Mongoose/Typescript - UnhandledPromiseRejectionWarning When Catch Is Present

I'm not sure why I'm seeing this UnhandledPromiseRejectionWarning. In this code, 'id' is a Mongoose Index, and I am testing inserting a duplicate ID which should be handled proper

Solution 1:

You basically did

var a = promise.then(…);
var b = promise.catch(…);

creating a branch in the chain. If promise is getting rejected now, the catch callback will be called and b will be a fulfilled promise just fine, but the a promise is getting rejected too and nobody handles that.

Instead, you should use both arguments of then and write

Requirement.create({id: id, data: req.body.data, deleted: false})
.then(requirement => {
    res.json(requirement);
}, reason => {
    let err = {'error': reason};
    res.json(err);
});

Solution 2:

catch catches an error from promise but not promise.then(...). If an error is thrown inside then, this would result in unhandled rejection. Even if it's not thrown but is propagated from promise, it is considered uncaught in this promise.

It should be:

promise
.then((requirement) => {
    return res.json(requirement);
})
.catch((reason) => {
    let err = {'error': reason};
    return res.json(err);
});