I know that Promises in JS are Immutable.
According to: What does it mean for promises to be immutable and their guaranteed value?
A promise is a one-way latch. Once it is resolved with a value or
rejected with a reason, its state and value/reason can never change.
So, no matter how many times you do .then() on the same promise, you
will always get the same result.
So I am confused about this image from: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise
It looks like after the Promise is either fulfilled or rejected (resolved) it returns a Promise that is (pending). Why is that?
Every call to .then() or .catch() returns a new promise. That promise will be resolved or rejected depending upon what happens in the .then() or .catch() handler that it derives from when that handler is called. The promise at the start of the chain is indeed immutable, but once someone else calls .then() or .catch() on it, they get a new promise back from each of those calls whose state is determined by what happens inside the code that runs in the .then() or .catch() handler.
When you get that new promise from .then() or .catch() it will always be in the pending state because the code inside the .then() or .catch() has not yet been called (the original promise has not yet called them). Those statements just register handlers on the original promise.
When those handlers are called, they will determine the state of the new promise. If they return a plain value, that new promise will be resolved with that value. If they throw, that new promise will be rejected. If they return a promise, then that new promise will follow the eventual state of that new promise that was returned form the .then() or .catch() handler and it will resolve or reject as that returned promise does.
Related
I'm getting confused with the different terminology thrown around. From my understanding, a promise can be:
fulfilled
rejected
pending
settled
resolved
defer
Does resolve mean settled? or does it mean its fulfilled? and what the heck is defer?
Terminology can be hard.
Let's take from the Promises/A+ specification and the respective ES6 section that there are 3 states:
pending - the promise does not have taken a value yet, it's future is still uncertain.
fulfilled - the promise successfully got a result value "assigned"
rejected - the promise is given a reason why no result could be acquired, typically an error.
The term settled is a hyperonym for fulfilled and rejected, meaning either - the opposite of pending.
The dynamic verbs fulfill and reject describe changing the state from pending into either the fulfilled or rejected. These transitions are called fulfillment or rejection of the promise.
Those were easy. Now, resolve is a different beast. It sometimes is used synonymous to "fulfill", but it would better be understood as settling the promise's fate to either fulfilled or rejected. The resolution (seldom: settlement) of a promise means that it leaves the pending state. But not even that is accurate - the problem is the recursive nature of the Promise Resolution Procedure:
resolving a promise with a "plain" value means fulfilling it
resolving a promise with a promise (or thenable) means adopting its state:
resolving with a fulfilled promise is a fulfillment
resolving with a rejected promise is a rejection
resolving with a pending promise means waiting for its resolution
Yes, if a promise is resolved it might not even be known whether it's going to be fulfilled or rejected. But it means the fate is no longer undetermined, as it's bound to the promise that we resolved with (notice that you can resolve a promise only once).
Ignoring this special case, a resolved promise usually means a settled promise.
Or, to cite the ECMAScript 6 Specification:
A promise is resolved if it is settled or if it has been “locked in” to match the state of another promise. Attempting to resolve or reject a resolved promise has no effect. A promise is unresolved if it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be pending, fulfilled or rejected.
and what the heck is defer?
Deferring a result means that you return an (asynchronous) promise for the result, and not the result directly (synchronously). And also return a deferred rejection instead of throwing synchronously.
Notice that "defer" is also used in some libraries (Q) as the method name to construct a Deferred object - see this answer on The differences between Deferred, Promise and Future for a good explanation.
Oh, and never trust variable names: defer might as well be an abbreviated "deferredObject".
The three promise states are listed in section 2.1 of the Promises/A+ specification.
From the specification:
So here are each of the terms you asked about:
Pending is the initial promise state. The operation represented by the promise has not yet been filfilled or rejected.
Fulfilled is another of the three promise states. It means that the promise has been resolved and now has its resolved value. The operation represented by the promise has been completed successfully.
Rejected is another of the three promise states. It means that the promise has been rejected and now has its rejected reason. The operation represented by the promise failed to obtain a value and thus has a reason for failing to do so (typically an error code or error object, but it can be anything).
