Why can I not throw inside a Promise.catch handler?

Question

Why can't I just throw an Error inside the catch callback and let the process handle the error as if it were in any other scope?

If I don't do console.log(err) nothing gets printed out and I know nothing about what happened. The process just ends...

Example:

function do1() {
    return new Promise(function(resolve, reject) {
        throw new Error('do1');
        setTimeout(resolve, 1000)
    });
}

function do2() {
    return new Promise(function(resolve, reject) {
        setTimeout(function() {
            reject(new Error('do2'));
        }, 1000)
    });
}

do1().then(do2).catch(function(err) {
    //console.log(err.stack); // This is the only way to see the stack
    throw err; // This does nothing
});

If callbacks get executed in the main thread, why the Error gets swallowed by a black hole?

Answer 1

As others have explained, the "black hole" is because throwing inside a .catch continues the chain with a rejected promise, and you have no more catches, leading to an unterminated chain, which swallows errors (bad!)

Add one more catch to see what's happening:

do1().then(do2).catch(function(err) {
    //console.log(err.stack); // This is the only way to see the stack
    throw err; // Where does this go?
}).catch(function(err) {
    console.log(err.stack); // It goes here!
});

A catch in the middle of a chain is useful when you want the chain to proceed in spite of a failed step, but a re-throw is useful to continue failing after doing things like logging of information or cleanup steps, perhaps even altering which error is thrown.

Trick

To make the error show up as an error in the web console, as you originally intended, I use this trick:

.catch(function(err) { setTimeout(function() { throw err; }); });

Even the line numbers survive, so the link in web console takes me straight to the file and line where the (original) error happened.

Why it works

Any exception in a function called as a promise fulfillment or rejection handler gets automatically converted to a rejection of the promise you're supposed to return. The promise code that calls your function takes care of this.

A function called by setTimeout on the other hand, always runs from JavaScript stable state, i.e. it runs in a new cycle in the JavaScript event loop. Exceptions there aren't caught by anything, and make it to the web console. Since err holds all the information about the error, including the original stack, file and line number, it still gets reported correctly.