How to implement send function so that calculate will be executed sequentially, with order of calls preserved?
async function calculate(value) {
return new Promise((resolve) => {
setTimeout(() => resolve(value * value), 1)
})
}
async function send(value) {
return await calculate(value)
}
Next call of calculate should not start until the previous call is finished.
Calls to calculate should arrive in the exact same order.
Results for await should be returned correctly.
It should work this way when the caller ignores result (we always return the result and don't care if it's used or not)
send(2)
send(3)
and for async calls too
;(async () => {
console.log(await send(2))
console.log(await send(3))
})()
P.S.
Why it's getting down-voted? It's a perfectly legitimate use case for a stateful remote service, where calculate will be the remote call. And you have to preserve the order because the remote service is stateful and results depends on the call order.
Here's how I set up async queues so that it processes things in order regardless of how they're called:
function calculate(value) {
var reject;
var resolve;
var promise = new Promise((r, rr) => {
resolve = r;
reject = rr;
})
queue.add({
value: value,
resolve: resolve,
reject: reject
});
return promise;
}
var calcluateQueue = {
list: [], // each member of list should have a value, resolve and reject property
add: function(obj) {
this.list.push(obj); // obj should have a value, resolve and reject properties
this.processNext();
},
processNext: async function() {
if (this.processing) return; // stops you from processing two objects at once
this.processing = true;
var next = this.list.unshift(); // next is the first element on the list array
if (!next) return;
try {
var result = await doSomeProcessing(next.value);
next.resolve(result);
this.processNext();
} catch(e) {
next.reject(e);
// you can do error processing here, including conditionally putting next back onto the processing queue if you want to
// or waiting for a while until you try again
this.processNext();
}
}
};
Recently I have been trying to implement my own LocalStorage implementation but instead of storing data in JS I'm trying to store data on iOS native side in NSUserDefaults. I will show u some code to make clear what I'm talking about. I have such method for my own LocalStorage prototype:
Storage.prototype.getItem = (function (key) {
var that = this;
var getValue = (function () {
return new Promise(resolve => {
// JS2Native request value for a specific key
sendWebKitMessage(LS_GET_ITEM, { key: key });
console.log("new Promise for key=" + key + " timeInMs=" + Date.now());
// create success callback
that.onKeyValueReceived = function (keyValuePair) {
if (keyValuePair !== null) {
console.log("onKeyValueReceived=" + keyValuePair.key + " value=" + keyValuePair.value);
resolve(keyValuePair.value);
}
else {
console.log("onKeyValueReceived=null for=" + key);
resolve(null);
}
};
});
});
// handle async response from the Native side
var waitFuntion = (async function () {
var value = await getValue();
console.log("value="+value);
return value;
});
return waitFuntion();
});
// Native2JS helper methods
Storage.prototype.pasKeyValuePair = function (key, value) {
this.onKeyValueReceived(key, value);
};
In getItem I assume that when a client is calling it, it will wait until the iOS native side will get this value from NSUserDefaults and pass it to Storage via Storage.prototype.pasKeyValuePair method.
But actually it look like that when sendWebKitMessage(LS_GET_ITEM, { key: key }) is called the 'thread' is not blocked and the client can call this method several time more and the call back is not working properly.
I'm a JS newbie guy, I expected that it will work like a mutex. Is this actually possible to implement in JS or am I building something this completely wrong.
I plan to do it using only vanilla JS.
I think I understand what you want to do. Please correct me if I am wrong.
Yes, we can wait for Storage.getItem before processing other code. But, for this we need to use async-await when calling getItem.
Please check this code:
var Storage = function() {};
Storage.prototype.getItem = function(key) {
var promise = new Promise(function(resolve, reject) {
//Fetch your item here and resolve the value. setTimeout is just for example.
setTimeout(function() {
resolve("Some value goes here.");
}, 2000);
});
return promise;
};
(async function() {
var storage = new Storage();
console.log("I am called BEFORE Storage.getItem");
var item = await storage.getItem("key");
console.log(item);
console.log("I am called AFTER Storage.getItem");
console.log("Storage.getItem second time");
item = await storage.getItem("key");
console.log(item);
//do something else
})();
This is not going to wait for Storage.getItem:
var storage = new Storage();
console.log("I am called BEFORE Storage.getItem");
var item = storage.getItem("key");
console.log(item);
console.log("I am called AFTER Storage.getItem");
Please check # CodePen https://codepen.io/animatedcreativity/pen/5a295fc445f97022b75c37189bf875fd
I cannot test your code.
