gulpfile.js

'use strict';

var gulp = require('gulp'),
    sass = require('gulp-sass'),
    browserSync = require('browser-sync');

gulp.task('sass', function () {
    return gulp.src('./css/*.scss')
        .pipe(sass())
        .pipe(gulp.dest('./css'));
});

gulp.task('sass:watch', function () {
    // gulp.watch('./css/*.scss', ['sass']);
    gulp.watch('./css/*.scss', gulp.series(['sass']));
});

gulp.task('browser-sync', function () {
    var files = [
        './*.html',
        './css/*.css',
        './img/*.{png,jpg,gif}',
        './js/*.js'
    ];

    browserSync.init(files, {
        server: {
            baseDir: "./"
        }
    });

});

// Default task
// gulp.task('default', ['browser-sync'], function () {
//     gulp.start('sass:watch');
// });

gulp.task('default', gulp.series(['browser-sync'], function(){
    gulp.start('sass:watch');
}));