Asynchronous programming sequencing problems in node.js - node.js

Question

I have picked up a project where when a node.js program starts for the first time, no database will exist. The program should create the database tables if they don't already exist.
However, in the sample program below, the data is not inserted if the database did not exist on first run because the select statement fails.
The output of the code below is:
$ node dbtest.js
finished initialise_database
program ended.
select err: { Error: SQLITE_ERROR: no such table: mytable errno: 1, code: 'SQLITE_ERROR' }
successfully created mytable table
database closed.
As you can see from the logging, the code assumes a synchronous execution.
I assume that what is happening is that the node.js runtime system uses different threads to schedule the database functions to run in parallel.
I need the CREATE TABLE command to complete before proceding. How would I achieve this?
Is there some standard way to achieve such a thing in node.js?
code below:
// npm install sqlite3 - to install sqlite3
const sqlite3 = require('sqlite3').verbose();
let db = initialise_database();
check_and_update(db); //Calling this function upon starting the server.
close_database(db);
console.log('program ended.');
function initialise_database() {
//Establishing a database connection.
let db = new sqlite3.Database('database1.db', (err)=>{
if(err) {
return console.error(err.message);
}
});
// // new db always succeeds even if no file exists - if empty file have to generate table structure
db.run("CREATE TABLE IF NOT exists 'mytable' ('num1' INTEGER, 'num2' INTEGER, 'text1' TEXT);", function(err) {
if (err) {
console.log("Create table error: ", err);
}
console.log("successfully created mytable table");
});
console.log("finished initialise_database");
return db;
}
function check_and_update(db) {
db.all("SELECT * FROM mytable", function(err, data){
if(err) {
console.log("select err: ", err);
} else {
db.run('INSERT INTO mytable (num1, num2, text1) VALUES (?, ?, ?)', [1, 2, 'hi guys!!!'], function(err){
if(err)
console.log("insert err: ", err);
});
}
});
}
function close_database(db) {
db.close((err) => {
if (err) {
return console.error(err.message);
}
console.log('database closed.');
});
}

Answer 1

Database requests are asynchronous, you have to deal with them in an asynchronous way.
Which can be :
Callback
Promise
Async/await
Otherwise you will try to perform a request on a database not initialized.
Here is an example using Promise.
const sqlite3 = require('sqlite3').verbose();
let dbPtr = false;
initialise_database()
.then((db) => {
dbPtr = db;
return check_and_update(dbPtr);
})
.then(() => {
close_database(dbPr);
// All is done
console.log('program ended.');
})
.catch((err) => {
// Deal with the error
});
function initialise_database() {
return new Promise((resolve, reject) => {
//Establishing a database connection.
const db = new sqlite3.Database('database1.db', (err) => {
if (err) {
console.error(err.message);
return reject(err);
}
db.run('...', function(err) {
if (err) {
console.log("Create table error: ", err);
return reject(err);
}
console.log("successfully created mytable table");
return resolve(db);
});
});
});
}
function check_and_update(db) {
return new Promise((resolve, reject) => {
db.all('...', function(err, data) {
if (err) {
console.log("select err: ", err);
return reject(err);
}
db.run('...', [1, 2, 'hi guys!!!'], function(err) {
if (err) {
console.log("insert err: ", err);
return reject(err);
}
return resolve();
});
});
});
}
function close_database(db) {
db.close();
}
#EDIT
Looking at the documentation it seems that db.close() do not take a callback in parameter. I've modified the snippet.