If function return does not recognize true, only false

Question

I have this code below, which was to if the function returns true, it runs if. But it only executes if if I change if to false, that is, by putting "if(!linkAlreadySent)" instead of "if(linkAlreadySent)". Does anyone know how to point out the problem?

var link = "http://www.google.com";

function linkAlreadySent(link) {
    mysql.handle.query('SELECT link FROM links WHERE link = ?', [link], function (err, res) {
        if(res.length == 1) {
            console.log('Retornando true, o link ja existe');
            return true;
        } else {
            console.log('Retornando false, o link não existe');
            return false;
        }
    });
}

if(linkAlreadySent(link)) {
  console.log('Entrou no if');
}

Answer 1

The problem is that your return is within the function of callback of mysql.handle.query. The callback function is returning a value, but linkAlreadySent nay.

Well, actually, linkAlreadySent is returning undefined, because every function without an explicit return returns undefined in JS. undefined is a "falsy" value, it is treated as false when you use it with condition. So how linkAlreadySent returns undefined, and you use this return in if(linkAlreadySent(link)) you never fall inside the if.

---

How to deal with the problem?

It’s not as simple as returning the return of mysql.handle.query. The thread JS main does not wait for input and output processes to continue running the code. When you query the database, the JS keeps running the function until the end, exits the function, and eventually when the database returns a response, the JS executes the callback function you declared. By now it is too late to return a value for the function linkAlreadySent, 'Cause she’s done rolling.

---

The functional approach

You can pass a function to linkAlreadySent execute when it receives the response from the database. As follows:

var link = "http://www.google.com";

function linkAlreadySent(link, callback) {
    mysql.handle.query('SELECT link FROM links WHERE link = ?', [link], function (err, res) {
        if(res.length == 1) {
            console.log('Retornando true, o link ja existe');
            // A função é invocada se você entrar na condição desejada
            callback(res);
        } else {
            console.log('Retornando false, o link não existe');
        }
    });
}

// Passo o link e uma função anonima como parametros
linkAlreadySent(link, (res) => {
    console.log('Entrou no "if"');
});

---

The procedural approach

With the use of promises, you can "pause" the function (but not the thread main) until you get the resolution of promise. Promise is basically an object that packs an answer, when you get the resolution of the answer, just unpack it from promise. We do it with the operator await.

// Módulo do NodeJS
var util = require('util');

var link = "http://www.google.com";

// Crio uma nova função que retorna promises
var mysqlQuery = util.promisify(mysql.handle.query);

// Funções precisam do modificador async para poderem utilizar await
// Toda função do tipo async retorna resultados empacotados em promises
async function linkAlreadySent(link) {
    // Espero pela resposta
    var res = await mysqlQuery('SELECT link FROM links WHERE link = ?', [link]);

    if (res.length == 1) {
        console.log('Retornando true, o link ja existe');
        return true;
    } else {
        console.log('Retornando false, o link não existe');
        return false;
    }
}

async verificaLink() {
    // Novamente, utilizamos await, pois o retorno de linkAlreadySent está empacotado
    if (await linkAlreadySent(link)) {
        console.log('Entrou no if');
    }
}

verificaLink();