Develop Reference

Setting Timeout to fs.copy function

global.resultArr = {};
global.failedArr = [];
global.successArr = [];
const writeFile = async function (dirResult, queryResult) {
for (i = 0; i < queryResult.recordset.length; i++) {
for (file of dirResult) {
if (
file.substring(file.length - 3) == "wav" &&
global.failedArr.indexOf(queryResult.recordset[i].ip_vch) == -1
) {
try {
const writeResult = await timeout(
fs.copy(
dir + "//" + file,
"//" +
queryResult.recordset[i].ip_vch +
"//RXWaveFiles//DynamicLibraries" +
"//" +
libid +
"//" +
file
),
5000
);
if (
writeResult &&
global.failedArr.indexOf(queryResult.recordset[i].ip_vch) == -1
) {
console.log(queryResult.recordset[i].ip_vch);
global.failedArr.push(queryResult.recordset[i].ip_vch);
await sql.query`update opower..dialers set fileMoveResult_int=0 where ip_vch =${queryResult.recordset[i].ip_vch}`;
} else if (
global.successArr.indexOf(queryResult.recordset[i].ip_vch) == -1 &&
global.failedArr.indexOf(queryResult.recordset[i].ip_vch) == -1
) {
global.successArr.push(queryResult.recordset[i].ip_vch);
await sql.query`update opower..dialers set fileMoveResult_int=1 where ip_vch =${queryResult.recordset[i].ip_vch}`;
console.log("success!" + queryResult.recordset[i].ip_vch);
}
} catch (error) {
console.error(error);
if (global.failedArr.indexOf(queryResult.recordset[i].ip_vch) == -1) {
global.failedArr.push(queryResult.recordset[i].ip_vch);
await sql.query`update opower..dialers set fileMoveResult_int=0 where ip_vch =${queryResult.recordset[i].ip_vch}`;
}
}
}
}
}
global.resultArr.success = successArr;
global.resultArr.failed = failedArr;
return global.resultArr;
};
// utility function that creates a promise that rejects after a certain time
function timeoutPromise(t, errMsg = "timeout") {
// create possible error object here to get appropriate stack trace
let e = new Error(errMsg);
e.timeout = true;
return new Promise((resolve, reject) => {
setTimeout(reject, t, e);
});
}
// wrap a promise with a timeout, pass promise, time in ms and
// optional timeout error message
function timeout(p, t, errMsg = "timeout") {
return Promise.race(p, timeoutPromise(t, errMsg));
}
I am using this await function in a for loop in which from a source directory I need to copy some files to multiple network directories, however the problem here with await is that for the directories it is failing it's taking almost a minute to resolve and then gives the control back for the next iteration, is there a way we could stop the current iteration after 5 seconds.

You can add an error timeout to any promise like this:
// utility function that creates a promise that rejects after a certain time
function timeoutPromise(t, errMsg = "timeout") {
// create possible error object here to get appropriate stack trace
let e = new Error(errMsg);
e.timeout = true;
return new Promise((resolve, reject) => {
setTimeout(reject, t, e);
});
}
// wrap a promise with a timeout, pass promise, time in ms and
// optional timeout error message
function timeout(p, t, errMsg = "timeout") {
return Promise.race([p, timeoutPromise(t, errMsg)]);
}
The, you would use this with your fs.copy() like this:
const writeResult = await timeout(fs.copy(...), 5000);
So, then if the fs.copy() is taking more than 5 seconds, the promise you are awaiting will reject and you can catch it in your catch handler and act accordingly. You will be able to see that the error object has a .timeout property.
The way this works is that the timeout() function creates a race between the promise you passed in and another promise that will reject after your timeout expires. The first one to complete controls the output of Promise.race() and thus controls what you are using await on. If the timeout wins, then the promise will be rejected.

Websocket waiting for server response with a queue

I'm using websocket to send and receive data (up to 30 small messages per seconds). I want the client to send a websocket payload and wait for a specific message from the server.
Flow:
The client send a request
It also store the requestId (163) in waitingResponse object as a new object with sent timestamp
waitingResponse = {
163: { sent: 1583253453549 }
}
When the server response, another function validate the payload and then append the result to that request object
waitingResponse = {
163: { sent: 1583253453549, action: "none" }
}
The client checks every x ms that object for the action key
I have a function sendPayload that sends the payload and then await for a value from awaitResponse (the function below). Right now this function doesn't work. I tried making 2 separate function, one that would be the setTimeout timer, the other was the promise. I also tried having both in the same function and decide if it was the loop or the promise with the original argument you can see below. Now I'm thinking that function should always return a promise even in the loop but I cannot seem to make that work with a timer and I'm afraid of the cost of multiple promise in one another. Let's say I check for a response every 5 ms and the timeout it 2000ms. That is a lot of promises.
public async sendPayload(details) {
console.log("sendPlayload", details);
this.waitingResponse[details.requestId] = { sent: +new Date() };
if (this.socket.readyState === WebSocket.OPEN) {
this.socket.send(JSON.stringify(details));
}
const bindAwaitResponse = this.awaitResponse.bind(this);
return new Promise(async function (resolve, reject) {
const result = await bindAwaitResponse(details.requestId, true);
console.log("RES", result);
console.info("Time took", (+new Date() - result.sent) / 1000);
resolve(result);
});
}
public async awaitResponse(requestId, original) {
// console.log(requestId, "awaitResponse")
return new Promise((resolve, reject) => {
// Is it a valid queued request
if (this.waitingResponse[requestId]) {
// Do we have an answer?
if (this.waitingResponse[requestId].hasOwnProperty("action")) {
console.log(requestId, "Got a response");
const tmp = this.waitingResponse[requestId];
delete this.waitingResponse[requestId]; // Cleanup
resolve(tmp);
} else {
// No answer yet from remote server
// console.log("no answer: ", JSON.stringify(this.waitingResponse));
// Check if request took too long
if (+new Date() - this.waitingResponse[requestId].sent > 5000) { // TODO: Option for time out
console.warn(requestId, "Request timed out");
// Timed out, result took too long
// TODO: Option, default action when timed out
delete this.waitingResponse[requestId]; // Cleanup
resolve({
action: "to" // For now, just sent a timeout action, maybe the default action should be outside of the Network class?
})
} else {
// console.log(requestId, "Still waiting for results");
console.log(JSON.stringify(this.waitingResponse));
// Still waiting, after x ms, recall function
return setTimeout(async () => { resolve(await this.awaitResponse(requestId, false)); }, 200);
}
}
}
});
}
private async processMessage(msg) {
console.log("WS received Message", JSON.stringify(msg.data));
console.log("Current: ", JSON.stringify(this.waitingResponse));
let data = JSON.parse(msg.data);
// console.log("Received: ", data);
if (data.hasOwnProperty("requestId") && this.waitingResponse[data.requestId]) {
// console.log("processMessage ID found");
this.waitingResponse[data.requestId] = { ...data, ...this.waitingResponse[data.requestId] };
}
}
Note: I put the websocket tag below because I looked hard for that. Maybe I came across the solution without even realising it, but if you have better tags for this question to be found easier, please edit them :)

