CODEWITHSUNDEEP
javascriptasync-await
richbai90
richbai90

Reputation: 5204

Async/Await Expected Behavior

I'm having trouble wrapping my head around async/await syntax

What I can't seem to figure out is why in this code, does "connected" get logged to the console even when there is an error in the connection to mysql.

mongoose.Promise = Promise;
function connection() {
    //return a promise to be consumed in an async function
    return new Promise((resolve, reject) => {
        mongoose.connect('mongodb://localhost/testdatabase');
        let db = mongoose.connection;
        let sw = mysql.createConnection({
            host: 'localhost',
            user: 'reporting',
            Promise: Promise,
            database: 'testdatabase'
        });

        //Only resolve if both database connections are made successfully
        sw.then(() => {
            db.on('error', (err) => {
                reject(err);
            });
            db.once('open', () => {
                resolve(db);
            });
        }).catch((e) => {

            reject(e);
        })
    });
}

//Await connection and return true or false for more synchronous code style
async function connect() {
    let connected = false;
    try {
        //should pause execution until resolved right?
        await connection();
        connected = true;

    } catch (e) {
        //Makes it here successfully
        console.error(e);
        connected = false
    } finally {
        return connected
    } 
}

if (connect()) {
    //This code also gets fired?
    console.log('connected');
}

Upvotes: 2

Views: 1766

Answers (2)

nem035
nem035

Reputation: 35481

Async functions return promises so this

if (connect()) { // <--- connect() will return a promise
  // and a promise always evaluates to a truthy value, 
  // that's why this code always runs
  console.log('connected');
}

Could be:

connect().then(isConnected => {
  if (isConnected) {
    console.log('connected');
  }
});

Or you could await for the promise returned from connect() as well, as Bergi demonstrated in his answer.

Upvotes: 1

Bergi
Bergi

Reputation: 664307

Remember that async functions are still asynchronous, they will not block anything or magically wait before they return. Calling connect() always returns a promise which is always truthy so "connected" will get logged immediately in every case.

If you want to wait for the connection, you will have to await the result and wrap the if statement in an async function as well:

async function connect() {
    try {
        await connection();
        return true;
    } catch (e) {
        console.error(e);
        return false
    } 
}

(async function() {
    if (await connect()) {
//      ^^^^^
        console.log('connected');
    }
}());

Upvotes: 3

Related Questions