I have written a library function pRead(Path), which returns a JavaScript Promise to read a file on the local computer under an Apache server, using Ajax. I won't include the code for this, as it uses standard technology that is well-known to anyone who can give a good answer to this question.
I want to write a second library function, pReadObj(Path), which will return a Promise to read a JSON file and provide its object value to asynchronous code. It should work like this:
pReadObj("test.json").then(then2).catch(pErr);
function then2(obj)
{
alert(JSON.stringify(obj)); // Shows the JSON obj
} // then2
Here is the code I wrote:
var globalPreviousResolve;
function pReadObj(Path) // Promise to read JSON from file
{
return new Promise(function(resolve,reject)
{
globalPreviousResolve=resolve;
pRead(Path).then(pReadObj2).catch(pErr);
});
} // pReadObj
function pReadObj2(JSONStr)
{
globalPreviousResolve(JSON.parse(JSONStr));
} // pReadObj2
function pTestDB() // Called from button
{
pReadObj("test.json").then(then2).catch(pErr);
} // pTestDB
This works, but has a problem: using a global variable to hold the resolve callback is not only ugly, but it will clearly malfunction if two calls to pReadObj happen within a short period of time and the disk read takes a longer time than that.
The resolve function needs to be stored inside the Promise in some way, I'm thinking.
There's no need to explicitly create a Promise; just return the one created by .then:
function pReadObj(Path) {
return pRead(Path).then(JSON.parse);
}
I'm having trouble understanding how Node operates regarding it's parallel processing and returning values from function calls.
FYI: The gulp function below is merely created as an example for this question.
Is it possible that the function could return the stream before the Read a large file statement has finished processing (the large file has been fully read from the file system and the stream has been added), or is Node smart enough to complete all statements before returning?
function moveFiles(){
var gulp = require('gulp'),
stream = require('merge-stream')();
// Read a large file
stream.add(gulp.src('src/large-file.txt')
.pipe(gulp.dest('dest/'))
);
// Read a small file
stream.add(gulp.src('src/small-file.txt')
.pipe(gulp.dest('dest/'))
);
return (stream.isEmpty() ? null : stream);
}
Could Node feasibly return a value from a function call before completing all operations within the function itself?
This is a tricky question. The answer is no, in a way that returning a value means that the function is finished executing, it's taken back from the stack and it will never do anything again - unless it's invoked another time of course, but the point is that this particular invocation is over.
But the tricky part is that it's the function that's finished executing and it doesn't mean that it couldn't schedule something else to happen in the future. It will get more complicated in a minute but first a very simple example.
function x() {
setTimeout(function () {
console.log('x1'));
}, 2000);
console.log('x2');
return;
console.log('x3');
}
Here when you call x() then it will schedule another function to run after 2 seconds, then it will print x2 and then it will return - at which point this function cannot do anything else ever again for that invocation.
It means that x3 will never get printed, but x1 will eventually get printed - because it's another function that will be called when the timeout fires. The anonymous function will get called not because the x() function can do anything after it returns, but because it managed to schedule the timeout before it returned.
Now, instead of just scheduling things to happen in the future, a function can return a promise that will get resolved some time later. For example:
function y() {
console.log('y1');
return new Promise(function (resolve, reject) {
setTimeout(function () {
resolve('message from y()');
}, 2000);
});
console.log('y2');
}
Now, when you run:
var promise = y();
what will happen is that y1 will get printed, a new promise will get returned and y2 will never get printed because at that point y() returned and cannot do anything else. But it managed to schedule a timeout that will resolve the promise after two seconds.
You can observe it with:
promise.then(function (value) {
console.log(value);
});
So with this example you can see that while the y() function itself returned and cannot do anything else, some other (anonymous in this case) function can be called in the future and finish the job that the y() function has initiated.
So I hope now it's clear why it's a tricky question. In a way a function cannot do anything after returning. But it could have scheduled some other functions as timeouts, event handlers etc. that can do something after the functions returns. And if the thing that the function returns is a promise then the caller can easily observe the value in the future when it's ready.
All of the examples could be simplified by using the arrow functions but I wanted to make it explicit that those are all separate functions, some of them are named, some are anonymous.
For more details see some of those answers:
A detailed explanation on how to use callbacks and promises
Explanation on how to use promises in complex request handlers
An explanation of what a promise really is, on the example of AJAX requests
An explanation of callbacks, promises and how to access data returned asynchronously
Note: I'm bootstrapping a reactjs app but this is a general JavaScript question.
