unhandledRejection nodejs

Question

I know there's quite a lot of posts about this error, most of them have the same answer but somehow I am still getting the warning.

I have read something like this In Node.js 7 what is the proper way to suppress UnhandledPromiseRejectionWarning? but instead of on I use once because of the event listener leak but somehow I still see the warning sometimes

I do want to get ride of the warning or solve it since its saying deprecated in the future but not sure when.

At first when I first run I would get this first

You have triggered an unhandledRejection, you may have forgotten to catch a Promise rejection: myrejectionmessage

then after, I will get this error

UnhandledPromiseRejectionWarning: myrejectionmessage UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 5)

this is my original code without what I tried with the posts I found, I am trying to get some files in aws s3 bucket but there is possible that the file in bucket does not exist

this function is to compare if there is file then compare the modified time, if the file does not exist reject

exports.compareObjectMT = (s3, Key, getFileStat) => {
    const s3GetParams = {
        Bucket: process.env.S3_BUCKET,
        Key,
    };

    return new Promise((res, rej) => {
        s3.getObject(s3GetParams, (err, data) => {
            if (err) rej('myrejecterror');
            if (data) {
                res(String(getFileStat.mtimeMs) === data.Metadata.mtimems);
            }
            res(false);
        });
    });
};

Thanks in advance for any suggestions

This is how I am using the function

exports.s3Put = async (path) => {
    try {
        fs.readFile(path, async (err, fileBinary) => {
            if (err) throw err;
            // console.log(data, 'data');
            const s3 = new AWS.S3();
            const Key = path.replace(process.env.WATCH_PATH, '');
            const getStat = await getFileStat(path);
            console.log(getStat, 'getstateeeeeeeeeeeeeeee');
            const compareObj = await compareObjectMT(s3, Key, getStat);
            console.log(compareObj, 'compareObj');
        });
    } catch (e) {
        console.log(e, 'errorrrrrrrrrrrrr');
    }
};

Solution

//calling compareObjectMT ,Your return value is a Promise Object either resolve/reject

//s3, Key, getFileStat aruments value you are passing

compareObjectMT(s3, Key, getFileStat).then((value)=>{do something}) 
                                     .catch((err)=>console.error(err))

what you are doing similar to this..Your callback after reading File inside try catch..It wont catch reject error from await

you mightput all await inside single try catch block

exports.s3Put = async (path) => {
try {
    fs.readFile(path, async (err, fileBinary) => {
        if (err) throw err;
        // console.log(data, 'data');
         try {
        const s3 = new AWS.S3();
        const Key = path.replace(process.env.WATCH_PATH, '');
        const getStat = await getFileStat(path);
        console.log(getStat, 'getstateeeeeeeeeeeeeeee');
        const compareObj = await compareObjectMT(s3, Key, getStat);
        console.log(compareObj, 'compareObj');
      }catch (e) {
    console.log(e, 'errorrrrrrrrrrrrr');
}
    });
} catch (e) {
    console.log(e, 'errorrrrrrrrrrrrr');
}

};

Comments

[CertainPerformance]

You need the consumer of compareObjectMT to catch errors.

[Dora]

@CertainPerformance I did try wrapping the inside of the function where const s3Get.....until the end of the function with try catch though

[CertainPerformance]

Not with try/catch, but with .catch, the Promise method

[Dora]

I think I know why I keep on getting the warning now, because I wanted to use async and await and I kept on doing const co = await compareObjectMT(s3, Key, getFileStat) If I want to do it this way, is it possible?

[Biswadev]

await must be used inside async function, need to put try{const co = await compareObjectMT(s3, Key, getFileStat)}catch(e){}

[Dora]

I edited my code to show how i actually used the compareObjectMT

[Biswadev]

const compareObj = await compareObjectMT(s3, Key, getStat); for this you donot have any error handler,it wont get caught outside catch block,for that you have to throw error,for outside catch block you have added

[Dora]

Thank you! I believe that worked like a charm. I always thought just one try / catch would be able to catch everything even if nested but this time it's because it's within another fs.readFile() that's why it's not catching it if I understand correctly. I will just have to figure the rest out since somehow doesn't work as expected anymore :|