New promise reject?

[Kevin Burton]

OK. Something I don't understand about promises. Here is the code:

function A() {
 return new Promise(function(resolve, reject) {
 setTimeout(function () {
 reject(1000);
 }, 1000);
 });
}
function B() {
 return new Promise(function(resolve, reject) {
 setTimeout(function () {
 reject(2000);
 }, 2000);
 });
}
function C() {
 return new Promise(function(resolve, reject) {
 setTimeout(function () {
 reject(3000);
 }, 3000);
 });
}
function D() {
 return new Promise(function(resolve, reject) {
 setTimeout(function () {
 reject(4000);
 }, 4000);
 });
}

function ABCD() {
 var workqueue = [
 A,
 B,
 C,
 D
 ];
 return Promise.all(workqueue.map(function(work) {
 return work();
 })).catch(function(err) {
 return err;
 });
}

ABCD().then(function(result) {
 console.log(result);
}).catch(function(err) {
 console.log(err);
});

Each function is called but my understanding is that when any (the first) is rejected then the 'all' promise is in error. What I am seeing from the above is that instead of the result showing up in the catch block the "rejection" shows up in the normal resolve. How can I get it to show up in the catch as an error?

Thank you.

[Kevin Burton]

Sorry I changed it to:

return Promise.all(workqueue.map(function(work) {
     return work().then(function(result) {
         return result;
     }).catch(function(err) {
         return err;
     });
})).catch(function(err) {
     return err;
});

Now it returns an array of rejection values. I would expect only one rejection.

[Tom Robinson]

The ".catch()" calls are catching the rejections, just like a "catch" clause in a normal try/catch would do, i.e. your revised version is equivalent to this:

try {
    return workqueue.map(function(work) {
        try {
            return work();
        } catch (err) {
            return err;
        }
    });
} catch (err) {
    return err;
}

Remove both calls to "catch" to allow the error to propagate: https://tonicdev.com/tlrobinson/promise-example 

Alternatively you can re-throw the error inside the catch clause.

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[Kevin Burton]

How do I "rethrow the error " ? Is throw ... Equivalent to reject? Thank you.

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[Tom Robinson]

Yes, "throw x;" inside a "then" or "catch" handler will cause the resulting promise to be rejected with x, analogous to re-throwing an error in a regular catch clause.

I recommend reading a detailed tutorial on promises, like this: http://www.html5rocks.com/en/tutorials/es6/promises/