But, here is what I can understand.
I am assuming that getValue's Promise is resolving properly.
Mistake is with the line: var value = await getValue();. Its not waiting for anything because it has nothing to wait. Its getting the Promise object right away.
So, please try this:
var waitFuntion = (async function () {
var value;
await getValue().then(function(_value) {
value = _value;
});
console.log("value="+value);
return value;
});
This makes sure to wait till getValue's Promise actually resolves the value.
I have two HTTP calls on a page and they are separate altogether.
vm.$onInit = function() {
....
....
//Get all the items only once during initialization
ds.getAllItems().then(function(result){
vm.items = result;
},function(error){
vm.errorInApi = true;
});
.....
.....
}
vm.getTimes = function(){
.....
.....
HttpWrapper.send(url,{"operation":'GET'}).then(function(times){
.....
}
If both the APIs fail then only I need to show a modal.
I can initiate a variable to true and on failure of the APIs, I can make that false and then only show the modal.
But then how long to wait for completion of all the APIs?
Hmm... simply invert the polarity of the promises and use Promise.all().
Promise.all() would normally resolve once all promises resolve, so once the promises are inverted, it resolves once all promises get rejected...
var invert = p => new Promise((v, x) => p.then(x, v));
Promise.all([Promise.reject("Error 404"), Promise.reject("Error WTF")].map(invert))
.then(v => console.log(v));
So as per #guest271314 comment i extend the solution in a silver spoon to show how inverting promises can be applied for this task.
var invert = p => new Promise((v, x) => p.then(x, v)),
prps = [Promise.reject("Error 404"), Promise.reject("Error WTF")]; // previously rejected promises
myButton.addEventListener('click', function(e){
setTimeout(function(...p){
p.push(Promise.reject("Error in Click Event Listener"));
Promise.all(p.map(invert))
.then(r => results.textContent = r.reduce((r,nr) => r + " - " + nr));
}, 200, ...prps);
});
<button id="myButton">Check</button>
<p id="results"></p>
If any of the promises including the previously obtained ones or the once in the event handler gets resolved you will get no output.
You can use async/await and .catch() to determine the number of rejected Promises, perform action if the number is equal to N, where N is the number of rejected Promise values required to perform action.
A prerequisite, as mentioned by #trincot, is to return the Promise from the function and return a value from the function passed to .then(), and throw an Error() from .catch() or function at second parameter of .then() see Why is value undefined at .then() chained to Promise?
const N = 2;
function onInit() {
return Promise.resolve("resolved")
}
function getTimes() {
return Promise.reject("rejected");
}
const first = onInit();
document.querySelector("button")
.onclick = async function() {
let n = 0;
const firstP = await first.catch(() => ++n);
const secondP = await getTimes().catch(() => ++n);
if (n === N) {
// do stuff if `n` is equal to `N`
} else {
// do other stuff
console.log(n, N)
}
};
<button>click</button>
You could use Promise.all.
First you should get the two promises in an array. To achieve that, it is probably useful if your two functions return the promise they create:
vm.$onInit = function() {
....
....
return ds.getAllItems().then(function(result){
vm.items = result;
},function(error){
vm.errorInApi = true;
});
}
vm.getTimes = function(){
.....
.....
return HttpWrapper.send(url,{"operation":'GET'}).then(function(times){
.....
});
}
The array would then be built from the return values:
var arr = [];
arr.push(vm.$onInit());
...
arr.push(vm.getTimes());
You write in a comment that getTimes "is called on some button click", so you would do that second push there.
Or, maybe you see another way to get these promises in an array... it does not matter much how you do it, as long as you achieve this.
Then (in that click handler) you need to detect the situation where both the promises are rejected. Promise.all can do that, but you need to flip the promise results:
// Flip promises so that a rejected one is regarded as fulfilled and vice versa:
arr = arr.map(p => p.then(res => { throw res }).catch(err => err));
// Detect that all original promises rejected, i.e. that the new promises all fulfill.