Yeah, you're mixing a lot of callback style functions with intermediate promises and async/await. Don't do polling when waiting for a response! Instead, when writing a queue, put the resolve function itself in the queue so that you can directly fulfill/reject the respective promise from the response handler.
In your case:
public async sendPayload(details) {
const request = this.waitingResponse[details.requestId] = { sent: +new Date() };
try {
if (this.socket.readyState === WebSocket.OPEN) {
this.socket.send(JSON.stringify(details));
}
const result = await new Promise(function(resolve) {
request.resolve = resolve;
setTimeout(() => {
reject(new Error('Timeout')); // or resolve({action: "to"}), or whatever
}, 5000);
});
console.info("Time took", (+new Date() - request.sent) / 1000);
return result; // or {...request, ...result} if you care
} finally {
delete this.waitingResponse[details.requestId];
}
}
private async processMessage(msg) {
let data = JSON.parse(msg.data);
if (data.hasOwnProperty("requestId") {
const request = this.waitingResponse[data.requestId]
if (request)
request.resolve(data)
else
console.warn("Got data but found no associated request, already timed out?", data)
} else {
console.warn("Got data without request id", data);
}
}
You might even do away with the request object altogether and only store the resolve function itself, if the processMessage function does not need any details about the request.

Can we make promise to wait until resolved and onreject call back the promise again

I'm learning about Promise's and have a little doubt assuming that I want to get resolved status out of Promises
and not want reject! Can I just call back the promise function inside
catch to make sure that I get only approved value! Is that possible or
will it throw an error or goes to loop iteration
let promisetocleantheroom = new Promise(function cleanroom(resolve, reject) {
//code to clean the room
//then as a result the clean variable will have true or flase
if (clean == "true") {
resolve("cleaned");
} else {
reject("not cleaned");
}
});
promisetocleantheroom.then(function cleanroom(fromResolve) {
// wait for the function to finish only then it would run the function then
console.log("the room is " + fromResolve);
}).catch(function cleanroom(fromReject) {
//calling back the promise again
cleanroom();
});

If you don't mind having higher order functions and recursivity, here is my proposed solution.
First you need to wrap your promise in a function to recreate it when it fails. Then you can pass it to retryPromiseMaker with a partial error handler to create another function that will act as retrier. And this function will return a Promise that will fulfill only if one of the inner promises fulfills.
Sounds complicated but I promise you it is not!
const retryPromiseMaker = (fn, errorfn = null) => {
const retryPromise = (retries = 3, err = null) => {
if (err) {
errorfn(err);
}
if (retries === 0) {
return Promise.reject(err);
}
return fn()
.catch(err => retryPromise(retries - 1, err));
};
return retryPromise;
}
const cleanTheRoom = (resolve, reject) => {
// simulate cleaning as a probability of 33%
const clean = Math.random() < 0.33;
setTimeout(() => {
if (clean) {
resolve("cleaned");
} else {
reject("not cleaned");
}
}, Math.random() * 700 + 200);
};
const promiseToCleanTheRoom = () => new Promise(cleanTheRoom);
const logStatus = end => value => {
let text = '';
if (end){
text += "at the end ";
}
text += "the room is " + value;
console.log(text);
};
retryPromiseMaker(promiseToCleanTheRoom, logStatus(false))(4)
.then(logStatus(true),logStatus(true));

Javascript - Defer Promise resolution

I have a service that produce some confirm-alert and wait for user interaction, managed by a Promise.
If a new confirm-alert is produced before the user has answered the first one, the service will store it in a sort of "waiting-list" and will show it as soon as the previous one will be dismissed by the user
let waitList = []
let activeAlert = null;
function createAlert(){
if(!activeAlert){
let pr = new Promise((resolve, reject) => {
// do stuff to show the alert
if('user click ok') resolve();
else reject();
});
activeAlert = pr;
return pr;
}
else {
let p = new Promise();
waitList.push(p);
return p;
}
}
let alert = createAlert();
alert.then(()=>{
// the user clicked OK
}).catch(()=>{
// the user click CANCEL
});
let alert2 = createAlert();
alert2.then(()=>{
// the user clicked OK on the second one
}).catch(()=>{
// the user click CANCEL on the second one
});
I am aware of the concept of Promise anti-pattern and that the Defer object is deprecated and considered obsolete. I can not understand how I can define the "resolve" and "reject" conditions for the Promise that is saved in the array.

