Develop Reference

Return value from multithreaded process in Node.js

I've designed a function diversify() to take some expensive function f() and run it in parallel on all the cores of the machine it's being run on. I've also set this up so that if f() returns a value on one core, all the other threads are automatically killed. The code for that is below:
let diversify = f => {
// Split a function `f` into a thread for each core of the machine.
// The first thread to finish will return its result and end all others.
if ( cluster.isMaster ) {
let children = []
for ( let i = 0; i < os.cpus().length; i++) {
children.push(cluster.fork())
}
cluster.on('message', (_, msg) => {
if ( msg.cmd == 'stop' ) { children.forEach(child => {
child.process.kill()
})}
return msg.out
})
} else {
let out = f()
process.send({ cmd: 'stop', out })
}
}
The problem is that after one process tells the master process to stop through msg.cmd = 'stop', there is no way to return the output msg.out. The way that it is being implemented in the code above, the return msg.out statement is inside an anonymous function nested inside the larger diversify function. Therefore, this return value is not exposed when you run something like diversify(() => { return true }), resulting in undefined instead. Is there any way to return what's being sent in msg.out?

Sending messages to workers is by design asynchronous.
There is technically no way to return a value from a callback that is an event handler, since the event may occur at any time.
The canonical solution to this problem is via promises. Simply wrap the function body in a promise and resolve the value instead of returning it.
const diversify = (f) =>
new Promise((resolve) => {
// Split a function `f` into a thread for each core of the machine.
// The first thread to finish will return its result and end all others.
if (cluster.isMaster) {
let children = [];
for (let i = 0; i < os.cpus().length; i++) {
children.push(cluster.fork());
}
cluster.on('message', (_, msg) => {
if (msg.cmd == 'stop') {
children.forEach((child) => {
child.process.kill();
});
}
resolve(msg.out);
});
} else {
let out = f();
process.send({ cmd: 'stop', out });
}
});
The result is then accessible in out:
const getPid = () => {
return process.pid;
};
diversify(() => {
console.log(getPid());
return true;
}).then((out) => {
console.log(out); // true
});
// or using async/await
const diversifyAsync = async () => {
const out = await diversify(() => {
console.log(getPid());
return true;
});
console.log(out); // true
};
diversifyAsync();

make an api call in foreach loop and push the result in another array

I am trying to make an api call in a for each loop. When I make an api call, the function returns the Promise. I want use that return response and check if the promise is resolved (then do something) and if rejected (then do something). But, everytime in the foreach loop, first the api makes both the calls and then goes to the then() part.
What I want is in foreach loop, make an api call -> check if resolved or rejected -> do something -> make an api call -> check if resolved or rejected -> do something and so on.
//cannot change the below implementation. I need to use this already created function
const callApiFunction = options => {
const callback = (resolve, reject) => {
something.call(options, resolve, reject)
};
return new Promise(callback);
}
//First attempt
const a =[];
const b =[];
list.forEach(listelement => {
callApiFunction(options)
.then(() => {
a.push(listelement);
})
.catch(() => {
b.push(listelement);
});
});
//Second attempt
const a =[];
const b =[];
list.forEach(listelement => {
let promise = new Promise((resolve, reject) => {
callApiFunction(options);
resolve(listelement);
reject(listelement);
});
promise.then((listelement) => {
a.push(listelement);
})
.catch((listelement) => {
b.push(listelement);
});
});
What happens in my second code block is, callApiFunction is first executed twice if list.length = 2 and then, then is executed twice.
I want the callApiFunction is executed and list[0] -> then or catch executes accordingly
callApiFunction is executed and list[1] -> then or catch executes accordingly

Does this variant with for-of loop and await work for you?
'use strict';
async function separate(list) {
const a = [];
const b = [];
for (const listelement of list) {
try {
await callApiFunction(options);
a.push(listelement);
} catch (err) {
b.push(listelement);
}
}
return [a, b];
}
separate([1, 2, 3]).then(([good, bad]) => {
console.log(`Success:\n${good.join('\n')}\n\nErrors:\n${bad.join('\n')}\n`);
});

Chained and Nested promises with For Loop

I am trying to get each property of my games within chained promises (Each of the property is coming from a different async calls).
Logic of my algorithm:
Check the Network and Get the smart contract address
Register the contract containing the addresses of all the Games
Get the number of Games
For each game, perform one aSync call
per property
Print all the games and details (here I am not able
to get the updated object)
Code:
var games = [];
window.addEventListener('load', function() {
// Check the Network and assign the smart contract address
web3.eth.net.getId()
.then(function(networkId) {
let contractAddressRegistry;
if (networkId == 1) {
contractAddressRegistry = "0xQWERTYUIOPQWERTYUIOPQWERTY"
} else {
contractAddressRegistry = "0x12345678901234567890123456"
}
return contractAddressRegistry;
})
.then(function(contractAddressRegistry) {
let contractRegistry = new web3.eth.Contract(contractAbiRegistry, contractAddressRegistry);
contractRegistry.methods.numberOfGames().call()
.then(function(numberOfGames) {
for (let i = 0; i < numberOfGames; i++) {
let game = {};
game.propertyA = aSyncCallGetPropertyA(i); // Promise
game.propertyB = aSyncCallGetPropertyB(i); // Promise
game.propertyC = aSyncCallGetPropertyC(i); // Promise
}
games.push(game);
})
})
.then(function() {
console.log(games) // Empty
})
})
I tried used Promises.all() but I am not able to sync it properly as some async calls are within a then().
How can I make sure to get the object Games filled with all its properties?

