How do promises work in JavaScript? - javascript

I just implemented my first function that returns a promise based on another promise in AngularJS, and it worked. But before I decided to just do it, I spent 2 hours reading and trying to understand the concepts behind promises. I thought if I could write a simple piece of code that simulated how promises worked, I would then be able to conceptually understand it instead of being able to use it without really knowing how it works. I couldn't write that code.
So, could someone please illustrate in vanilla JavaScript how promises work?

A promise is basically an object with two methods. One method is for defining what to do, and one is for telling when to do it. It has to be possible to call the two methods in any order, so the object needs to keep track of which one has been called:
var promise = {
isDone: false,
doneHandler: null,
done: function(f) {
if (this.isDone) {
f();
} else {
this.doneHandler = f;
}
},
callDone: function() {
if (this.doneHandler != null) {
this.doneHandler();
} else {
this.isDone = true;
}
}
};
You can define the action first, then trigger it:
promise.done(function(){ alert('done'); });
promise.callDone();
You can trigger the action first, then define it:
promise.callDone();
promise.done(function(){ alert('done'); });
Demo: http://jsfiddle.net/EvN9P/
When you use a promise in an asynchronous function, the function creates the empty promise, keeps a reference to it, and also returns the reference. The code that handles the asynchronous response will trigger the action in the promise, and the code calling the asynchronous function will define the action.
As either of those can happen in any order, the code calling the asynchronous function can hang on to the promise and define the action any time it wants.

For the simplicity to understand about the promises in Javascript.
You can refer below example. Just copy paste in a new php/html file and run.
<!DOCTYPE HTML>
<html>
<head>
<script type="text/javascript">
function test(n){
alert('input:'+n);
var promise = new Promise(function(fulfill, reject) {
/*put your condition here */
if(n) {
fulfill("Inside If! match found");
}
else {
reject(Error("It broke"));
}
});
promise.then(function(result) {
alert(result); // "Inside If! match found"
}, function(err) {
alert(err); // Error: "It broke"
});
}
</script>
</head>
<body>
<input type="button" onclick="test(1);" value="Test"/>
</body>
</html>
Click on Test button,
It will create new promise,
if condition will be true it fulfill the response,
after that promise.then called and based on the fulfill it will print the result.
In case of reject promise.then returns the error message.

Probably the simplest example of promises usage looks like that:
var method1 = (addings = '') => {
return new Promise(resolve => {
console.log('method1' + addings)
resolve(addings + '_adding1');
});
}
var method2 = (addings = '') => {
return new Promise(resolve => {
console.log('method2' + addings)
resolve(addings + '_adding2');
});
}
method1().then(method2).then(method1).then(method2);
// result:
// method1
// method2_adding1
// method1_adding1_adding2
// method2_adding1_adding2_adding1
That's basic of basics. Having it, you can experiment with rejects:
var method1 = (addings = '*') => {
return new Promise((resolve, reject) => {
console.log('method1' + addings)
resolve(addings + '_adding1');
});
}
var method2 = (addings = '*') => {
return new Promise((resolve, reject) => {
console.log('method2' + addings)
reject();
});
}
var errorMethod = () => {
console.log('errorMethod')
}
method1()
.then(method2, errorMethod)
.then(method1, errorMethod)
.then(method2, errorMethod)
.then(method1, errorMethod)
.then(method2, errorMethod);
// result:
// method1*
// method2*_adding1
// errorMethod
// method2*
// errorMethod
// method2*
As we can see, in case of failure error function is fired (which is always the second argument of then) and then next function in chain is fired with no given argument.
For advanced knowledge I invite you here.

please check this simple promise code. this will help you to better understand of promise functionality.
A promise is an object that may produce a single value some time in the future: either a resolved value, or a reason that it’s not resolved. A promise may be in one of 3 possible states: fulfilled, rejected, or pending. Promise users can attach callbacks to handle the fulfilled value or the reason for rejection.
let myPromise = new Promise((resolve, reject)=>{
if(2==2){
resolve("resolved")
}else{
reject("rejected")
}
});
myPromise.then((message)=>{
document.write(`the promise is ${message}`)
}).catch((message)=>{
document.write(`the promise is ${message}`)
})
check this out

Related

How do use Javascript Async-Await as an alternative to polling for a statechange?