Promises cannot be resolved from outside their scope without saving some references first. If you want to solve them from outside the array, you should also push the related resolve and reject callbacks into an object so that they can be used from outside that scope.
EX:
let myRefHolder = {};
let toResolve = new Promise((res, rej) => {
myRefHolder.resolutionCallback = res;
//other code
});
And then in the even handler:
clickHandler = () => {
myRefHolder.resolutionCallback(resolutionValue);
//other code
}
Then you just have to change this structure a bit to accomodate all your promises and callbacks... Instead of an myRefHolder object, you could have an array of objects shaped like myRefHolders, once for each Promise!
An alternative is assigning the button event handler directly in the Promise:
new Promise((rej, res) => yourButtonElement.addEventListener(‘click’, res));
This will be solved when you click on the button. Instead of ref you could also pass a more complicated inline function that has more complicated logic.
Since an event can have multiple handlers, you can keep adding to that event for each of your promises - but it could get messy because you then have to remove the listener before actually resolving the Promise (the example 1 line code I used it’s likely going to leak memory because that handler stays alive, and keeps Garbage Collector from freeing up the Promise memory)
If what you're asking is how do I then/catch a specific promise in an array of promises, it's as easy as myArray[index].then(() => {})
If you're waiting on an array of promises, as easy as Promise.all(myArray)

You can keep a promise chain for previous alerts (separate from the alert's own promise, so you can resolve/reject those as appropriate), and then have each promise resolve when the alert is completed:
// Kick things off with a resolved promise
let lastPromise = Promise.resolve();
function createAlert(msg) {
// Create the promise for this alert, but don't start it yet
let resolve, reject;
const p = new Promise((_resolve, _reject) => {
resolve = _resolve;
reject = _reject;
});
// Wait until the last alert is complete before starting this one
lastPromise = lastPromise.then(() => {
// Start this alert, and when it's done, resolve our chain promise
// ...
// later, if user confirms or whatever:
resolve();
// or if they cancel or whatever:
reject();
// Slave our chain to the resolution/rejection of the alert
return p.catch(e => undefined).then(v => undefined);
});
// Return the alert promise
return p;
}
Running example with a very crude "dialog" with OK and Cancel buttons, which resolve and reject the promise respectively:
// Kick things off with a resolved promise
let lastPromise = Promise.resolve();
function createAlert(msg) {
console.log("createAlert('" + msg + "')");
// Create the promise for this alert, but don't start it yet
let resolve, reject;
const p = new Promise((_resolve, _reject) => {
resolve = _resolve;
reject = _reject;
});
// Wait until the last alert is complete before starting this one
lastPromise = lastPromise.then(() => {
// Start this alert, and when it's done, resolve our chain promise
// This is obviousyl VERY CRUDE, just for demo purposes
console.log("showing '" + msg + "'");
function handler(e) {
switch (e.target.value) {
case "OK":
console.log("user clicked OK on '" + msg + "'");
close();
resolve();
break;
case "Cancel":
console.log("user clicked Cancel on '" + msg + "'");
close();
reject();
break;
}
}
function close() {
alert.style.display = "none";
alert.querySelector(".message").innerHTML = "";
alert.removeEventListener("click", handler);
}
const alert = document.getElementById("alert");
alert.querySelector(".message").appendChild(
document.createTextNode(msg)
);
alert.addEventListener("click", handler);
alert.style.display = "";
// Slave our chain to the resolution/rejection of the alert
return p.catch(e => undefined).then(v => undefined);
});
// Return the alert promise
return p;
}
function test(msg) {
createAlert(msg)
.then(() => {
console.log("alert resolved for '" + msg + "'");
})
.catch(() => {
console.log("alert rejected for '" + msg + "'");
});
}
test("one");
test("two");
test("three");
.as-console-wrapper {
max-height: 100% !important;
}
<div id="alert" style="display: none">
<div class="message"></div>
<input type="button" value="OK">
<input type="button" value="Cancel">
</div>

How do I access <state> and <value> properties of a Promise? [duplicate]

I have a pure JavaScript Promise (built-in implementation or poly-fill):
var promise = new Promise(function (resolve, reject) { /* ... */ });
From the specification, a Promise can be one of:
'settled' and 'resolved'
'settled' and 'rejected'
'pending'
I have a use case where I wish to interrogate the Promise synchronously and determine:
is the Promise settled?
if so, is the Promise resolved?
I know that I can use #then() to schedule work to be performed asynchronously after the Promise changes state. I am NOT asking how to do this.
This question is specifically about synchronous interrogation of a Promise's state. How can I achieve this?

No such synchronous inspection API exists for native JavaScript promises. It is impossible to do this with native promises. The specification does not specify such a method.
Userland libraries can do this, and if you're targeting a specific engine (like v8) and have access to platform code (that is, you can write code in core) then you can use specific tools (like private symbols) to achieve this. That's super specific though and not in userland.

Nope, no sync API, but here's my version of the async promiseState (with help from #Matthijs):
function promiseState(p) {
const t = {};
return Promise.race([p, t])
.then(v => (v === t)? "pending" : "fulfilled", () => "rejected");
}
var a = Promise.resolve();
var b = Promise.reject();
var c = new Promise(() => {});
promiseState(a).then(state => console.log(state)); // fulfilled
promiseState(b).then(state => console.log(state)); // rejected
promiseState(c).then(state => console.log(state)); // pending

promise-status-async does the trick. It is async but it does not use then to wait the promise to be resolved.
const {promiseStatus} = require('promise-status-async');
// ...
if (await promiseStatus(promise) === 'pending') {
const idle = new Promise(function(resolve) {
// can do some IDLE job meanwhile
});
return idle;
}

