What kind of errors does this try-catch block handle?

Question

//mongoose connection function

const connectDB = (url) => {
    return mongoose.connect(url, {
        useNewUrlParser: true,
        useUnifiedTopology: true
    })
    .then(() => console.log('Connected to database'))
    .catch(err=>console.log(`DB CONNECTION ERR ${err}`))
}

const port = process.env.PORT || 5000
const mongoURI = process.env.MONGO_URI

//start

const start = async () => {
    try {
        await connectDB(mongoURI)
        app.listen(port, () => console.log(`Server running at ${port}`))
    } catch (error) {
        console.log(error);
    }
    
}

start()

This is my basic server setup. I tried messing with mongoose connection setup and app.listen() too but the catch block (in try-catch) didn't handle anything. Does the catch block only handle the errors we throw?

You already caught all errors in `.catch` method of promise inside `connectDS`, so your `await` don't throw any — Mr. Hedgehog, Oct 08 '21 at 11:39

score 2 · Accepted Answer · answered Oct 08 '21 at 11:40

The try/catch doesn't handle anything because you prevent it from seeing errors with this code in connectDB:

.catch(err=>console.log(`DB CONNECTION ERR ${err}`))

That converts rejection into fulfillment with the value undefined (the return value of console.log).

Just as you should avoid catching exceptions too early, you should avoid handling rejections too early. That's easiest (IMHO) if you consistently use async functions where possible:

const connectDB = async (url) => {          // *** Make it an `async` function
    await mongoose.connect(url, {           // *** Await connection
        useNewUrlParser: true,
        useUnifiedTopology: true
    });
    console.log('Connected to database');   // *** Connection worked
};

const port = process.env.PORT || 5000;
const mongoURI = process.env.MONGO_URI;

//start

const start = async () => {
    try {
        await connectDB(mongoURI);          // ** Now you'll see errors here
        app.listen(port, () => console.log(`Server running at ${port}`));
    } catch (error) {
        console.log(error);
    }
};

start();

score 0 · Answer 2 · answered Oct 08 '21 at 11:50

"promise"(async funcs if you want) - throws error in .catch(), if not specified, it acts as usual
"usual" - throws an error into try{}catch(){} block

const port = process.env.PORT || 5000;
const mongoURI = process.env.MONGO_URI;

//
//
//

mongoose.connect(mongoURI, {
    useNewUrlParser: true,
    useUnifiedTopology: true,
    // useFindAndModify: false,
    // useCreateIndex: true,
}).catch((e)=>{
    console.log('db error', e.message);
});

//
//
//

(async()=>{
    // main code
})();

app.listen(port, ()=>console.log(`Server running at http://localhost:${port}/`));

I refactored the code a little, it was painful to see

Specify local address in the "listen log", good practice, shortcut from the IDE console to browser
Two keys for db, I used them, maybe you will need
Self-executing function, more

What kind of errors does this try-catch block handle?

2 Answers2