I'd like to accomplish the following using promises: only execute further once the state of something is ready. I.e. like polling for an external state-change.
I've tried using promises and async-await but am not getting the desired outcome. What am I doing wrong here, and how do I fix it?
The MDN docs have something similar but their settimeout is called within the promise--that's not exactly what I'm looking for though.
I expect the console.log to show "This function is now good to go!" after 5 seconds, but instead execution seems to stop after calling await promiseForState();
var state = false;
function stateReady (){
state = true;
}
function promiseForState(){
var msg = "good to go!";
var promise = new Promise(function (resolve,reject){
if (state){
resolve(msg);
}
});
return promise;
}
async function waiting (intro){
var result = await promiseForState();
console.log(intro + result)
}
setTimeout(stateReady,5000);
waiting("This function is now ");
What you're doing wrong is the promise constructor executor function executes immediately when the promise is created, and then never again. At that point, state is false, so nothing happens.
Promises (and async/await) are not a replacement for polling. You still need to poll somewhere.
The good news: async functions make it easy to do conditional code with loops and promises.
But don't put code inside promise constructor executor functions, because of their poor error handling characteristics. They are meant to wrap legacy code.
Instead, try this:
var state = false;
function stateReady() {
state = true;
}
const wait = ms => new Promise(resolve => setTimeout(resolve, ms));
async function promiseForState() {
while (!state) {
await wait(1000);
}
return "good to go!";
}
async function waiting(intro) {
var result = await promiseForState();
console.log(intro + result)
}
setTimeout(stateReady,5000);
waiting("This function is now ");
Based on your comments that you are waiting for messages from a server it appears you are trying to solve an X/Y problem. I am therefore going to answer the question of "how do I wait for server messages" instead of waiting for global variable to change.
If your network API accepts a callback
Plenty of networking API such as XMLHttpRequest and node's Http.request() are callback based. If the API you are using is callback or event based then you can do something like this:
function myFunctionToFetchFromServer () {
// example is jQuery's ajax but it can easily be replaced with other API
return new Promise(function (resolve, reject) {
$.ajax('http://some.server/somewhere', {
success: resolve,
error: reject
});
});
}
async function waiting (intro){
var result = await myFunctionToFetchFromServer();
console.log(intro + result);
}
If your network API is promise based
If on the other hand you are using a more modern promise based networking API such as fetch() you can simply await the promise:
function myFunctionToFetchFromServer () {
return fetch('http://some.server/somewhere');
}
async function waiting (intro){
var result = await myFunctionToFetchFromServer();
console.log(intro + result);
}
Decoupling network access from your event handler
Note that the following are only my opinion but it is also the normal standard practice in the javascript community:
In either case above, once you have a promise it is possible to decouple your network API form your waiting() event handler. You just need to save the promise somewhere else. Evert's answer shows one way you can do this.
However, in my not-so-humble opinion, you should not do this. In projects of significant size this leads to difficulty in tracing the source of where the state change comes form. This is what we did in the 90s and early 2000s with javascript. We had a lot of events in our code like onChange and onReady or onData instead of callbacks passed as function parameters. The result was that sometimes it takes you a long time to figure out what code is triggering what event.
Callback parameters and promises forces the event generator to be in the same place in the code as the event consumer:
let this_variable_consumes_result_of_a_promise = await generate_a_promise();
this_function_generate_async_event((consume_async_result) => { /* ... */ });
From the wording of your question you seem to be wanting to do this instead;
..somewhere in your code:
this_function_generate_async_event(() => { set_global_state() });
..somewhere else in your code:
let this_variable_consumes_result_of_a_promise = await global_state();
I would consider this an anti-pattern.
Calling asynchronous functions in class constructors
This is not only an anti-pattern but an impossibility (as you've no doubt discovered when you find that you cannot return the asynchronous result).
There are however design patterns that can work around this. The following is an example of exposing a database connection that is created asynchronously:
class MyClass {
constructor () {
// constructor logic
}
db () {
if (this.connection) {
return Promise.resolve(this.connection);
}
else {
return new Promise (function (resolve, reject) {
createDbConnection(function (error, conn) {
if (error) {
reject(error);
}
else {
this.connection = conn; // cache the connection
resolve(this.connection);
}
});
});
}
}
}
Usage:
const myObj = new MyClass();
async function waiting (intro){
const db = await myObj.db();
db.doSomething(); // you can now use the database connection.
}
You can read more about asynchronous constructors from my answer to this other question: Async/Await Class Constructor
The way I would solve this, is as follows. I am not 100% certain this solves your problem, but the assumption here is that you have control over stateReady().
let state = false;
let stateResolver;
const statePromise = new Promise( (res, rej) => {
stateResolver = res;
});
function stateReady(){
state = true;
stateResolver();
}
async function promiseForState(){
await stateResolver();
const msg = "good to go!";
return msg;
}
async function waiting (intro){
const result = await promiseForState();
console.log(intro + result)
}
setTimeout(stateReady,5000);
waiting("This function is now ");
Some key points:
The way this is written currently is that the 'state' can only transition to true once. If you want to allow this to be fired many times, some of those const will need to be let and the promise needs to be re-created.
I created the promise once, globally and always return the same one because it's really just one event that every caller subscribes to.
I needed a stateResolver variable to lift the res argument out of the promise constructor into the global scope.
Here is an alternative using .requestAnimationFrame().
It provides a clean interface that is simple to understand.
var serverStuffComplete = false
// mock the server delay of 5 seconds
setTimeout(()=>serverStuffComplete = true, 5000);
// continue until serverStuffComplete is true
function waitForServer(now) {
if (serverStuffComplete) {
doSomethingElse();
} else {
// place this request on the next tick
requestAnimationFrame(waitForServer);
}
}
console.log("Waiting for server...");
// starts the process off
requestAnimationFrame(waitForServer);
//resolve the promise or whatever
function doSomethingElse() {
console.log('Done baby!');
}