You can make a race with Promise.resolve
It's not synchronous but happens now
function promiseState(p, isPending, isResolved, isRejected) {
Promise.race([p, Promise.resolve('a value that p should not return')]).then(function(value) {
if (value == 'a value that p should not return') {
(typeof(isPending) === 'function') && isPending();
}else {
(typeof(isResolved) === 'function') && isResolved(value);
}
}, function(reason) {
(typeof(isRejected) === 'function') && isRejected(reason);
});
}
A little script for testing and understand their meaning of asynchronously
var startTime = Date.now() - 100000;//padding trick "100001".slice(1) => 00001
function log(msg) {
console.log((""+(Date.now() - startTime)).slice(1) + ' ' + msg);
return msg;//for chaining promises
};
function prefix(pref) { return function (value) { log(pref + value); return value; };}
function delay(ms) {
return function (value) {
var startTime = Date.now();
while(Date.now() - startTime < ms) {}
return value;//for chaining promises
};
}
setTimeout(log, 0,'timeOut 0 ms');
setTimeout(log, 100,'timeOut 100 ms');
setTimeout(log, 200,'timeOut 200 ms');
var p1 = Promise.resolve('One');
var p2 = new Promise(function(resolve, reject) { setTimeout(resolve, 100, "Two"); });
var p3 = Promise.reject("Three");
p3.catch(delay(200)).then(delay(100)).then(prefix('delayed L3 : '));
promiseState(p1, prefix('p1 Is Pending '), prefix('p1 Is Resolved '), prefix('p1 Is Rejected '));
promiseState(p2, prefix('p2 Is Pending '), prefix('p2 Is Resolved '), prefix('p2 Is Rejected '));
promiseState(p3, prefix('p3 Is Pending '), prefix('p3 Is Resolved '), prefix('p3 Is Rejected '));
p1.then(prefix('Level 1 : ')).then(prefix('Level 2 : ')).then(prefix('Level 3 : '));
p2.then(prefix('Level 1 : ')).then(prefix('Level 2 : ')).then(prefix('Level 3 : '));
p3.catch(prefix('Level 1 : ')).then(prefix('Level 2 : ')).then(prefix('Level 3 : '));
log('end of promises');
delay(100)();
log('end of script');
results with delay(0) (comment the while in delay)
00001 end of promises
00001 end of script
00001 Level 1 : One
00001 Level 1 : Three
00001 p1 Is Resolved One
00001 p2 Is Pending undefined
00001 p3 Is Rejected Three
00001 Level 2 : One
00001 Level 2 : Three
00001 delayed L3 : Three
00002 Level 3 : One
00002 Level 3 : Three
00006 timeOut 0 ms
00100 timeOut 100 ms
00100 Level 1 : Two
00100 Level 2 : Two
00101 Level 3 : Two
00189 timeOut 200 ms
and the results of this test with firefox(chrome keep the order)
00000 end of promises
00100 end of script
00300 Level 1 : One
00300 Level 1 : Three
00400 p1 Is Resolved One
00400 p2 Is Pending undefined
00400 p3 Is Rejected Three
00400 Level 2 : One
00400 Level 2 : Three
00400 delayed L3 : Three
00400 Level 3 : One
00400 Level 3 : Three
00406 timeOut 0 ms
00406 timeOut 100 ms
00406 timeOut 200 ms
00406 Level 1 : Two
00407 Level 2 : Two
00407 Level 3 : Two
promiseState make .race and .then : Level 2

in node, say undocumented internal process.binding('util').getPromiseDetails(promise)
> process.binding('util').getPromiseDetails(Promise.resolve({data: [1,2,3]}));
[ 1, { data: [ 1, 2, 3 ] } ]
> process.binding('util').getPromiseDetails(Promise.reject(new Error('no')));
[ 2, Error: no ]
> process.binding('util').getPromiseDetails(new Promise((resolve) => {}));
[ 0, <1 empty item> ]

You can use an (ugly) hack in Node.js until a native method is offered:
util = require('util');
var promise1 = new Promise (function (resolve) {
}
var promise2 = new Promise (function (resolve) {
resolve ('foo');
}
state1 = util.inspect (promise1);
state2 = util.inspect (promise2);
if (state1 === 'Promise { <pending> }') {
console.log('pending'); // pending
}
if (state2 === "Promise { 'foo' }") {
console.log ('foo') // foo
}

Updated: 2019
Bluebird.js offers this: http://bluebirdjs.com/docs/api/isfulfilled.html
var Promise = require("bluebird");
let p = Promise.resolve();
console.log(p.isFulfilled());
If you'd prefer to create your own wrapper, here is a nice blog about it.
Because JavaScript is single-threaded, it's hard to find a common enough use case to justify putting this in the spec. The best place to know if a promise is resolved is in .then(). Testing if a Promise is fullfilled would create a polling loop which is most likely the wrong direction.
async/await is a nice construct if you'd like to reason async code synchronously.
await this();
await that();
return 'success!';
Another useful call is Promise.all()
var promise1 = Promise.resolve(3);
var promise2 = 42;
var promise3 = new Promise(function(resolve, reject) {
setTimeout(resolve, 100, 'foo');
});
Promise.all([promise1, promise2, promise3]).then(function(values) {
console.log(values);
});
// expected output: Array [3, 42, "foo"]
When I first reached for this answer, that is the use case I was looking for.

It's indeed quite annoying that this basic functionality is missing. If you're using node.js then I know of two workarounds, neither of 'em very pretty. Both snippets below implement the same API:
> Promise.getInfo( 42 ) // not a promise
{ status: 'fulfilled', value: 42 }
> Promise.getInfo( Promise.resolve(42) ) // fulfilled
{ status: 'fulfilled', value: 42 }
> Promise.getInfo( Promise.reject(42) ) // rejected
{ status: 'rejected', value: 42 }
> Promise.getInfo( p = new Promise(() => {}) ) // unresolved
{ status: 'pending' }
> Promise.getInfo( Promise.resolve(p) ) // resolved but pending
{ status: 'pending' }
There doesn't seem to be any way to distinguish the last two promise states using either trick.
1. Use the V8 debug API
This is the same trick that util.inspect uses.
const Debug = require('vm').runInDebugContext('Debug');
Promise.getInfo = function( arg ) {
let mirror = Debug.MakeMirror( arg, true );
if( ! mirror.isPromise() )
return { status: 'fulfilled', value: arg };
let status = mirror.status();
if( status === 'pending' )
return { status };
if( status === 'resolved' ) // fix terminology fuck-up
status = 'fulfilled';
let value = mirror.promiseValue().value();
return { status, value };
};
2. Synchronously run microtasks
This avoids the debug API, but has some frightening semantics by causing all pending microtasks and process.nextTick callbacks to be run synchronously. It also has the side-effect of preventing the "unhandled promise rejection" error from ever being triggered for the inspected promise.
Promise.getInfo = function( arg ) {
const pending = {};
let status, value;
Promise.race([ arg, pending ]).then(
x => { status = 'fulfilled'; value = x; },
x => { status = 'rejected'; value = x; }
);
process._tickCallback(); // run microtasks right now
if( value === pending )
return { status: 'pending' };
return { status, value };
};

await usage to #jib's answer, with idiomatic prototyping.
Object.defineProperty(Promise.prototype, "state", {
get: function(){
const o = {};
return Promise.race([this, o]).then(
v => v === o ? "pending" : "resolved",
() => "rejected");
}
});
// usage: console.log(await <Your Promise>.state);
(async () => {
console.log(await Promise.resolve(2).state); // "resolved"
console.log(await Promise.reject(0).state); // "rejected"
console.log(await new Promise(()=>{}).state); // "pending"
})();
note that this async function execute "almost" immediately like synced function (or actually possibly be instantly).