I have a special module "locationUtils" that I am trying to keep in it's own package, but keeping that code separate is causing an eyesore with callbacks.
When I access one of it's methods I have to send a callback with it that only has one of its parameters initially defined, and in that method I get the other data parameter to initalize the other parameter.
Can I add in undefined parameters later like that in JavaScript, and is it good practice to initial parameters for a callback method as you go down the callback chain in general, or am I making a convoluted newbie mistake?
/********************Module 1******************************/
var bootStrapUI = function(callback) {
locationUtils.findData(findOtherData(callback));
}
//This gets called last to finalize bootstraping
var findOtherData = function(callback,originalFetchedData){
//use originalFetchedData to get more data
//bootStraping program with all rendering data
callback() //sends back a boolean confirming all fetched
}
/**********************Module2**********************************/
var findData = function(findOtherData){
var data = magicGetData();
findOtherData(findOtherData,data);//I initialized a param late here!
}
It's a good Javascript question, callbacks can become a serious hell for the uninitiated, particularly when they are nested and / or the order in which they return is important.
This is where promises come in: they are an essential tool for Javascript development and about to become part of the standard (in EcmaScript 6).
In essence: a promise is an object that is returned from a function with a method (callback) that is called when the asynchronous action (e.g. API call) has been completed. The difference between a promise and a callback is that promises allow you to structure how you handle the callbacks and, importantly, in what order.
I recently wrote a method that had to make 30 api calls with each call dependent on the results of the previous one (this was not a well designed api). Can you imagine trying to do that with callbacks? As it was, I created an array of promises and used jQuery.when() to handle things when all the api calls had completed.
For the moment we need to use a library for promises. jQuery: https://api.jquery.com/jquery.deferred/ is the obvious one but there are various other implementations that do much the same thing.
Update:
The question relates more specifically to the passing of arguments between callbacks and modifying the arguments as execution moves between them. This can be done easily by passing whatever info you need as an argument to your resolve method.
Typically it looks something like this (using jQuery):
var myAsyncMethod = function(info){
var deferred = $.Deferred();
$.getJSON(myUrl,
function(dataFromServer) {
// Do stuff with data
var newData = doSomething(dataFromServer);
deferred.resolve(newData);
});
});
return deferred.promise();
};
// Make initial method call
myAsyncMethod(myInitialData).then(
function(transformedData){
// transformed data from server is returned here.
}
);
I have created a web api method that carries out calculations using a json object I post to the method. My understanding is that a jquery post is asynchronous? Assuming this is so, I'd like to be able to chain together multiple calls to the js function that invokes this api method because certain calls are order-critical.
There are 80 calls to this asynchronous api method in a row. I don't want to do:
asyncMethodCall1(myParams).then(asyncMethodCall2(myParams).then...))
...
as there is no need to chain them like this as none of them depend on the other and can all run simultaneously. However at the end I need to do some other calculations that DO depend on the previous results having finished.
Is there a way of doing a sort of group of calls followed by a "then"? Is it simply a case of having something like:
mySynchronousFunction(params).then(function() {...do other calcs});
function mySynchronousFunction(params) {
asyncmethodcall1(myparams);
asyncmethodcall2(myparams);
asyncmethodcall3(myparams);
...
asyncmethodcall76(myparams);
}
or do I need to have a "setTimeOut" in there somewhere? JavaScript is something I'm tryign to learn and it's all a bit odd to me at the moment!
EDIT 1
Well I'm stumped.
I know it's a fundamental lack of understanding on my part that's causing the problem but it's very hard to find basic introductory stuff that someone coming from a synchronous language can follow and understand. Currently a resource I'm working through is this one and it seems to make some sense but it's still not sinking in: http://blog.mediumequalsmessage.com/promise-deferred-objects-in-javascript-pt1-theory-and-semantics
Currently I have this:
$.when(
individualImpactCalc('byWeightAndFactor', 'CO2e', articleResults().material_CO2e),
individualImpactCalc('byWeightAndFactor', 'Water', articleResults().material_Water),
individualImpactCalc('byWeightAndFactor', 'pH', articleResults().material_pH),
...lots of other calls in here...
).then(function () {
//do calculation totalling
alert("I'm done!");
}).fail(function() {
alert("Argh! I failed!");
});
...and it simply doesn't work as I want it to. I never get an alert showing. The impact calculations are done, the observables are updated, my page values change, but no alerts.