Linking promise chains

I have a working promise chain:
function startSync(db) {
var promise = new Promise(function(resolve, reject) {
syncCats(db)
.then(syncTrees(db))
.then(syncCars(db))
...
.then(resolve());
});
return promise;
}
This works great. It performs each of those function calls, waiting for each one to complete before firing off another. Each of those functions returns a promise, like so:
function syncCaesar(db) {
var promise = new Promise(resolve, reject) {
// do aysnc db calls, and whatnot, resolving/rejecting appropriately
}
return promise;
}
I need to run this chain on a series of databases, sequentially.
I've tried other solutions here, but they would fire off syncCats() all at once.
For instance:
var promise = Promise.resolve();
dbs.forEach(function(db) {
promise = promise.then(startSync(db));
}
Fires off syncCats(db[0]) and syncCats(db[1]) simultaneously.
Edit:
Performing startSync(dbs[0]).then(startSync(dbs[1])); acts the same.
Edit2: Also performs the same:
dbs.forEach(function(db) {
promise = promise.then(function() {
return startSync(db);
}
}
You're calling startSync and then passing its return value in to then. So naturally, if you do that twice, it's going to start the process twice in parallel.
Instead, pass in a function that doesn't call startSync until it's called:
var promise = Promise.resolve();
dbs.forEach(function(db) {
promise = promise.then(function() { startSync(db); });
});
or with ES2015:
let promise = Promise.resolve();
dbs.forEach(function(db) {
promise = promise.then(_ => startSync(db));
});
Separately, three things jump out about startSync:
It starts all its operations in parallel, not sequentially
It exhibits the promise creation anti-pattern. There's no reason for startSync to create a new promise; it already has a promise it can work with
It ensures that its resolution value is undefined
If you really want the operations running in parallel like that, I suggest being more explicit about it:
function startSync(db) {
return Promise.all([
syncCats(db),
syncTrees(db),
syncCars(db)
])
.then(_ => undefined); // This does #3
}
...but you could also do to avoid the anti-pattern:
// Still run in parallel!
function startSync(db) {
return syncCats(db)
.then(syncTrees(db))
.then(syncCars(db))
.then(_ => undefined); // Does #3
}
If you meant for them to be sequential, you need to pass functions, not the result of calling them:
function startSync(db) {
return syncCats(db)
.then(_ => syncTrees(db))
.then(_ => syncCars(db))
.then(_ => undefined); // Again, does #3
}
If you made syncCats, syncTrees, and syncCars resolve their promises with db, like this:
function syncCats(db) {
return startSomethingAsync().then(_ => db);
}
...then it could be:
function startSync(db) {
return syncCats(db)
.then(syncTrees)
.then(syncCars)
.then(_ => undefined); // Only here for #3
}
...since each would receive db as its first argument.
Finally, if you don't need to force undefined as the promise resolution value, I suggest dropping that part from the above. :-)
With appreciation to T.J and Jaromanda, I was able to solve it by fixing how startSync() was resolved:
function startSync(db) {
var promise = new Promise(function(resolve, reject) {
syncCats(db)
.then(syncTrees(db)) // not correct way to call, see below
.then(syncCars(db))
...
.then(function() { resolve(); });
});
return promise;
}
Now, as T.J points out, the way each of the .then's is being called is incorrect. He explains it better in his answer. I'm being saved by some misunderstood database layer queueing. syncTrees() and syncCars() should be running in parallel.
If I understood it correctly, you will be having an array of databases and you want to sync them one by one i.e. sync for dbs[0], once that is complete sync for dbs[1], and so on. If that's correct you can do something like this.
var syncMultipleDBs = (dataBases) {
var db = dataBases.shift();
if (!db) {
return;
}
startSync(db).then(syncMultipleDBs.bind(null, dataBases));
};
syncsyncMultipleDBs(dbs.slice());
I hope this will help.

