gulpfile.js

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

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

gulp.task('browser-sync', ['sass'], function(){
    browserSync({
        server: {
            baseDir: 'app'
        },
    })
});

gulp.task('watch', ['browser-sync'], function(){
    gulp.watch('scss/*.scss', ['sass']);
    gulp.watch('app/*.html', browserSync.reload );
});


gulp.task('default', ['watch'])

https://css-tricks.com/gulp-for-beginners/