Promise.all(arr).then(function() {
// execute whatever you need to execute when both ajax calls failed
}).catch(err => err); // ignore
There is a lot of information on how to handle errors when using promise.all() using catch but what I'm trying to achieve is to handle every time a promise inside of this promise.all() resolves. The reason I'm trying to do this is because I am attempting to setup a custom progress bar in console and I need to call the tick method every time a promise is resolved.
this.getNewSources = function () {
var bar = new ProgressBar(':bar', {total: this.getSourceMap().size});
var timer = setInterval(function () {
bar.tick();
if (bar.complete) {
console.log('\ncomplete\n');
clearInterval(timer);
}
}, 100);
let promiseArr = [];
for (let x of this.getSourceMap().values()) {
promiseArr.push(this.requestArticles(x.getName(), x.getCat(), x.getKey()));
}
return Promise.all(promiseArr).then(() => {
console.log("Articles loaded this round: " + this.articles.size);
console.log('all sources updated');
this.loadedArticles = true;
console.log(this.articleCount);
console.log(this.articles.size);
}).catch(e => {
console.log(e);
});
};
I'm trying to figure out a way of being able to call the bar.tick() method when each individual promise resolves.
(Answering my own question.)
I handled it by adding a then handler where I'm getting the promise from requestArticles (where I'm pushing them into the promiseArr array). I had to be sure to pass the value that handler receives out of the handler so it propagates to Promise.all, see *** lines:
this.getNewSources = function () {
var bar = new ProgressBar(':bar', {total: this.getSourceMap().size});
var timer = setInterval(function () {
if (bar.complete) {
console.log('\ncomplete\n');
clearInterval(timer);
}
}, 100);
function updateProgressBar() {
bar.tick()
}
let promiseArr = [];
for (let x of this.getSourceMap().values()) {
promiseArr.push(this.requestArticles(x.getName(), x.getCat(), x.getKey())
.then(value => { // ***
updateProgressBar(); // ***
return value; // ***
}) // ***
);
}
return Promise.all(promiseArr).then(() => {
console.log("Articles loaded this round: " + this.articles.size);
console.log('all sources updated');
this.loadedArticles = true;
console.log(this.articleCount);
console.log(this.articles.size);
}).catch(e => {
console.log(e);
});
};
That way, my handlers get called as the promises complete individually, and since I'm returning the value I receive, the promise created by my call to then will resolve with that value, which Promise.all will see. Rejections will skip over that handler and go straight to the handlers hooked up by Promise.all.
The ascii progress library on npm
Result output in console:
(With thanks to T.J. Crowder for his initial explanation, which made me realize I could do this where I'm pushing onto the array. He said he preferred deleting that answer and having me post this instead.)
Can anyone recommend a pattern for instantly retrieving data from a function that returns a Promise?
My (simplified) example is an AJAX preloader:
loadPage("index.html").then(displayPage);
If this is downloading a large page, I want to be able to check what's happening and perhaps cancel the process with an XHR abort() at a later stage.
My loadPage function used to (before Promises) return an id that let me do this later:
var loadPageId = loadPage("index.html",displayPage);
...
doSomething(loadPageId);
cancelLoadPage(loadPageId);
In my new Promise based version, I'd imagine that cancelLoadPage() would reject() the original loadPage() Promise.
I've considered a few options all of which I don't like. Is there a generally accepted method to achieve this?
Okay, let's address your bounty note first.
[Hopefully I'll be able to grant the points to someone who says more than "Don't use promises"... ]
Sorry, but the answer here is: "Don't use promises". ES6 Promises have three possible states (to you as a user): Pending, Resolved and Rejected (names may be slightly off).
There is no way for you to see "inside" of a promise to see what has been done and what hasn't - at least not with native ES6 promises. There was some limited work (in other frameworks) done on promise notifications, but those did not make it into the ES6 specification, so it would be unwise of you to use this even if you found an implementation for it.
A promise is meant to represent an asynchronous operation at some point in the future; standalone, it isn't fit for this purpose. What you want is probably more akin to an event publisher - and even that is asynchronous, not synchronous.
There is no safe way for you to synchronously get some value out of an asynchronous call, especially not in JavaScript. One of the main reasons for this is that a good API will, if it can be asynchronous, will always be asynchronous.
Consider the following example:
const promiseValue = Promise.resolve(5)
promiseValue.then((value) => console.log(value))
console.log('test')
Now, let's assume that this promise (because we know the value ahead of time) is resolved synchronously. What do you expect to see? You'd expect to see:
> 5
> test
However, what actually happens is this:
> test
> 5
This is because even though Promise.resolve() is a synchronous call that resolves an already-resolved Promise, then() will always be asynchronous; this is one of the guarantees of the specification and it is a very good guarantee because it makes code a lot easier to reason about - just imagine what would happen if you tried to mix synchronous and asynchronous promises.