You can wrap your promises in this way
function wrapPromise(promise) {
var value, error,
settled = false,
resolved = false,
rejected = false,
p = promise.then(function(v) {
value = v;
settled = true;
resolved = true;
return v;
}, function(err) {
error = err;
settled = true;
rejected = true;
throw err;
});
p.isSettled = function() {
return settled;
};
p.isResolved = function() {
return resolved;
};
p.isRejected = function() {
return rejected;
};
p.value = function() {
return value;
};
p.error = function() {
return error;
};
var pThen = p.then, pCatch = p.catch;
p.then = function(res, rej) {
return wrapPromise(pThen(res, rej));
};
p.catch = function(rej) {
return wrapPromise(pCatch(rej));
};
return p;
}

I looked through the solutions proposed to this question and could not see one that corresponds to a simple approach that I have used in Node.js.
I have defined a simple class PromiseMonitor, which takes a promise as the single parameter to its constructor, and has a string property .status which returns the standard string values corresponding to the promise status, "pending", "resolved" or "rejected", and four boolean properties .pending, .resolved, .rejected and .error. The property .error is set true only if .rejected is true and the reject callback was passed an Error object.
The class simply uses .then() on the promise to change the status of the PromiseMonitor when the promise is resolved or rejected. It does not interfere with any other use of the original promise. Here is the code:
class PromiseMonitor {
constructor(prm){
this._status = "pending";
this._pending = true;
this._resolved = false;
this._rejected = false;
this._error = false;
prm
.then( ()=>{
this._status = "resolved";
this._resolved = true;
this._pending = false;
}
, (err)=>{
this._status = "rejected";
this._pending = false;
this._rejected = true;
this._error = err instanceof Error ? true: false ;
}
);
}
get status(){ return this._status; };
get pending(){ return this._pending; };
get resolved(){ return this._resolved; };
get rejected(){ return this._rejected; };
get error(){ return this._error };
};
To monitor the status of a Promise, simply create an instance of PromiseMonitor, passing the promise in as a parameter, for example:
let promiseObject = functionThatReturnsAPromise();
let promiseMonitor = new PromiseMonitor( promiseObject );
Now you can syncrhonously check all the properties of promiseMonitor, which will track the status of the original promise. Here is a test script that demonstrates the three possible resolutions of a promise being monitored.
let ticks = 0;
let tickerID = setInterval( ()=>{++ticks; console.log(`..tick ${ticks}`)}, 1000);
async function run(){
console.log("Start");
let delay = prmDelay(2000);
let delayMonitor = new PromiseMonitor(delay);
// normal handling of delay promise
delay.then((result)=>( console.log("Normal resolution of delay using .then()") ) );
console.log("delay at start:\n", delay);
console.log("delayMonitor at start:\n", delayMonitor);
await delay;
console.log("delay finished:\n", delay);
console.log("delayMonitor finished:\n", delayMonitor);
console.log("\n\n TEST2: Rejection without an Error test ================================")
let rejDelay = prmDelay(3000, "reject");
let rejMonitor = new PromiseMonitor(rejDelay);
// normal handling of reject result on promise
rejDelay.then((result)=>( console.log("Normal resolution of rejDelay using .then will not happen") )
, (err)=>( console.log("Rejection of rejDelay handled using .then")));
console.log("rejDelay at start:\n", rejDelay);
console.log("rejMonitor at start:\n", rejMonitor);
await rejDelay.catch( (err)=>{ console.log( "Caught error using .catch on rejDelay" ); });
console.log("rejDelay finished:\n", rejDelay);
console.log("rejMonitor finished:\n", rejMonitor);
console.log("\n\n TEST3: Rejection with an Error test ================================")
let errMonitor ;
let errDelay;
try{
errDelay = prmDelay(1000, "error");
errMonitor = new PromiseMonitor(errDelay);
// normal handling of results of the original promise
errDelay.then(
(result)=>{
console.log("Normal expiry of errDelay");
console.log("Monitor Status is " + errMonitor.status )
}
, (err)=>{
console.log("** Rejection of errDelay handled using .then()");
console.log(" Monitor Status is " + errMonitor.status )
}
);
console.log("errDelay at start:\n", errDelay);
console.log("errMonitor at start:\n", errMonitor);
await errDelay;
console.log("**** This should never be run");
} catch(err) {
console.log( "** Caught error on errDelay using try{}catch{}:" );
console.log( " Monitor Status is " + errMonitor.status )
};
console.log("errDelay finished:\n", errDelay);
console.log("errMonitor finished:\n", errMonitor);
clearInterval(tickerID);
}
/**
* Creates a new promise with a specific result
* #param {*} tt
* #param {*} exitType ("resolve", "reject" or "error")
*/
function prmDelay (tt, exitType) {
return new Promise(function(resolve, reject) {
if( exitType == 'reject' ){
setTimeout(()=>{ reject("REJECTED")}, tt);
} else if( exitType== 'error'){
setTimeout(()=>{ reject(new Error( "ERROR Rejection") ); }, tt);
} else {
setTimeout(()=>{ resolve("RESOLVED") }, tt);
} ;
});
};
run();

You can add a method to Promise.prototype. It looks like this:
Edited: The first solution is not working properly, like most of the answers here. It returns "pending" until the asynchronous function ".then" is invoked, which is not happen immediately. (The same is about solutions using Promise.race). My second solution solves this problem.
if (window.Promise) {
Promise.prototype.getState = function () {
if (!this.state) {
this.state = "pending";
var that = this;
this.then(
function (v) {
that.state = "resolved";
return v;
},
function (e) {
that.state = "rejected";
return e;
});
}
return this.state;
};
}
You can use it on any Promise. For exemple:
myPromise = new Promise(myFunction);
console.log(myPromise.getState()); // pending|resolved|rejected
Second (and correct) solution:
if (window.Promise) {
Promise.stateable = function (func) {
var state = "pending";
var pending = true;
var newPromise = new Promise(wrapper);
newPromise.state = state;
return newPromise;
function wrapper(resolve, reject) {
func(res, rej);
function res(e) {
resolve(e);
if (pending) {
if (newPromise)
newPromise.state = "resolved";
else
state = "resolved";
pending = false;
}
}
function rej(e) {
reject(e);
if (pending) {
if (newPromise)
newPromise.state = "rejected";
else
state = "rejected";
pending = false;
}
}
}
};
}
And use it:
Notice: In this solution you doesn't have to use the "new" operator.
myPromise = Promise.stateable(myFunction);
console.log(myPromise.state); // pending|resolved|rejected