What am I fundamentally missing here?
Edit 2
What I was fundamentally missing was the difficulty in debugging chained promises! It actually was working once I cured a hidden reference error that wasn't bubbling up. Lots of painstaking stepping through javascript finally found it.
Just to add in another element to the mix of answers, I used the Q library as that was what is used in Durandal/Breeze anyway and it's just easy to keep the code consistent. I tried it with $.when and it worked just as well. I tried it with Promise and it failed with "Promise is not defined".
My working Q implementation:
var calcPromise = Q.all([
individualImpactCalc('byWeightAndFactor', 'CO2e', articleResults().material_CO2e),
individualImpactCalc('byWeightAndFactor', 'Water', articleResults().material_Water),
individualImpactCalc('byWeightAndFactor', 'pH', )]);
return calcPromise
.then(calcsDone)
.fail(calcsFailed);
function calcsDone() {
alert("all calcs done");
}
function calcsFailed() {
alert("Argh! Calc failure...");
}
What you are looking for is the jQuery when method:
http://api.jquery.com/jQuery.when/
In the case where multiple Deferred objects are passed to jQuery.when,
the method returns the Promise from a new "master" Deferred object
that tracks the aggregate state of all the Deferreds it has been
passed. The method will resolve its master Deferred as soon as all the
Deferreds resolve, or reject the master Deferred as soon as one of the
Deferreds is rejected.
Specifically you put the calls that don't depend on each other in when, and the ones that depend on them in the then.
$.when(/* independent calls */).then(/* dependent calls */);
So as an example if you want to run deffered 1 and 2 in paralel, then run 3, then run 4 you can do:
$.when(def1, def2).then(def3).then(def4);
You can use Promise.when() to wait for all of them.
function mySynchronousFunction(params) {
return Promise.when(asyncmethodcall1(myparams),
asyncmethodcall2(myparams),
...);
}
Promise.when returns a new promise, so you can chain this with .then().
See Asynchronous Programming in JavaScript with “Promises”
As others have pointed out, you can use $.when(...) to call all of the methods at once and only continue with .then() when all are done. But that doesn't solve your problem, because the methods are started all at once and there is no chaining. Calls to your web api can still come in and finish in random order.
Even though your example shows different async methods, your description sounds like you have only one method with different parameter values. So why not collecting those parameters in an array and then chaining the calls in a loop?
var params = [ 27, 38, 46, 83, 29, 22 ];
var promise = asyncmethodcall(params[0]);
for (var i=1; i<params.length; i++) {
promise = promise.then(buildAsyncmethodcall(params[i]));
}
function buildAsyncmethodcall(param) {
return function() {
return asyncmethodcall(param);
}
}
http://jsfiddle.net/qKS5e/ -- see here why I had to build the function using another function
If you really want to call different methods, you could write a jQuery plugin eg. $.whenSequential(...) to which you pass an array of functions that return Deferred objects like this:
$.whenSequential( // your cool new plugin
function() { return callasyncmethod1(123); },
function() { return callasyncmethod2(345); },
function() { return callasyncmethod3(678); }
);
The plugin method would work just like the for-loop above, but instead of params you iterate the function arguments and pass one by one to then().
Either way, you won't get around wrapping all calls in a function somehow to chain them, because otherwise they would get executed immediately (like with $.when).
I’ve been seeing code that looks like:
myObj.doSome("task").then(function(env) {
// logic
});
Where does then() come from?
The traditional way to deal with asynchronous calls in JavaScript has been with callbacks.
Say we had to make three calls to the server, one after the other, to set up our
application. With callbacks, the code might look something like the following (assuming
a xhrGET function to make the server call):
// Fetch some server configuration
xhrGET('/api/server-config', function(config) {
// Fetch the user information, if he's logged in
xhrGET('/api/' + config.USER_END_POINT, function(user) {
// Fetch the items for the user
xhrGET('/api/' + user.id + '/items', function(items) {
// Actually display the items here
});
});
});
In this example, we first fetch the server configuration. Then based on that, we fetch
information about the current user, and then finally get the list of items for the current
user. Each xhrGET call takes a callback function that is executed when the server
responds.
Now of course the more levels of nesting we have, the harder the code is to read, debug,
maintain, upgrade, and basically work with. This is generally known as callback hell.
