What is an unhandled promise rejection?

226

For learning Angular 2, I am trying their tutorial.

I am getting an error like this:

(node:4796) UnhandledPromiseRejectionWarning: Unhandled promise rejection (r                                                                                                     ejection id: 1): Error: spawn cmd ENOENT
[1] (node:4796) DeprecationWarning: Unhandled promise rejections are deprecated.
In the future, promise rejections that are not handled will terminate the Node.
js process with a non-zero exit code.

I went through different questions and answers in SO but could not find out what an "Unhandled Promise Rejection" is.

Can anyone simply explain me what it is and also what Error: spawn cmd ENOENT is, when it arises and what I have to check to get rid of this warning?

This question is tagged with javascript angular es6-promise spawn unhandled-exception

~ Asked on 2016-11-09 05:13:47

The Best Answer is


222

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.

~ Answered on 2016-12-16 08:38:08


39

This is when a Promise is completed with .reject() or an exception was thrown in an async executed code and no .catch() did handle the rejection.

A rejected promise is like an exception that bubbles up towards the application entry point and causes the root error handler to produce that output.

See also

~ Answered on 2016-11-09 05:52:13


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