Caveat: This method uses undocumented Node.js internals and could be changed without warning.
In Node you can synchronously determine a promise's state using process.binding('util').getPromiseDetails(/* promise */);.
This will return:
[0, ] for pending,
[1, /* value */] for fulfilled, or
[2, /* value */] for rejected.
const pending = new Promise(resolve => setTimeout(() => resolve('yakko')));;
const fulfilled = Promise.resolve('wakko');
const rejected = Promise.reject('dot');
[pending, fulfilled, rejected].forEach(promise => {
console.log(process.binding('util').getPromiseDetails(promise));
});
// pending: [0, ]
// fulfilled: [1, 'wakko']
// rejected: [2, 'dot']
Wrapping this into a helper function:
const getStatus = promise => ['pending', 'fulfilled', 'rejected'][
process.binding('util').getPromiseDetails(promise)[0]
];
getStatus(pending); // pending
getStatus(fulfilled); // fulfilled
getStatus(rejected); // rejected

There's another elegant & hacky way of checking if a promise is still pending just by converting the whole object to string and check it with the help of inspect like this: util.inspect(myPromise).includes("pending").
Tested on Node.js 8,9,10,11,12,13
Here's a full example
const util = require("util")
function sleep(ms) {
return new Promise(resolve => setTimeout(resolve, ms));
}
(async ()=>{
let letmesleep = sleep(3000)
setInterval(()=>{
console.log(util.inspect(letmesleep).includes("pending"))
},1000)
})()
Result:
true
true
false
false
false

what you can do, is to use a variable to store the state, manually set the state to that variable, and check that variable.
var state = 'pending';
new Promise(function(ff, rjc) {
//do something async
if () {//if success
state = 'resolved';
ff();//
} else {
state = 'rejected';
rjc();
}
});
console.log(state);//check the state somewhere else in the code
of course, this means you must have access to the original code of the promise. If you don't, then you can do:
var state = 'pending';
//you can't access somePromise's code
somePromise.then(function(){
state = 'resolved';
}, function() {
state = 'rejected';
})
console.log(state);//check the promise's state somewhere else in the code
My solution is more coding, but I think you probably wouldn't have to do this for every promise you use.

As of Node.js version 8, you can now use the wise-inspection package to synchronously inspect native promises (without any dangerous hacks).

I made a package for this. Unlike most of the other answers here, it doesn't swallow unhandled rejections.
npm install p-state
import timers from 'timers/promises';
import {promiseStateSync} from 'p-state';
const timeoutPromise = timers.setTimeout(100);
console.log(promiseStateSync(timeoutPromise));
//=> 'pending'
await timeoutPromise;
console.log(promiseStateSync(timeoutPromise));
//=> 'fulfilled'

It looks like somehow nobody came up with one of the simplest solution that doesn't require any hacks:
define a variable to indicate that the promise is running
Add a .finally clause to the promise that sets the variable to false (you can do it at any time after the promise is created)
After that in your code just check if the above variable is true or false, to see whether the Promise is still running.
If you want to know not just whether it's finished or not then instead of .finally add a .then and a .catch clauses that set the variable to "resolved" or "rejected".
The only drawback is that the state variable doesn't get set right away (synchronously) when you add the clauses, in case the promise has already finished. Because of this, it's best to add this to the earliest possible place after the creation of the promise.
Example:
async function worker(){
// wait a very short period of time
await (new Promise(resolve => setTimeout(resolve, 100)))
//...
}
const w1=worker()
let w1_running=true
w1.finally( ()=> {w1_running=false});
//...
//Then check if it's running
(async ()=>{
while(true){
if (w1_running) {
console.log("Still Busy :(")
} else {
console.log("All done :)")
break
}
await (new Promise(resolve => setTimeout(resolve, 10)))
}
})()
// Note we need some async action started otherwise the event loop would never reach the code in the function `worker` or in the `.finally` clause

Here is a more fleshed out es6 version of the QueryablePromise, allowing the ability to chain then and catch after the first resolve and to immediately resolve or reject to keep the api consistent with the native Promise.
const PROMISE = Symbol('PROMISE')
const tap = fn => x => (fn(x), x)
const trace = label => tap(x => console.log(label, x))
class QueryablePromise {
resolved = false
rejected = false
fulfilled = false
catchFns = []
constructor(fn) {
this[PROMISE] = new Promise(fn)
.then(tap(() => {
this.fulfilled = true
this.resolved = true
}))
.catch(x => {
this.fulfilled = true
this.rejected = true
return Promise.reject(x)
})
}
then(fn) {
this[PROMISE].then(fn)
return this
}
catch(fn) {
this[PROMISE].catch(fn)
return this
}
static resolve(x) {
return new QueryablePromise((res) => res(x))
}
static reject(x) {
return new QueryablePromise((_, rej) => rej(x))
}
}
const resolvedPromise = new QueryablePromise((res) => {
setTimeout(res, 200, 'resolvedPromise')
})
const rejectedPromise = new QueryablePromise((_, rej) => {
setTimeout(rej, 200, 'rejectedPromise')
})
// ensure our promises have not been fulfilled
console.log('test 1 before: is resolved', resolvedPromise.resolved)
console.log('test 2 before: is rejected', rejectedPromise.rejected)
setTimeout(() => {
// check to see the resolved status of our promise
console.log('test 1 after: is resolved', resolvedPromise.resolved)
console.log('test 2 after: is rejected', rejectedPromise.rejected)
}, 300)
// make sure we can immediately resolve a QueryablePromise
const immediatelyResolvedPromise = QueryablePromise.resolve('immediatelyResolvedPromise')
// ensure we can chain then
.then(trace('test 3 resolved'))
.then(trace('test 3 resolved 2'))
.catch(trace('test 3 rejected'))
// make sure we can immediately reject a QueryablePromise
const immediatelyRejectedPromise = QueryablePromise.reject('immediatelyRejectedPromise')
.then(trace('test 4 resolved'))
.catch(trace('test 4 rejected'))
<script src="https://codepen.io/synthet1c/pen/KyQQmL.js"></script>