Also, if we needed to handle errors, we need to possibly pass in another function to each
xhrGET call to tell it what it needs to do in case of an error. If we wanted to have just one
common error handler, that is not possible.
The Promise API was designed to solve this nesting problem and the
problem of error handling.
The Promise API proposes the following:
Each asynchronous task will return a promise object.
Each promise object will have a then function that can take two arguments, a success
handler and an error handler.
The success or the error handler in the then function will be called only once, after
the asynchronous task finishes.
The then function will also return a promise, to allow chaining multiple calls.
Each handler (success or error) can return a value, which will be passed to the next
function as an argument, in the chain of promises.
If a handler returns a promise (makes another asynchronous request), then the next
handler (success or error) will be called only after that request is finished.
So the previous example code might translate to something like the following, using
promises and the $http service(in AngularJs):
$http.get('/api/server-config').then(
function(configResponse) {
return $http.get('/api/' + configResponse.data.USER_END_POINT);
}
).then(
function(userResponse) {
return $http.get('/api/' + userResponse.data.id + '/items');
}
).then(
function(itemResponse) {
// Display items here
},
function(error) {
// Common error handling
}
);
Propagating Success and Error
Chaining promises is a very powerful technique that allows us to accomplish a lot of
functionality, like having a service make a server call, do some postprocessing of the
data, and then return the processed data to the controller. But when we work with
promise chains, there are a few things we need to keep in mind.
Consider the following hypothetical promise chain with three promises, P1, P2, and P3.
Each promise has a success handler and an error handler, so S1 and E1 for P1, S2 and
E2 for P2, and S3 and E3 for P3:
xhrCall()
.then(S1, E1) //P1
.then(S2, E2) //P2
.then(S3, E3) //P3
In the normal flow of things, where there are no errors, the application would flow
through S1, S2, and finally, S3. But in real life, things are never that smooth. P1 might
encounter an error, or P2 might encounter an error, triggering E1 or E2.
Consider the following cases:
• We receive a successful response from the server in P1, but the data returned is not
correct, or there is no data available on the server (think empty array). In such a
case, for the next promise P2, it should trigger the error handler E2.
• We receive an error for promise P2, triggering E2. But inside the handler, we have
data from the cache, ensuring that the application can load as normal. In that case,
we might want to ensure that after E2, S3 is called.
So each time we write a success or an error handler, we need to make a call—given our
current function, is this promise a success or a failure for the next handler in the promise
chain?
If we want to trigger the success handler for the next promise in the chain, we can just
return a value from the success or the error handler
If, on the other hand, we want to trigger the error handler for the next promise in the
chain, we can do that using a deferred object and calling its reject() method
Now What is deferred object?
Deferred objects in jQuery represents a unit of work that will be
completed later, typically asynchronously. Once the unit of work
completes, the deferred object can be set to resolved or failed.
A deferred object contains a promise object. Via the promise object
you can specify what is to happen when the unit of work completes. You
do so by setting callback functions on the promise object.
Deferred objects in Jquery : https://api.jquery.com/jquery.deferred/
Deferred objects in AngularJs : https://docs.angularjs.org/api/ng/service/$q
then() function is related to "Javascript promises" that are used in some libraries or frameworks like jQuery or AngularJS.
A promise is a pattern for handling asynchronous operations. The promise allows you to call a method called "then" that lets you specify the function(s) to use as the callbacks.
For more information see: http://wildermuth.com/2013/8/3/JavaScript_Promises
And for Angular promises: http://liamkaufman.com/blog/2013/09/09/using-angularjs-promises/
As of ECMAScript6
The .then() method has been included with pure JavaScript with Promises.
From the Mozilla documentation:
The then() method returns a Promise. It takes two arguments: callback
functions for the success and failure cases of the Promise.
The Promise object, in turn, is defined as
The Promise object is used for deferred and asynchronous
computations. A Promise represents an operation that hasn't completed
yet, but is expected in the future.
That is, the Promise acts as a placeholder for a value that is not yet computed, but shall be resolved in the future. And the .then() function is used to associate the functions to be invoked on the Promise when it is resolved - either as a success or a failure.
Before ECMAScript6
To my knowledge, there isn't a built-in then() method in javascript (at the time of this writing).
It appears that whatever it is that doSome("task") is returning has a method called then.
If you log the return result of doSome() to the console, you should be able to see the properties of what was returned.
console.log( myObj.doSome("task") ); // Expand the returned object in the
// console to see its properties.
