Asynchroniczność

database_async.js

const FORCE_ERROR = false;

function countUsersInDatabase() {
    return new Promise(function(resolve, reject) {
        setTimeout(function() {
            if(FORCE_ERROR) {
                return reject(new Error('database error'));
            } else {
                return resolve(44);
            }
        }, 3000);
    });
}

async function runCode() {
    var numberOfUsers;
    try {
        result = await countUsersInDatabase();
        numberOfUsers = result;
        console.log(numberOfUsers);
    } catch(err) {
        console.error(err);
    }
}


var sumOfTwoNumbers;
runCode();
sumOfTwoNumbers = 10 + 5;

console.log(sumOfTwoNumbers);

database_callback.js

const FORCE_ERROR = false;

function countUsersInDatabase(callback) {
    setTimeout(function() {
        if(FORCE_ERROR) {
            return callback(new Error('database error'));
        } else {
            return callback(null, 44);
        }
    }, 3000);
}


var numberOfUsers;
var sumOfTwoNumbers;

countUsersInDatabase(function(err, response) {
    if(err) {
        return console.error(err);
    }
    numberOfUsers = response;
    console.log(numberOfUsers);
});

sumOfTwoNumbers = 10 + 5;

console.log(sumOfTwoNumbers);

database_promise.js

const FORCE_ERROR = false;

function countUsersInDatabase() {
    return new Promise(function(resolve, reject) {
        setTimeout(function() {
            if(FORCE_ERROR) {
                return reject(new Error('database error'));
            } else {
                return resolve(44);
            }
        }, 3000);
    });
}


var numberOfUsers;
var sumOfTwoNumbers;

countUsersInDatabase()
    .then(function(response) {
        numberOfUsers = response;
        console.log(numberOfUsers);
    })
    .catch(function(err){
        console.error(err);
    });

sumOfTwoNumbers = 10 + 5;

console.log(sumOfTwoNumbers);