2019:
The simple way to do that as I know is thenable , super thin wrapper around promise or any async job.
const sleep = (t) => new Promise(res => setTimeout(res,t));
const sleeping = sleep(30);
function track(promise){
let state = 'pending';
promise = promise.finally( _=> state ='fulfilled');
return {
get state(){return state},
then: promise.then.bind(promise), /*thentable*/
finally:promise.finally.bind(promise),
catch:promise.catch.bind(promise),
}
}
promise = track(sleeping);
console.log(promise.state) // pending
promise.then(function(){
console.log(promise.state); // fulfilled
})

You can extend the Promise class to create a new queryable Promise
class.
You can create your own subclass, say QueryablePromise, by inheriting from the natively available Promise class, the instances of which would have a status property available on it that you can use to query the status of the promise objects synchronously. An implementation of it can be seen below or refer this for a better explanation.
class QueryablePromise extends Promise {
constructor (executor) {
super((resolve, reject) => executor(
data => {
resolve(data)
this._status = 'Resolved'
},
err => {
reject(err)
this._status = 'Rejected'
},
))
this._status = 'Pending'
}
get status () {
return this._status
}
}
// Create a promise that resolves after 5 sec
var myQueryablePromise = new QueryablePromise((resolve, reject) => {
setTimeout(() => resolve(), 5000)
})
// Log the status of the above promise every 500ms
setInterval(() => {
console.log(myQueryablePromise.status)
}, 500)

CAVEAT: process.binding('util').getPromiseDetails is undefined on node 16!
Benchmark:
Candidates:
/**
* https://stackoverflow.com/a/47009572/5318303
*/
const isPromisePending1 = (() => { // noinspection JSUnresolvedFunction
const util = process.binding('util') // noinspection JSUnresolvedFunction
return promise => !util.getPromiseDetails(promise)[0]
})()
/**
* https://stackoverflow.com/a/35852666/5318303
*/
const isPromisePending2 = (promise) => util.inspect(promise) === 'Promise { <pending> }'
/**
* https://stackoverflow.com/a/35820220/5318303
*/
const isPromisePending3 = (promise) => {
const t = {}
return Promise.race([promise, t])
.then(v => v === t, () => false)
}
Test promises:
const a = Promise.resolve()
const b = Promise.reject()
const c = new Promise(() => {})
const x = (async () => 1)()
Run benchmark:
const n = 1000000
console.time('isPromisePending1')
for (let i = 0; i < n; i++) {
isPromisePending1(a)
isPromisePending1(b)
isPromisePending1(c)
isPromisePending1(x)
}
console.timeEnd('isPromisePending1')
console.time('isPromisePending2')
for (let i = 0; i < n; i++) {
isPromisePending2(a)
isPromisePending2(b)
isPromisePending2(c)
isPromisePending2(x)
}
console.timeEnd('isPromisePending2')
console.time('isPromisePending3')
for (let i = 0; i < n; i++) {
await isPromisePending3(a)
await isPromisePending3(b)
await isPromisePending3(c)
await isPromisePending3(x)
}
console.timeEnd('isPromisePending3')
Result:
isPromisePending1: 440.694ms
isPromisePending2: 3.354s
isPromisePending3: 4.761s
Obviously isPromisePending1() is too faster (8~10 times)! But it's not usable on node 16! (see above caveat).

If you're using ES7 experimental you can use async to easily wrap the promise you want to listen.
async function getClient() {
let client, resolved = false;
try {
client = await new Promise((resolve, reject) => {
let client = new Client();
let timer = setTimeout(() => {
reject(new Error(`timeout`, 1000));
client.close();
});
client.on('ready', () => {
if(!resolved) {
clearTimeout(timer);
resolve(client);
}
});
client.on('error', (error) => {
if(!resolved) {
clearTimeout(timer);
reject(error);
}
});
client.on('close', (hadError) => {
if(!resolved && !hadError) {
clearTimeout(timer);
reject(new Error("close"));
}
});
});
resolved = true;
} catch(error) {
resolved = true;
throw error;
}
return client;
}

I've written a little npm package, promise-value, which provides a promise wrapper with a resolved flag:
https://www.npmjs.com/package/promise-value
It also gives synchronous access to the promise value (or error). This doesn't alter the Promise object itself, following the wrap rather than extend pattern.

