Derived Promises don't propagate reject correctly

[heymatthew]

If you

1. create a deferred object
2. and derive a promise P2 from it's internal promise P1
3. and P2 is rejected
4. then a chain P1.then(P2).then... will not cascade the P2 reject. Instead this will behave like P2 was resolved.

Given the following sample snippet

q = require('q');

var deferredA = q.defer();
var promiseA = deferredA.promise;

var derivedPromiseA = promiseA.then(function() {
  var equation = 1+1;
  throw 'fuuuuuuuuuuuuuu';
});

promiseA
  .then( derivedPromiseA )
  .then(function onSuccess() {
    console.log("Unexpected :(");
  })
  .fail(function onFail() {
    console.log("Expected state");
  })
;

deferredA.resolve('go');

It prints "Unexpected :("

Conversely, if you run

derivedPromiseA
  .then(function onSuccess() {
    console.log("Unexpected :(");
  })
  .fail(function onFail() {
    console.log("Expected state");
  })
;

This prints "Expected State"

The use case for having a longer chain is that you can get explicit dependency declaration despite the redundancy. This becomes more obvious if you have branching dependency chains.

---

Detail:

• Node v0.10.29
• Q v1.0.1

[domenic]

.then takes functions, not promises

---

From: Matthewmailto:notifications@github.com
Sent: ý2014-ý07-ý20 19:49
To: kriskowal/qmailto:q@noreply.github.com
Subject: [q] Derived Promises don't propagate reject correctly (#557)

Givent the following snippet

q = require('q');

var deferredA = q.defer();
var promiseA = deferredA.promise;

var derivedPromiseA = promiseA.then(function() {
var equation = 1+1;
throw 'fuuuuuuuuuuuuuu';
});

promiseA
.then( derivedPromiseA )
.then(function onSuccess() {
console.log("Unexpected :(");
})
.fail(function onFail() {
console.log("Expected state");
})
;

deferredA.resolve('go');

What actually happens is it prints "Unexpected :("

Conversely, if you run

derivedPromiseA
.then(function onSuccess() {
console.log("Unexpected :(");
})
.fail(function onFail() {
console.log("Expected state");
})
;

This prints "Expected State"

The use case for having a longer chain is that you can get explicit dependency declaration despite the redundancy.

---

Detail:

• Node v0.10.29
• Q v1.0.1

—
Reply to this email directly or view it on GitHubhttps://github.com//issues/557.