Settled is a term that means the promise is either fulfilled or rejected (e.g. it's not pending any more), but it is not a separate state just a descriptive term to indicate it is no longer pending.
Resolved is a term that is often used to mean the same as fulfilled, but the two are not exactly the same. A promise can be resolved with a value which leads to fulfillment or it can be resolved with a rejected promise (which leads to rejection of this promise) or it can be resolved with a pending promise (which means it will now be waiting on the eventual state of some other promise).
It's hard to say exactly what you mean by defer. Promises are often classified as deferred objects in that they are an object that represents an action and result that is deferred to the future (it will occur in the future). In some promises implementations, there are actually two types of objects, a deferred object and a promise object. The deferred object is a superset of the promise object. Both can observe when the action is resolved or rejected with .then() handlers. But, only the deferred object can actually change the state to resolved or rejected.
In jQuery, you can create a deferred object with $.Deferred(). In other implementations such as ES6 promises, you just have promise objects with a constructor callback that has reject and resolve functions. The world is presumably moving toward what ES6 will have.
jQuery example that uses a deferred object:
function delay(t) {
var defer = $.Deferred();
setTimeout(function() {
defer.resolve();
}, t);
return defer.promise()
}
delay(200).then(function() {
// run my delayed function now
doMyThing();
});
ES6 promise example:
function delay(t) {
return new Promise(function(resolve, reject) {
setTimeout(function() {
resolve();
}, t);
});
}
delay(200).then(function() {
// run my delayed function now
doMyThing();
});
Domenic Denicola's "States and Fates" is a good, pithy summary.
States:
a promise is fulfilled if promise.then(f) will call f "as soon as possible"
a promise is rejected if promise.then(undefined, r) will call r "as soon as possible"
a promise is pending if it is neither fulfilled nor rejected.
Fates:
a promise is resolved if trying to resolve or reject it has no effect, i.e. the promise has been "locked in" to either follow another promise, or has been fulfilled or rejected
a promise is unresolved if it is not resolved, i.e. if trying to resolve or reject it will have an impact on the promise.
Follow the link for details 'relating states and fates'.
This question already has answers here:
Why does the Promise constructor require a function that calls 'resolve' when complete, but 'then' does not - it returns a value instead?
(5 answers)
Closed 1 year ago.
So this works
new Promise(res => {
console.log("1")
res(res);
}).then(res => {
console.log("2")
}).then(res => {
console.log("3")
})
But if I exclude the res(res), it will not chain. But it chains fine afterwards without any additional res().
Why is that, why does the first block matter if it has that res() and the subsequent blocks don't need it in order to chain onwards ?
Thanks.
It chains just fine without the call to res() in that the succeeding .then() handlers are chained onto the original promise.
But, the original promise will never be resolved if you don't execute res(someValue) so therefore, it will never call your .then() handlers. At some point, you have to resolve the original promise at the start of the chain if you want any of the .then() handlers to get called. Until you call res(someValue), you have a promise chain where the head of the chain is sitting in the pending state waiting to be resolved or rejected so it can then run the rest of the chain.
But if I exclude the res(res), it will not chain. But it chains fine afterwards without any additional res().
The original promise has to be resolved before the first .then() handler will get called. From then on, just returning from a .then() handler resolves the next promise in the chain - you no longer have to manually resolve it.
From within a .then() handler, you can trigger different outcomes three different ways:
Return from the .then() handler. This resolves that chained promise and triggers its .then() handlers to get called. If you return a value, that value becomes the new resolved value of this part of the chain. If you don't return a value or there's an implicit return, then the resolved value is undefined.
Throw an exception from within the .then() handler. This will be automatically caught by the promise infrastructure and will cause this promise in the promise chain to reject - trigger any .catch() handler to get called.
Return a promise from the .then() handler. This causes the current promise in the chain to track this new promise and it will resolve or reject based on what the newly returned promise does.
Also, a given promise can only be resolved or rejected once. It's a one-way state machine. Once it has been resolved or rejected, it's state is fixed and can not be changed again. So, you would only ever call res(someValue) once. Attempting to call it more than once just does nothing as the promise has already been changed to the fulfilled state and can no longer be changed.
