How to slow down javascript execution until callback is received?

Question

JavaScript

In order to get the data for the page, the server sends a request to the database. The result comes in the form of a callback, with a variable that stores the JSON object.
The JSON object must be used when rendering the page. But at startup, the page renders faster than the callback arrives. How can I pause a function until a Callback is received?
javascript code:

var Datastore, JPug, Pug, app, async, db, engine, findPage, koa, page;

async = require('async');
Datastore = require('nedb');

db = {};
db.pages = new Datastore({
    filename: 'DB/pages/index.db',
    autoload: true
});

koa = require('koa');
app = koa();

Pug = require('koa-pug');
engine = new Pug({
    viewPath: './views',
    debug: false,
    pretty: true,
    compileDebug: false,
    locals: {},
    basedir: __dirname + '/public',
    app: app
});

findPage = function(req, json) {
    db.pages.find({
        url: req
    }, function(err, docs) {
        console.log(docs);
        json = docs; // Искомые данные для страницы
    });
};

app.use(function *() {
    'use strict';
    var json, req;
    req = this.request.url;
    console.log(req);
    async.series([
        findPage(req, json), 
        page = this.render('index', {page: json}, true) // рендер страницы
    ], function() {
        return page;
    });
});

app.listen(3000);

When running the code, a rendering error occurs, because the json variable is not defined. And then the received callback is issued:

I hope for your ideas and comments. Thanks in advance.

Answer 1

No way.
The request is executed asynchronously. You need to start rendering after receiving the response, or rewrite everything so that this data is optional at the time the render starts.

Answer 2

findPage = function(req, json, cb) {
    db.pages.find({
        url: req
    }, function(err, docs) {
        if (err) cd(err);
        console.log(docs);
        json = docs; // Искомые данные для страницы
        cb(null, json)
    });
};

app.use(function *() {
    'use strict';
    var json, req;
    req = this.request.url;
    console.log(req);
    findPage(req, json, function(err, data) {
        if (err) throw err;
        page = this.render('index', {page: data}, true) // рендер страницы
    });
});

And you are using async here, it would be used for sequential execution of tasks.
https://github.com/caolan/async#seriestasks-callback