javascript - Unhandled promise rejection despite catching the promise

Question

I don't understand... Is it me or is this a bug in node?

This is fine as expected:

const a = new Promise((resolve, reject) => {
  setTimeout(() => reject('timeout'), 1000);
});
a.catch(console.log);

And this is throwing a warning:

const a = new Promise((resolve, reject) => {
  setTimeout(() => reject('timeout'), 1000);
});
a.then(console.log);
a.catch(console.log);

I get

timeout
(node:40463) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): timeout
(node:40463) [DEP0018] 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.

Answer 1

Annotated and slightly modified source:

// promise A is created
const a = new Promise((resolve, reject) => {
  setTimeout(() => reject('timeout'), 1000);
});

// promise A is chained with .then()
// means new promise is created
// and only resolve catched here
const aThenPromise = a.then(console.log);

// promise A is chained with .catch()
// means new promise is created
// and only reject catched here
const aCatchPromise = a.catch(console.log);

// aThenPromise !== aCatchPromise

When promise a is rejected:

• aCatchPromise works as expected, and timeout is logged to console
• aThenPromise does nothing, as it works only with resolve(), and reject is passed through it, and not handled because it is different Promise. This leads to UnhandledRejection

You need to add catch to aThenPromise,

One possible option is a.then(console.log).catch(console.log) this will handle rejection passed through .then