You should use Promise.all like this. Basically, you need to wrap all three aSyncCallGetProperty async calls in Promise.all for waiting until they really finish then assign the results to object game.
whatever
.then(function(contractAddressRegistry) {
let contractRegistry = new web3.eth.Contract(contractAbiRegistry, contractAddressRegistry);
return contractRegistry.methods.numberOfGames().call();
})
.then(function(numberOfGames) {
return Promise.all(numberOfGames.map(() => {
return Promise.all([
aSyncCallGetPropertyA(),
aSyncCallGetPropertyB(),
aSyncCallGetPropertyC()
]).then(results => {
let game = {};
game.propertyA = results[0];
game.propertyB = results[1];
game.propertyC = results[2];
return game;
});
}));
})
.then(function(games) {
console.log(JSON.stringify(games));
})

#Lewis' code seems right but I can not make sure what numberOfGames is. Assuming that it's an integer as used in your question (not an array as treated in the other answer) here is a further rephrased version without nested .then()s.
window.addEventListener('load', function() {
web3.eth.net.getId()
.then(networkId => networkId === 1 ? "0xQWERTYUIOPQWERTYUIOPQWERTY"
: "0x12345678901234567890123456")
.then(contractAddressRegistry => new web3.eth.Contract(contractAbiRegistry, contractAddressRegistry).methods.numberOfGames().call())
.then(numberOfGames => Promise.all(Array(numberOfGames).fill()
.map(_ => Promise.all([aSyncCallGetPropertyA(),
aSyncCallGetPropertyB(),
aSyncCallGetPropertyC()]))))
.then(function(games){
games = games.map(game => ({propertyA: game[0],
propertyB: game[1],
propertyC: game[2]}));
doSomethingWith(games);
});
});

Promise not returning expected value

Ive been learning about promises and I have a question. I have a function named getNumber which return an array of number (for the sake of understanding). The I used that function to iterate over that array and make a http request for each value (with a setTimeout to make a delay between calls)
Then I want to used that information gathered in a then function, but it's giving me a 'undefined error'. obviously something is wrong here, but I cant see it. Do you guy know how can I fix this and what is wrong?
var getNumbers = () => {
return new Promise(function(resolve, reject) {
console.log("In function getNumbers");
var data = [1,2,3,4,5,6,7,8,9];
resolve(data);
});
};
getNumbers()
.then(numbersArray => {
//Supposed to return array of posts title
return numbersArray.map(number => {
console.log("Reading number" + number);
setTimeout(() => {
//make a http request
return getHtml("https://jsonplaceholder.typicode.com/posts/"+number)
.then(function(post) {
return post.title;
})
}, 10000);//make a request each ten seconds
});
})
.then(postTitlesArray => {
//Shows array of undefined
console.log(postTitlesArray)
});
function getHtml(webUrl) {
return fetch(webUrl)
.then(function(res) {
return res.json();
});
}

