How can I respond to the frond-end request only after getting expected response from my database server?

Question

I am trying create a pure Node.js HTTP server to connect my AngularJS Front-End to Postgres Database. But I unable to send a valid response to Front-End as the response to the Front-End gets sent before I could response from DB server.

My Back-End server code is as follows.

const http = require('http');
const pg = require('pg');

let menu = [];

const headers = {
    'Access-Control-Allow-Origin': '*',
    'Access-Control-Allow-Methods': 'OPTIONS, POST, GET',
    'Access-Control-Max-Age': 2592000,
    'Content-Type':'application/JSON',
};

http.createServer((req, res) =>{

    client.connect();
    client.query('select * from menu;')
     .then(res => {
        for(row of res.rows) {
            let obj =   {
                            'name': row.name,
                            'price': row.price,
            };
            menu.push(obj);
        }
        console.log(menu);
        client.end().then(()=>{console.log("Database disconnected");});
     }).catch(e => console.error(e.stack));
    console.log(menu);
    res.writeHead(200, headers);
    res.write(JSON.stringify(menu));
    res.end();
}).listen(3500, ()=>{
    console.log("BackEnd Server online");
});

The console logs from by Back-End is as follows. Kindly suggest me a possible fix.

BackEnd Server online
[]
[
  { name: 'Panner Butter Masala', price: 130 },
  { name: 'Dosa', price: 80 },
  { name: 'Roti', price: 40 },
  { name: 'Pulka', price: 30 },
  { name: 'Sambar vada', price: 70 }
]
Database disconnected

As you can see, before I can get a response from my DB, the response to my front-end already gets sent. So, I am unable to get the requested data.

Answer 1

Use async/await to wait for the response and then send to the client

const http = require('http');
const pg = require('pg');

let menu = [];

const headers = {
    'Access-Control-Allow-Origin': '*',
    'Access-Control-Allow-Methods': 'OPTIONS, POST, GET',
    'Access-Control-Max-Age': 2592000,
    'Content-Type':'application/JSON',
};

http.createServer(async (req, res) =>{

    client.connect();
    try {
        const res = await client.query('select * from menu;');
        for(row of res.rows) {
            let obj = {
                'name': row.name,
                'price': row.price,
            };
            menu.push(obj);
        }   
    } catch (e) {
        console.error(e.stack);
    }
    await client.end();
    console.log("Database disconnected")
    res.writeHead(200, headers);
    res.write(JSON.stringify(menu));
    res.end();
}).listen(3500, ()=>{
    console.log("BackEnd Server online");
});