Gulp 4 error: "Did you forget to signal async completion?" - node.js

Question

I am working on a small project that needs .sass files compiled into .css files. I have used Gulp a while ago and loved it, but my old gulpfile.js does not work anymore, because Gulp has changed since version 4.
I have made a new gulpfile.js:
var gulp = require('gulp'),
sass = require('gulp-sass'),
rename = require('gulp-rename');
var paths = {
styles: {
src: 'src/scss/*.scss',
dest: 'build/css'
}
};
function styles() {
return gulp
.src(paths.styles.src, {
sourcemaps: true
})
.pipe(sass())
.pipe(rename({
basename: 'main',
suffix: '.min'
}))
.pipe(gulp.dest(paths.styles.dest));
}
function watch() {
gulp
.watch(paths.styles.src, styles);
}
var syncConfig = {
server: {
baseDir : './',
index : 'index.html'
},
port : 3000,
open : false
};
// browser-sync
function server() {
init(syncConfig);
}
var build = gulp.parallel(styles, watch, server);
gulp
.task(build);
gulp
.task('default', build);
I have a "Did you forget to signal async completion?" error returned by the console.
Where is my mistake?

Answer 1

There ware two things missing: the browsersync variable browsersync = require('browser-sync').create() and the callback for the server function:
function server(done) {
if (browsersync) browsersync.init(syncConfig);
done();
}
It compiles SASS to CSS, still there is this problem: the page does not reload automatically when changes are made in the main.scss.
Here is the entire code:
var gulp = require('gulp'),
browsersync = require('browser-sync').create(),
sass = require('gulp-sass'),
rename = require('gulp-rename');
var paths = {
styles: {
src: 'src/scss/*.scss',
dest: 'build/css'
}
};
function styles() {
return gulp
.src(paths.styles.src, {
sourcemaps: true
})
.pipe(sass())
.pipe(rename({
basename: 'main',
suffix: '.min'
}))
.pipe(gulp.dest(paths.styles.dest));
}
function watch() {
gulp
.watch(paths.styles.src, styles);
}
var syncConfig = {
server: {
baseDir : './',
index : 'index.html'
},
port : 3000,
open : false
};
// browser-sync
function server(done) {
if (browsersync) browsersync.init(syncConfig);
done();
}
var build = gulp.parallel(styles, watch, server);
gulp
.task(build);
gulp
.task('default', build);