FYI, the three states of a promise are pending, fulfilled and rejected. When you call res(someValue), you change the state of the original promise in your chain from pending to fulfilled and the internals of the promise will then call its .then() handlers on the next tick.
Something that people often don't realize is that each call to .then() creates and returns a new promise that is linked to the earlier promise chain. So, the code in your question actually creates 3 promises, one from new Promise() and two from the two calls to .then(). These subsequent promises from calls to .then() get resolved or rejected only when the parent promise gets resolved or rejected and the .then() or .catch() handler gets called and then there's a return value from that .then() or .catch() handler. Those return values from those handlers determine what happens next in the chain.
So I am wondering if this works?
S3.getObject()
.promise()
.then()
.catch() // catch error from the first then() statement
.then()
.catch() // catch error from the second then() statement
or do I need to place all 'catches' in the end? Can I have multiple catch then? Will they be fired in the order of the 'then' statements throwing errors?
It depends of your actual goals.
As a matter of fact, .then() method takes two parameters:
onFullfilled: Callback to be invoked when the promise is fulfilled.
onRejected: Callback to be invoked when the promise is rejected.
In fact, .catch(fn) is just a shorthand for .then(null, fn).
Both .then() and .catch() each return a new promise which resolves to its return value. In other words:
A resolved promise of that value if it isn't a promise.
The actual return value if it is already a promise (that will be fulfilled or rejected).
A rejected promise if the return value is a rejected promise (as previous point says) or any error is thrown.
The main reason behind the use of .then(onFullfill).catch(onReject) pattern instead of .then(onFullfill, onReject) is that, in the former (which is equivalent to .then(onFullfill).then(null, onReject)), we are chaining the onReject callback to the promise returned by first .then() instead of directly to the original promise.
The consequence of this is that if en error is thrown inside the onFullfill callback (or it returns a promise which happen to resolve to a rejected state), it will be catched by the chained .catch() too.
So, answering to your question, when you do something like:
P.then(...)
.then(...)
.then(...)
.catch(...)
;
You are chaining promises "supposing" all will go fine "and only check at the end". That is: Whenever any step fails, all subsequent .then()s are bypassed up to the next (in this case the last) .catch().
On the other hand, if you insert more .catch()s in between, you would be able to intercept rejected promises earlier and, if appropriate, solve whatever were going on and turn it into a resolved state again in order to resume the chain.
I'm getting confused with the different terminology thrown around. From my understanding, a promise can be:
fulfilled
rejected
pending
settled
resolved
defer
Does resolve mean settled? or does it mean its fulfilled? and what the heck is defer?
Terminology can be hard.
Let's take from the Promises/A+ specification and the respective ES6 section that there are 3 states:
pending - the promise does not have taken a value yet, it's future is still uncertain.
fulfilled - the promise successfully got a result value "assigned"
rejected - the promise is given a reason why no result could be acquired, typically an error.
The term settled is a hyperonym for fulfilled and rejected, meaning either - the opposite of pending.
The dynamic verbs fulfill and reject describe changing the state from pending into either the fulfilled or rejected. These transitions are called fulfillment or rejection of the promise.
Those were easy. Now, resolve is a different beast. It sometimes is used synonymous to "fulfill", but it would better be understood as settling the promise's fate to either fulfilled or rejected. The resolution (seldom: settlement) of a promise means that it leaves the pending state. But not even that is accurate - the problem is the recursive nature of the Promise Resolution Procedure:
resolving a promise with a "plain" value means fulfilling it
resolving a promise with a promise (or thenable) means adopting its state:
resolving with a fulfilled promise is a fulfillment
resolving with a rejected promise is a rejection
resolving with a pending promise means waiting for its resolution
Yes, if a promise is resolved it might not even be known whether it's going to be fulfilled or rejected. But it means the fate is no longer undetermined, as it's bound to the promise that we resolved with (notice that you can resolve a promise only once).
Ignoring this special case, a resolved promise usually means a settled promise.