There are several conceptual things in the way of your approach doing what you want.
First, .map() is synchronous. That means it runs to completion and doesn't wait for any async operations to finish.
Second, setTimeout() is non-blocking. It just schedules a timer for some time in the future and then your .map() callback returns immediately, returning nothing.
So, your approach doesn't work at all.
From your comments, it appears that what you're trying to accomplish is to make a bunch of network calls in a loop, but put a delay between them so you don't get rate limited. There are a bunch of ways to do that.
There are two basic concepts you need to make that work:
Make your async operations be sequential so the next one doesn't get initiated until the prior one is done.
Put a delay that works with promises before starting the next one.
I'll first show an ES7 approach using async/await as it probably looks conceptually the simplest.
Using async/await to sequence asynchronous array access
function delay(t) {
return new Promise(resolve => {
setTimeout(resolve, t);
});
}
getNumbers().then(async function(numbersArray) {
//Supposed to return array of posts title
let results = [];
let delayT = 0; // first delay is zero
for (let number of numbersArray) {
console.log("Reading number" + number);
let r = await delay(delayT).then(() => {
delayT = 10 * 1000; // 10 seconds for subsequent delays
return getHtml("https://jsonplaceholder.typicode.com/posts/"+number).then(function(post) {
return post.title;
});
});
results.push(r);
}
return results;
});
Using .reduce() to sequence asynchronous array acess
If you wanted to do it without async/await, then you could use the .reduce() design pattern for sequencing async iteration of an array:
function delay(t) {
return new Promise(resolve => {
setTimeout(resolve, t);
});
}
getNumbers().then(numbersArray => {
//Supposed to return array of posts title
let results = [];
let delayT = 0; // first delay is zero
return numersArray.reduce((p, number) => {
return p.then(() => {
return delay(delayT).then(() => {
delayT = 10 * 1000; // 10 seconds for subsequent delays
return getHtml("https://jsonplaceholder.typicode.com/posts/"+number).then(function(post) {
results.push(post.title);
});
});
});
}, Promise.resolve()).then(() => {
// make array of results be the resolved value of the returned promise
return results;
});
});
Note that both of these algorithms are coded to not delay the first operation since presumably you don't need to, so it only delays between successive operations.
As coded, these are modeled after Promise.all() and they will reject if any of your getHtml() calls reject. If you want to return all results, even if some reject, then you can change:
return getHtml(...).then(...)
to
return getHtml(...).then(...).catch(err => null);
which will put null in the returned array for any result that failed, or if you want to log the error, you would use:
return getHtml(...).then(...).catch(err => {
console.log(err);
return null;
});
Generic Helper Function
And, since this is a somewhat generic problem, here's a generic helper function that lets you iterate an array calling an async operation on each item in the array and accumulating all the results into an array:
// Iterate through an array in sequence with optional delay between each async operation
// Returns a promise, resolved value is array of results
async iterateArrayAsync(array, fn, opts = {}) {
const options = Object.assign({
continueOnError: true,
delayBetweenAsyncOperations: 0,
errPlaceHolder: null
}, opts);
const results = [];
let delayT = 0; // no delay on first iteration
for (let item of array) {
results.push(await delay(delayT).then(() => {
return fn(item);
}).catch(err => {
console.log(err);
if (options.continueOnError) {
// keep going on errors, let options.errPlaceHolder be result for an error
return options.errPlaceHolder;
} else {
// abort processing on first error, will reject the promise
throw err;
}
}));
delayT = options.delayBetweenAsyncOperations; // set delay between requests
}
return results;
}
This accepts options that let you continueOnError, lets you set the delay between each async operation and lets you control the placeholder in the array of results for any failed operation (only used if continueOnError is set). All the options are optional.

I assume what you want to do is: 1) Get a list of numbers using getNumbers. 2) Iterate through each number from step one and form a url, with which an http request is made every ten seconds. 3) If a request is successfully sent, wait for its response. 4) Get post.title from response. 5) Wait until the iteration in step 2 ends, and return an array of all post.titles received from each call.
With the above assumptions in mind, I edit your code a bit and the following solution will work. See in jsfiddle.
I think the main problem with your code is that the map method doesn't return anything.
const getNumbers = () => {
return new Promise(function(resolve, reject) {
console.log("In function getNumbers");
var data = [1,2,3,4,5,6,7,8,9];
resolve(data);
});
};
const delay = (number, t) => {
return new Promise((resolve) => {
setTimeout(() => {
//make a http request
resolve(
getHtml("https://jsonplaceholder.typicode.com/posts/"+number)
.then(function(post) {
console.log('title', post.title)
return post.title;
})
)
}, t)
})
}
const getHtml = (webUrl) => {
return fetch(webUrl)
.then(function(res) {
return res.json();
});
}
getNumbers()
.then(numbersArray => {
//Supposed to return array of posts title
return Promise.all(numbersArray.map((number, i) => {
console.log("Reading number" + number);
return delay(number, 10000*(i+1));//make a request each ten seconds
}))
.then(postTitlesArray => {
console.log(postTitlesArray)
});
})

You can use Promise.all, assuming numbers are not in the thousands or else you can use batched Promise.all.
Then use throttlePeriod from here to make sure only 1 request is made every 10 seconds.
And then resolve failed requests with a special value so you don't loose all successes if one fails:
var getNumbers = () => {
return new Promise(function (resolve, reject) {
console.log("In function getNumbers");
var data = [1, 2, 3, 4, 5, 6, 7, 8, 9];
resolve(data);
});
};
function getHtml(webUrl) {
return fetch(webUrl)
.then(function (res) {
return res.json();
});
}
const Fail = function(reason){this.reason=reason;};
const isFail = x=>(x&&x.constructor)===Fail;
const notFail = x=>!isFail(x);
//maximum 1 per 10 seconds
//you can get throttle period from here:
//https://github.com/amsterdamharu/lib/blob/master/src/index.js
const max1Per10Seconds = lib.throttlePeriod(1,10000)(getHtml);
getNumbers()
.then(
numbersArray =>
Promise.all(//process all numbers
numbersArray
.map(//map number to url
number =>
`https://jsonplaceholder.typicode.com/posts/${number}`
)
//map url to promise
//max1Per10Seconds calls getHtml maximum 1 time per 10 seconds
// (will schedule the calls)
.map(max1Per10Seconds)
.map(//map promise to promise that does not reject
p=>//instead of rejecting promise, resolve with Fail value
//these Fail values can be filtered out of the result later.
//(see last then)
p.catch(err=>new Fail([err,number]))
)
)
).then(
//got the results, not all results may be successes
postTitlesArray => {
//is a comment really needed here?
const successes = postTitlesArray.filter(notFail);
const failed = postTitlesArray.filter(isFail);
}
);

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?