Passing an extra parameter in the middle of a Promise chain

I need to throw in a userID param in the middle of a promise chain(it is the only promise that needs it). All the promises should execute in a synchronous order.
SideNote- All the similar examples on stackoverflow are all a bit different- like using lambda functions(I use declared functions).So I'm still not quite sure.
var initialize = function(userID) {
var firstPromise = Module2.getFirstPromise();
firstPromise.then(getSecondPromise)
.then(getThirdPromise)
.then(getFourthPromise) //<----Fourth promise needs userID
.then(getFifthPromise)
.then(Utils.initializeComplete);
}
All the promises are functions that look like this:
var thirdPromise = function() {
return new Promise(function(resolve, reject) {
//fetch data
//On Return, Store it
resolve() //Nothing needed to passed down from this promise
});
});
}
I'm trying this, and it "works", but I'm not sure if that is how I am "suppose" to handle something like this. :)
var initialize = function(userID) {
var firstPromise = Module2.getFirstPromise();
firstPromise.then(getSecondPromise)
.then(getThirdPromise)
.then(function(){ return fourthPromise(userID)})
.then(getFourthPromise)
.then(Utils.initializeComplete);
}
Note: getFirstPromise is coming from a different module in my code. That shouldn't be important to the question though :)
Assuming that firstPromise is really a promise but secondPromise and so on are actually functions returning promises, then yes, what you've done is how you're supposed to do that. Here's a live example on Babel's REPL, which looks like this:
function doSomething(userID) {
getFirstPromise()
.then(getSecondPromise)
.then(getThirdPromise)
.then(() => getFourthPromise(userID))
// Or .then(function() { return getFourthPromise(userID); })
.then(getFifthPromise)
.catch(err => {
console.log("Error: " + err.message);
});
}
doSomething("foo");
function getFirstPromise() {
console.log("getFirstPromise called");
return new Promise(resolve => {
setTimeout(() => {
resolve("one");
}, 0);
});
}
// and then second, third, fourth (with user ID), and fifth
(If you don't use arrow functions, just replace them with the function form.)
Note the catch in the example above. Unless you have a really good reason not to, a promise chain should always have a .catch if you don't return the result.
Your solution is perfectly fine. It might be easier to understand with concrete signatures.
If your thirdPromise doesn't take anything and doesn't return anything its signature might be written (pseudocode assuming a -> b is a function from a to b) as _ -> Promise (_). If it returns some value a, it would be _ -> Promise (a). If it took something and returned something it might be a -> Promise (b)
So you can reason about your promise chains as about functions taking some value and returning some other value wrapped in a promise. However, your fourthPromise looks differently:
fourthPromise : UserId -> a -> Promise (b)
Which can be written as:
fourthPromise : UserId -> (a -> Promise (b))
It takes one parameter before becoming an actual promise you can chain. In a way, it's a template of a promise.
If you want the plain .then chain in the main function, try to write getFourthPromise as a factory function
function getSomethingByUserID(userId) {
return function() {
return new Promise(function(resolve) {
//your actual async job
resolve('a result');
});
};
}
Then you will get plan list of thens
var firstPromise = Module2.getFirstPromise()
.then(getSecondPromise)
.then(getSomethingByUserID(userId))
.then(Utils.initializeComplete);
Do not forget that if you missed to provide userId to getSomethingByUserID, it will not work.