Or, to cite the ECMAScript 6 Specification:
A promise is resolved if it is settled or if it has been “locked in” to match the state of another promise. Attempting to resolve or reject a resolved promise has no effect. A promise is unresolved if it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be pending, fulfilled or rejected.
and what the heck is defer?
Deferring a result means that you return an (asynchronous) promise for the result, and not the result directly (synchronously). And also return a deferred rejection instead of throwing synchronously.
Notice that "defer" is also used in some libraries (Q) as the method name to construct a Deferred object - see this answer on The differences between Deferred, Promise and Future for a good explanation.
Oh, and never trust variable names: defer might as well be an abbreviated "deferredObject".
The three promise states are listed in section 2.1 of the Promises/A+ specification.
From the specification:
So here are each of the terms you asked about:
Pending is the initial promise state. The operation represented by the promise has not yet been filfilled or rejected.
Fulfilled is another of the three promise states. It means that the promise has been resolved and now has its resolved value. The operation represented by the promise has been completed successfully.
Rejected is another of the three promise states. It means that the promise has been rejected and now has its rejected reason. The operation represented by the promise failed to obtain a value and thus has a reason for failing to do so (typically an error code or error object, but it can be anything).
Settled is a term that means the promise is either fulfilled or rejected (e.g. it's not pending any more), but it is not a separate state just a descriptive term to indicate it is no longer pending.
Resolved is a term that is often used to mean the same as fulfilled, but the two are not exactly the same. A promise can be resolved with a value which leads to fulfillment or it can be resolved with a rejected promise (which leads to rejection of this promise) or it can be resolved with a pending promise (which means it will now be waiting on the eventual state of some other promise).
It's hard to say exactly what you mean by defer. Promises are often classified as deferred objects in that they are an object that represents an action and result that is deferred to the future (it will occur in the future). In some promises implementations, there are actually two types of objects, a deferred object and a promise object. The deferred object is a superset of the promise object. Both can observe when the action is resolved or rejected with .then() handlers. But, only the deferred object can actually change the state to resolved or rejected.
In jQuery, you can create a deferred object with $.Deferred(). In other implementations such as ES6 promises, you just have promise objects with a constructor callback that has reject and resolve functions. The world is presumably moving toward what ES6 will have.
jQuery example that uses a deferred object:
function delay(t) {
var defer = $.Deferred();
setTimeout(function() {
defer.resolve();
}, t);
return defer.promise()
}
delay(200).then(function() {
// run my delayed function now
doMyThing();
});
ES6 promise example:
function delay(t) {
return new Promise(function(resolve, reject) {
setTimeout(function() {
resolve();
}, t);
});
}
delay(200).then(function() {
// run my delayed function now
doMyThing();
});
Domenic Denicola's "States and Fates" is a good, pithy summary.
States:
a promise is fulfilled if promise.then(f) will call f "as soon as possible"
a promise is rejected if promise.then(undefined, r) will call r "as soon as possible"
a promise is pending if it is neither fulfilled nor rejected.
Fates:
a promise is resolved if trying to resolve or reject it has no effect, i.e. the promise has been "locked in" to either follow another promise, or has been fulfilled or rejected
a promise is unresolved if it is not resolved, i.e. if trying to resolve or reject it will have an impact on the promise.
Follow the link for details 'relating states and fates'.
I read in docs.angularjs.org that on a deferred object, a promise is resolved with a value and here are my couple of questions.
var dfd = $q.deferred();
dfd.resolve() - what happens when resolving without a value? Is it a rejection?
resolve(value) – resolves the derived promise with the value. If the value is a rejection constructed via $q.reject, the promise will be rejected instead what does it mean?
Can anyone please clarify?
dfd.resolve() - what happens when resolving without a value? Is it a
rejection?
No, it's just the same as dfd.resolve(undefined) - you call a function with a parameter without arguments.
resolve(value) "If the value is a rejection constructed via
$q.reject, the promise will be rejected instead" - what does it mean?
If the value is a promise, that promise's state will be adopted. $q.reject does construct a rejected promise. It'll work with fulfilled promises as well, like resolve($q.when(value))