This is older question but I was trying to do something similar. I need to keep n workers going. They are structured in a promise. I need to scan and see if they are resolved, rejected or still pending. If resolved, I need the value, if rejected do something to correct the issue or pending. If resolved or rejected I need to start another task to keep n going. I can't figure a way to do it with Promise.all or Promise.race as I keep working promises in an array and can find no way to delete them. So I create a worker that does the trick
I need a promise generator function that returns a promise which resolves or rejects as necessary. It is called by a function that sets up the framework to know what the promise is doing.
In the code below the generator simply returns a promise based on setTimeout.
Here it is
//argObj should be of form
// {succeed: <true or false, nTimer: <desired time out>}
function promiseGenerator(argsObj) {
let succeed = argsObj.succeed;
let nTimer = argsObj.nTimer;
return new Promise((resolve, reject) => {
setTimeout(() => {
if (succeed) {
resolve('ok');
}
else {
reject(`fail`);
}
}, nTimer);
})
}
function doWork(generatorargs) {
let sp = { state: `pending`, value: ``, promise: "" };
let p1 = promiseGenerator(generatorargs)
.then((value) => {
sp.state = "resolved";
sp.value = value;
})
.catch((err) => {
sp.state = "rejected";
sp.value = err;
})
sp.promise = p1;
return sp;
}
doWork returns an object containing the promise and the its state and returned value.
The following code runs a loop that tests the state and creates new workers to keep it at 3 running workers.
let promiseArray = [];
promiseArray.push(doWork({ succeed: true, nTimer: 1000 }));
promiseArray.push(doWork({ succeed: true, nTimer: 500 }));
promiseArray.push(doWork({ succeed: false, nTimer: 3000 }));
function loopTimerPromise(delay) {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve('ok');
}, delay)
})
}
async function looper() {
let nPromises = 3; //just for breaking loop
let nloop = 0; //just for breaking loop
let i;
//let continueLoop = true;
while (true) {
await loopTimerPromise(900); //execute loop every 900ms
nloop++;
//console.log(`promiseArray.length = ${promiseArray.length}`);
for (i = promiseArray.length; i--; i > -1) {
console.log(`index ${i} state: ${promiseArray[i].state}`);
switch (promiseArray[i].state) {
case "pending":
break;
case "resolved":
nPromises++;
promiseArray.splice(i, 1);
promiseArray.push(doWork({ succeed: true, nTimer: 1000 }));
break;
case "rejected":
//take recovery action
nPromises++;
promiseArray.splice(i, 1);
promiseArray.push(doWork({ succeed: false, nTimer: 500 }));
break;
default:
console.log(`error bad state in i=${i} state:${promiseArray[i].state} `)
break;
}
}
console.log(``);
if (nloop > 10 || nPromises > 10) {
//should do a Promise.all on remaining promises to clean them up but not for test
break;
}
}
}
looper();
Tested in node.js
BTW Not in this answer so much but in others on similar topics, I HATE it when someone says "you don't understand" or "that's not how it works" I generally assume the questioner knows what they want. Suggesting a better way is great. A patient explanation of how promises work would also be good.

Old question with many answers but none seem to suggest what I think is the simplest solution: set a bool indicator on promise resolution/rejection.
class Promise2 {
constructor(...args) {
let promise = new Promise(...args);
promise.then(() => promise._resolved_ = true);
promise.catch(() => promise._rejected_ = true);
return promise;
}
}
let p = new Promise2(r => setTimeout(r, 3000));
setInterval(() => {
console.log('checking synchronously if p is resolved yet?', p._resolved_);
}, 1000);

This is the Future pattern I use: (https://github.com/Smallscript-Corp)
enables sync and async fn usage
enables event patterns to be unified with async behavior
class XPromise extends Promise {
state = 'pending'
get settled() {return(this.state !== 'pending')}
resolve(v,...a) {
this.state = 'resolved'
return(this.resolve_(this.value = v,...a))
}
reject(e,...a) {
this.state = 'rejected'
return(this.reject_(this.value = (e instanceof Error) ? e : XPromise.Error(e),...a))
}
static Error(e) {const v = Error('value-rejected'); v.value = e; return(v)}
static Future(fn,...args) { // FactoryFn
let r,t,fv = new XPromise((r_,t_) => {r=r_;t=t_})
fv.resolve_ = r; fv.reject_ = t;
switch(typeof fn) {
case 'undefined': break; case 'function': fn(fv,...args); break;
default: fv.resolve(fn)
}
return(fv)
}
}
global.Future = XPromise.Future
Then you can create future-value instances that can be resolved using sync and async functions; enables handling events uniformly.
You can use it to write a pattern like:
async doSomething() {
// Start both - logically async-parallel
const fvIsNetworkOnLine = this.fvIsNetworkOnline
const fvAuthToken = this.fvAuthToken
// await both (order not critical since both started/queued above)
await fvAuthToken
await fvIsNetworkOnLine
// ... we can check the future values here if needed `fv.resolved`, `fv.state` etc
// ... do dependent workflow here ...
}
onNetworkOnLine(fIsOnline) {
// We utilize the `fv.settled` below, and use the event to `settle` it etc
if(fIsOnline) {
if(this.fvNetworkAvailable_)
this.fvNetworkAvailable_.resolve(true)
this.fvNetworkAvailable_ = undefined
}
else if(this.fvNetworkAvailable_.settled) {
this.fvNetworkAvailable_ = undefined
}
}
get fvNetworkAvailable() {
if(navigator.onLine)
return true
else if(this.fvNetworkAvailable_)
return this.fvNetworkAvailable_
return (this.fvNetworkAvailable_ = Future())
}
get fvAuthToken() {
if(this.fvAuthToken_)
return this.fvAuthToken_
const authTokenFv = async fv => {
// ... handle retry logic etc here ...
}
return(this.fvAuthToken_ = Future(authTokenFv))
}

I found this solution to be simple and allow me to continue using native promises but add useful synchronous checks. I also didn't have to pull in an entire promise library.
CAVEAT: This only works if there is some sort of break in the current execution thread to allow the promises to execute BEFORE checking the synchronous constructs. That makes this of more limited usefulness than I'd initially thought -- still useful for my use case though (Thanks Benjamin Gruenbaum for pointing this out)
/**
* This function allow you to modify a JS Promise by adding some status properties.
* Based on: http://stackoverflow.com/questions/21485545/is-there-a-way-to-tell-if-an-es6-promise-is-fulfilled-rejected-resolved
* But modified according to the specs of promises : https://promisesaplus.com/
*/
function MakeQuerablePromise(promise) {
// Don't modify any promise that has been already modified.
if (promise.isFulfilled) return promise;
// Set initial state
var isPending = true;
var isRejected = false;
var isFulfilled = false;
// Observe the promise, saving the fulfillment in a closure scope.
var result = promise.then(
function(v) {
isFulfilled = true;
isPending = false;
return v;
},
function(e) {
isRejected = true;
isPending = false;
throw e;
}
);
result.isFulfilled = function() { return isFulfilled; };
result.isPending = function() { return isPending; };
result.isRejected = function() { return isRejected; };
return result;
}
wrappedPromise = MakeQueryablePromise(Promise.resolve(3));
setTimeout(function() {console.log(wrappedPromise.isFulfilled())}, 1);
From https://ourcodeworld.com/articles/read/317/how-to-check-if-a-javascript-promise-has-been-fulfilled-rejected-or-resolved which based their answer on Is there a way to tell if an ES6 promise is fulfilled/rejected/resolved?