What is an unhandled promise rejection?

TLDR: A promise has resolve and reject, doing a reject without a catch to handle it is deprecated, so you will have to at least have a catch at top level.

4 years ago

Try not closing the connection before you send data to your database. Remove client.close(); from your code and it'll work fine.

4 years ago

The origin of this error lies in the fact that each and every promise is expected to handle promise rejection i.e. have a .catch(...) . you can avoid the same by adding .catch(...) to a promise in the code as given below.

for example, the function PTest() will either resolve or reject a promise based on the value of a global variable somevar

var somevar = false;

var PTest = function () {

    return new Promise(function (resolve, reject) {

        if (somevar === true)

            resolve();

        else

            reject();

    });

}

var myfunc = PTest();

myfunc.then(function () {

     console.log("Promise Resolved");

}).catch(function () {

     console.log("Promise Rejected");

});

In some cases, the "unhandled promise rejection" message comes even if we have .catch(..) written for promises. It's all about how you write your code. The following code will generate "unhandled promise rejection" even though we are handling catch.

var somevar = false;

var PTest = function () {

    return new Promise(function (resolve, reject) {

        if (somevar === true)

            resolve();

        else

            reject();

    });

}

var myfunc = PTest();

myfunc.then(function () {

     console.log("Promise Resolved");

});

// See the Difference here

myfunc.catch(function () {

     console.log("Promise Rejected");

});

The difference is that you don't handle .catch(...) as chain but as separate. For some reason JavaScript engine treats it as promise without un-handled promise rejection.

4 years ago