This applies to all asynchronous calls, by the way: any action in JavaScript that could potentially be asynchronous will be asynchronous. As a result, there is no way for you do any kind of synchronous introspection in any API that JavaScript provides.
That's not to say you couldn't make some kind of wrapper around a request object, like this:
function makeRequest(url) {
const requestObject = new XMLHttpRequest()
const result = {
}
result.done = new Promise((resolve, reject) => {
requestObject.onreadystatechange = function() {
..
}
})
requestObject.open(url)
requestObject.send()
return requestObject
}
But this gets very messy, very quickly, and you still need to use some kind of asynchronous callback for this to work. This all falls down when you try and use Fetch. Also note that Promise cancellation is not currently a part of the spec. See here for more info on that particular bit.
TL:DR: synchronous introspection is not possible on any asynchronous operation in JavaScript and a Promise is not the way to go if you were to even attempt it. There is no way for you to synchronously display information about a request that is on-going, for example. In other languages, attempting to do this would require either blocking or a race condition.
Well. If using angular you can make use of the timeout parameter used by the $http service if you need to cancel and ongoing HTTP request.
Example in typescript:
interface ReturnObject {
cancelPromise: ng.IPromise;
httpPromise: ng.IHttpPromise;
}
#Service("moduleName", "aService")
class AService() {
constructor(private $http: ng.IHttpService
private $q: ng.IQService) { ; }
doSomethingAsynch(): ReturnObject {
var cancelPromise = this.$q.defer();
var httpPromise = this.$http.get("/blah", { timeout: cancelPromise.promise });
return { cancelPromise: cancelPromise, httpPromise: httpPromise };
}
}
#Controller("moduleName", "aController")
class AController {
constructor(aService: AService) {
var o = aService.doSomethingAsynch();
var timeout = setTimeout(() => {
o.cancelPromise.resolve();
}, 30 * 1000);
o.httpPromise.then((response) => {
clearTimeout(timeout);
// do code
}, (errorResponse) => {
// do code
});
}
}
Since this approach already returns an object with two promises the stretch to include any synchronous operation return data in that object is not far.
If you can describe what type of data you would want to return synchronously from such a method it would help to identify a pattern. Why can it not be another method that is called prior to or during your asynchronous operation?
You can kinda do this, but AFAIK it will require hacky workarounds. Note that exporting the resolve and reject methods is generally considered a promise anti-pattern (i.e. sign you shouldn't be using promises). See the bottom for something using setTimeout that may give you what you want without workarounds.
let xhrRequest = (path, data, method, success, fail) => {
const xhr = new XMLHttpRequest();
// could alternately be structured as polymorphic fns, YMMV
switch (method) {
case 'GET':
xhr.open('GET', path);
xhr.onload = () => {
if (xhr.status < 400 && xhr.status >= 200) {
success(xhr.responseText);
return null;
} else {
fail(new Error(`Server responded with a status of ${xhr.status}`));
return null;
}
};
xhr.onerror = () => {
fail(networkError);
return null;
}
xhr.send();
return null;
}
return xhr;
case 'POST':
// etc.
return xhr;
// and so on...
};
// can work with any function that can take success and fail callbacks
class CancellablePromise {
constructor (fn, ...params) {
this.promise = new Promise((res, rej) => {
this.resolve = res;
this.reject = rej;
fn(...params, this.resolve, this.reject);
return null;
});
}
};
let p = new CancellablePromise(xhrRequest, 'index.html', null, 'GET');
p.promise.then(loadPage).catch(handleError);
// times out after 2 seconds
setTimeout(() => { p.reject(new Error('timeout')) }, 2000);
// for an alternative version that simply tells the user when things
// are taking longer than expected, NOTE this can be done with vanilla
// promises:
let timeoutHandle = setTimeout(() => {
// don't use alert for real, but you get the idea
alert('Sorry its taking so long to load the page.');
}, 2000);
p.promise.then(() => clearTimeout(timeoutHandle));
Promises are beautiful. I don't think there is any reason that you can not handle this with promises. There are three ways that i can think of.
