CODEWITHSUNDEEP
node.jsasync-await
Mr Wannabe
Mr Wannabe

Reputation: 151

logging errors in async functions

I have this code:

async function getURL() {
   try {
       await fetch("http://www.blah.com");
       return 0;
   } catch (err) {
       return err;
   }
}

getURL().then( result => {
    if (result === 0) console.log("success");
    else console.log(result);
});

The fetch will fail and the error is logged to the console. How do I rework the code so it uses async and try/catch everywhere? That is, I'm looking to avoid doing getURL().then for the sake of consistency.

EDIT:

For those downvoting me, await getURL() won't work as it's invalid syntax.

EDIT2:

Tried this but it didn't catch the error:

async function getURL() {
    return await fetch("http://www.blah.com");
}

let result = async function() {return await getURL();}

try {
    result();
} catch (e) {
    console.log(e);
}

Upvotes: 0

Views: 1148

Answers (3)

Mr Wannabe
Mr Wannabe

Reputation: 151

I realize now async functions always return a promise. Even if you throw an error it still gets wrapped up into a promise. Therefore using try/catch won't help. This is how I ended up writing the code:

async function getURL() {
    return await fetch("http://fake");
 }

 getURL().then( () => console.log("success")).catch( (e) => console.log(e));

Upvotes: 0

NAVIN
NAVIN

Reputation: 3317

Every time you need to catch an error from promise, either using new Promise, async-await or generator you need to use .then() or you can do something like this another async-await.

async function getURL() {
    try {
        await fetch("http://www.blah.com");
        return 0; // EDIT: just returning value which is success
    } catch (err) {
        return err; // EDIT: returning value not rejecting a promise
    }
}

async function main () {
    try {
        let result = await getURL();
        if (result === 0) console.log("success");
        console.log(result); // EDIT: error will be print.
    }
    catch (err) { // EDIT: getURL() never rejects so always success.
        console.log(err);
    }
});

main();

This situation doesn't really occurs as while our main function in server-side or client-side are async and handling this for us.

Like using express:

app.post('/api', async (req, res) => {
    try {
        let result = await getURL();
        res.send(async);
    }
    catch(err) {
        res.send(err);
    }
});

EDIT: asyn-await doesn't reject or resolve a call, just return a value. thus must be used carefully.

function fetch(url) {
    return new Promise( (resolve, reject) => {
        let x = Math.floor(Math.random() * Math.floor(9) + 1);
        // 50-50 resolve or reject
        if(x%2===0) return resolve(false); //resolve with `false` statement
        reject(true); // reject with `true` still a reject
    });
}

async function getURL() {
    try {
        await fetch("http://www.blah.com"); 
        return 0; // if fetch resolve
    } catch (err) { //only if fetch reject
        return err;
    }
}

async function main () {
    try {
        let result = getURL();
        if (result === 0) console.log("success"); //getURL never reject any call
        console.log(result);
    }
    catch (err) { // getURL doesnt reject
        console.log(err);
    }
};

main();

Upvotes: 1

Baboo
Baboo

Reputation: 4278

You can wrap your whole code inside an instantly executed async function like this:

// service.js
async function getURL() {
  return await fetch("http://www.blah.com");
}

// your.module.js
(async function() {
  // do things...

  try {
    let result = await getURL();
  } catch (e) {
    console.log(e);
  }

  // do things...

  res.send({});
});

Upvotes: 1

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