(conf.paths.src, '/app/**/*.js'), path.join(conf.paths.src, '/app/**/*.json')], function (event) {
      if (isOnlyChange(event)) {
        gulp.start('scripts-reload');
      } else {
        gulp.start('inject-reload');
      }
    });

    gulp.watch(path.join(conf.paths.src, '/app/**/*.html'), function (event) {
      browserSync.reload(event.path);
    });
  };

  gulp.task('watch', gulp.series(watch));
})();

I am getting the following error Error: watching src*.html: watch task has to be a function (optionally generated by using gulp.parallel or gulp.series)

Do I need to add series and parallel for each gulp.watch? How to migrate this, is there any sample code available for reference.

The error states that the watch task has to be a function, while yours is an array. Change it to:

gulp.watch([path.join(conf.paths.src, '/*.html')], gulp.series('inject-reload'));

For more info, have a look at this question.