The simplest way to handle this is within the executer. If you would like to cancel the promise (like for instance because of timeout) you just define a timeout flag in the executer and turn it on with a setTimeout(_ => timeout = true, 5000) instruction and resolve or reject only if timeout is false. ie (!timeout && resolve(res) or !timeout && reject(err)) This way your promise indefinitely remains unresolved in case of a timeout and your onfulfillment and onreject functions at the then stage never gets called.
The second is very similar to the first but instead of keeping a flag you just invoke reject at the timeout with proper error description. And handle the rest at the then or catch stage.
However if you would like to carry the id of your asych operation to the sync world then you can also do it as follows;
In this case you have to promisify the async function yourself. Lets take an example. We have an async function to return the double of a number. This is the function
function doubleAsync(data,cb){
setTimeout(_ => cb(false, data*2),1000);
}
We would like to use promises. So normally we need a promisifier function which will take our async function and return another function which when run, takes our data and returns a promise. Right..? So here is the promisifier function;
function promisify(fun){
return (data) => new Promise((resolve,reject) => fun(data, (err,res) => err ? reject(err) : resolve(res)));
}
Lets se how they work together;
function promisify(fun){
return (data) => new Promise((resolve,reject) => fun(data, (err,res) => err ? reject(err) : resolve(res)));
}
function doubleAsync(data,cb){
setTimeout(_ => cb(false, data*2),1000);
}
var doubleWithPromise = promisify(doubleAsync);
doubleWithPromise(100).then(v => console.log("The asynchronously obtained result is: " + v));
So now you see our doubleWithPromise(data) function returns a promise and we chain a then stage to it and access the returned value.
But what you need is not only a promise but also the id of your asynch function. This is very simple. Your promisified function should return an object with two properties; a promise and an id. Lets see...
This time our async function will return a result randomly in 0-5 secs. We will obtain it's result.id synchronously along with the result.promise and use this id to cancel the promise if it fails to resolve within 2.5 secs. Any figure on console log Resolves in 2501 msecs or above will result nothing to happen and the promise is practically canceled.
function promisify(fun){
return function(data){
var result = {id:null, promise:null}; // template return object
result.promise = new Promise((resolve,reject) => result.id = fun(data, (err,res) => err ? reject(err) : resolve(res)));
return result;
};
}
function doubleAsync(data,cb){
var dur = ~~(Math.random()*5000); // return the double of the data within 0-5 seconds.
console.log("Resolve in " + dur + " msecs");
return setTimeout(_ => cb(false, data*2),dur);
}
var doubleWithPromise = promisify(doubleAsync),
promiseDataSet = doubleWithPromise(100);
setTimeout(_ => clearTimeout(promiseDataSet.id),2500); // give 2.5 seconds to the promise to resolve or cancel it.
promiseDataSet.promise
.then(v => console.log("The asynchronously obtained result is: " + v));
You can use fetch(), Response.body.getReader(), where when .read() is called returns a ReadableStream having a cancel method, which returns a Promise upon cancelling read of the stream.
// 58977 bytes of text, 59175 total bytes
var url = "https://gist.githubusercontent.com/anonymous/"
+ "2250b78a2ddc80a4de817bbf414b1704/raw/"
+ "4dc10dacc26045f5c48f6d74440213584202f2d2/lorem.txt";
var n = 10000;
var clicked = false;
var button = document.querySelector("button");
button.addEventListener("click", () => {clicked = true});
fetch(url)
.then(response => response.body.getReader())
.then(reader => {
var len = 0;
reader.read().then(function processData(result) {
if (result.done) {
// do stuff when `reader` is `closed`
return reader.closed.then(function() {
return "stream complete"
});
};
if (!clicked) {
len += result.value.byteLength;
}
// cancel stream if `button` clicked or
// to bytes processed is greater than 10000
if (clicked || len > n) {
return reader.cancel().then(function() {
return "read aborted at " + len + " bytes"
})
}
console.log("len:", len, "result value:", result.value);
return reader.read().then(processData)
})
.then(function(msg) {
alert(msg)
})
.catch(function(err) {
console.log("err", err)
})
});
<button>click to abort stream</button>
The method I am currently using is as follows:
var optionalReturnsObject = {};
functionThatReturnsPromise(dataToSend, optionalReturnsObject ).then(doStuffOnAsyncComplete);
console.log("Some instant data has been returned here:", optionalReturnsObject );
For me, the advantage of this is that another member of my team can use this in a simple way:
functionThatReturnsPromise(data).then(...);
And not need to worry about the returns object. An advanced user can see from the definitions what is going on.