How to combine browersync with my build?

Question

gulp.js

// include the gulp module
const gulp = require('gulp');
const browserSync = require('browser-sync').create();
const sass = require('gulp-sass');
const concat = require('gulp-concat');
const autoprefixer = require('gulp-autoprefixer');
const csso = require('gulp-csso');
//The order of including css files
const sassFiles = [
'./app/src/sass/main.sass'
]
//The order of including js files
const jsFiles = [
'./app/src/js/lib.js',
'. /app/src/css/main.js'
]
// gulp.task('serve', ['sass'], function() {
// browserSync.
// gulp.watch("app/src/sass/*.scss", ['sass']);
// gulp.watch("app/*.html").on('change', browserSync.reload);
// });
function styles() {
return gulp.src(sassFiles)
.pipe(sass().on('error', sass.logError))
.pipe(concat('style.css'))
.pipe(autoprefixer({
cascade: false
}))
.pipe(csso({
restructure: false,
sourceMap: true,
debug: true
}))
.pipe(gulp.dest('./app/build/css'))
}
function scripts() {
return gulp .src(jsFiles)
.pipe(concat('
.pipe(gulp.dest('./app/build/js'))
}
gulp.task('styles', styles);
gulp.task('scripts', scripts);
in TERMINAL writes
C:\Users\For_back\Desktop\Gulp-Sass-New>gulp styles
assert.js:374
throw err;
^
AssertionError [ERR_ASSERTION]: Task function must be specified
at Gulp.set [as _setTask] (C:\Users\For_back\Desktop\Gulp-Sass-New\node_modules\undertaker\lib\set-task.js:10:3 )
at Gulp.task (C:\Users\For_back\Desktop\Gulp-Sass-New\node_modules\undertaker\lib\task.js:13:8)
at Object. (C:\Users\For_back\Desktop\Gulp-Sass-New\gulpfile.js:19:6)
at Module._compile (internal/modules/cjs/loader.js:959:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:995:10)
at Module.load (internal/modules/cjs/loader.js:815:32)
at Function.Module._load( internal/modules/cjs/loader.js:727:14)
at Module.require (internal/modules/cjs/loader.js:852:19)
at require (internal/modules/cjs/helpers.js:74:18)
at execute (C:\Users\For_back\AppData\Roaming\npm\node_modules\gulp\node_modules\gulp-cli\lib\versioned\^4.0.0\index.js:36:18) {
generatedMessage: false,
code: 'ERR_ASSERTION',
actual: false,
expected: true,
operator: '=='
}
C:\Users\For_back\Desktop\Gulp-Sass-New>