Parse.Query.each() chained promises

I'm writing a background job function on Parse.com CloudCode. The job needs to call the same function (that includes a Parse.Query.each()call) several times with different parameters, and I want to chain these calls with promises. Here's what I have so far:
Parse.Cloud.job("threadAutoReminders", function(request, response) {
processThreads(parameters1).then(function() {
return processThreads(parameters2);
}).then(function() {
return processThreads(parameters3);
}).then(function() {
return processThreads(parameters4);
}).then(function() {
response.success("Success");
}, function(error) {
response.error(JSON.stringify(error));
});
});
Below is the processThreads() function:
function processThreads(parameters) {
var threadQuery = new Parse.Query("Thread");
threadQuery... // set up query using parameters
return threadQuery.each(function(thread) {
console.log("Hello");
// do something
});
}
My questions are:
Am I chaining function calls using promises correctly?
What happens in threadQuery.each() returns zero results? Will the promise chain continue with execution? I'm asking because at the moment "Hello" never gets logged..
Am I chaining function calls using promises correctly?
Yes.
What happens in threadQuery.each() returns zero results? Will the promise chain continue with execution? I'm asking because at the moment "Hello" never gets logged.
I think I'm right in saying that, if "do something" is synchronous, then zero "Hello" messages can only happen if :
an uncaught error occurs in "do something" before a would-be "Hello" is logged, or
every stage gives no results (suspect your data, your query or your expectation).
You can immunise yourself against uncaught errors by catching them. As Parse promises are not throw-safe, you need to catch them manually :
function processThreads(parameters) {
var threadQuery = new Parse.Query("Thread");
threadQuery... // set up query using parameters
return threadQuery.each(function(thread) {
console.log("Hello");
try {
doSomething(); // synchronous
} catch(e) {
//do nothing
}
});
}
That should ensure that the iteration continues and that a fulfilled promise is returned.
The following example shows as use promises inside your function using a web browser implementation.
function processThreads(parameters) {
var promise = new Promise();
var threadQuery = new Parse.Query("Thread");
threadQuery... // set up query using parameters
try {
threadQuery.each(function(thread) {
console.log("Hello");
if (condition) {
throw "Something was wrong with the thread with id " + thread.id;
}
});
} catch (e) {
promise.reject(e);
return promise;
}
promise.resolve();
return promise;
}
Implementations of promise:
Web Browser https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise
jQuery https://api.jquery.com/promise/
Angular https://docs.angularjs.org/api/ng/service/$q

Jquery .then not working

I have an accordion, and want to trigger something when it has finished transitioning from one state to another. The following code is throwing up an error Uncaught TypeError, I am just trying to console.log when it has finished for now:
$(document).ready(function () {
$('.accordion-tabs').each(function() {
$(this).children('li').first().children('a').addClass('is-active').next().addClass('is-open').show();
});
$('.accordion-tabs').on('click', 'li > a.tab-link', function(event) {
if (!$(this).hasClass('is-active')) {
event.preventDefault();
var accordionTabs = $(this).closest('.accordion-tabs');
accordionTabs.find('.is-open').removeClass('is-open').hide();
$(this).next().toggleClass('is-open').toggle();
accordionTabs.find('.is-active').removeClass('is-active');
$(this).addClass('is-active').then(
function() {
console.log( "Accordion Finished" );
});
} else {
event.preventDefault();
}
});
});
Where am I going wrong? This is the first time I have used .then!
yes it's not working, this is not the way of using it you need to learn promises first
It's used to replace (or provide an alternate way) the old callback mechanism with a cleaner way to handle asynchronous requests, instead of passing your callbacks as parameters, you can chain your function with .then, given function will be executed once the promise gets resolved.
Anyhow, this is just a basic explanation, you should really get into the books of promises for more info.
a simple example of :
var promise = new Promise(function(resolve, reject) {
if (true /* everything turned out fine */) {
resolve("Stuff worked!");
}
else {
reject(Error("It broke"));
}
});
promise.then(function (x) { // Suppose promise returns "abc"
console.log(x);
return 123;
}).then(function (x){
console.log(x);
}).then(function (x){
console.log(x)
})

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