Here is a thing I made for myself to clear out how things work. I guess others too can find this concrete example useful:
doit().then(function() { log('Now finally done!') });
log('---- But notice where this ends up!');
// For pedagogical reasons I originally wrote the following doit()-function so that
// it was clear that it is a promise. That way wasn't really a normal way to do
// it though, and therefore Slikts edited my answer. I therefore now want to remind
// you here that the return value of the following function is a promise, because
// it is an async function (every async function returns a promise).
async function doit() {
log('Calling someTimeConsumingThing');
await someTimeConsumingThing();
log('Ready with someTimeConsumingThing');
}
function someTimeConsumingThing() {
return new Promise(function(resolve,reject) {
setTimeout(resolve, 2000);
})
}
function log(txt) {
document.getElementById('msg').innerHTML += txt + '<br>'
}
<div id='msg'></div>
Here is a small JS_Fiddle.
then is a method callback stack which is available after a promise is resolved it is part of library like jQuery but now it is available in native JavaScript and below is the detail explanation how it works
You can do a Promise in native JavaScript : just like there are promises in jQuery, Every promise can be stacked and then can be called with Resolve and Reject callbacks, This is how you can chain asynchronous calls.
I forked and Edited from MSDN Docs on Battery charging status..
What this does is try to find out if user laptop or device is charging battery. then is called and you can do your work post success.
navigator
.getBattery()
.then(function(battery) {
var charging = battery.charging;
alert(charging);
})
.then(function(){alert("YeoMan : SINGH is King !!");});
Another es6 Example
function fetchAsync (url, timeout, onData, onError) {
…
}
let fetchPromised = (url, timeout) => {
return new Promise((resolve, reject) => {
fetchAsync(url, timeout, resolve, reject)
})
}
Promise.all([
fetchPromised("http://backend/foo.txt", 500),
fetchPromised("http://backend/bar.txt", 500),
fetchPromised("http://backend/baz.txt", 500)
]).then((data) => {
let [ foo, bar, baz ] = data
console.log(`success: foo=${foo} bar=${bar} baz=${baz}`)
}, (err) => {
console.log(`error: ${err}`)
})
Definition :: then is a method used to solve Asynchronous callbacks
this is introduced in ES6
Please find the proper documentation here Es6 Promises
.then returns a promise in async function.
Good Example would be:
var doSome = new Promise(function(resolve, reject){
resolve('I am doing something');
});
doSome.then(function(value){
console.log(value);
});
To add another logic to it, you can also add the reject('I am the rejected param') call the function and console.log it.
It's about the use of curly braces {} in our arrow functions:
Those 3 examples are doing the same thing (nothing, but have valid grammar, and are a valid Promise chain!)
new Promise(function(ok) {
ok(
/* myFunc1(param1, param2, ..) */
)
}).then(function(){
/* myFunc1 succeed */
/* Launch something else */
/* console.log(whateverparam1) */
/* myFunc2(whateverparam1, otherparam, ..) */
}).then(function(){
/* myFunc2 succeed */
/* Launch something else */
/* myFunc3(whatever38, ..) */
})
console.log("This code has no errors GG!")
The same logic using arrow functions shorthand without {}
new Promise((ok) =>
ok(
/* myFunc1(param1, param2, ..) */
).then(() =>
0 // HEY DID YOU NOTICE! A number that does nothing,
// but otherwise the parsing will fail!
// The code is pretty clean but have a major downside
// As arrow functions without {} can contains only one declaration
// console.log("something") will FAIL here
).then(() =>
"" // HEY DID YOU NOTICE! An empty string that does nothing,
// but otherwise the parsing will fail!
// As arrow functions without {} can contains only one declaration
// We can't add more code here, hence:
// console.log("something")
// Will break the whole promise
// This is likely the error in y(our) code ;)
))
console.log("This code has no errors GG!")
Arrow function with {}
new Promise( (ok) => {
ok(
/* myFunc1(param1, param2, ..) */
)
}).then( () => {
/* myFunc1 succeed */
/* Launch something else */
}).then( () => {
/* myFunc2 succeed */
/* Launch something else */
/* myFunc3(whatever38, ..) */
console.log("something")
/* More console logs! */
console.log("something else")
})
console.log("This code has no errors GG!")
I suspect doSome returns this, which is myObj, which also has a then method. Standard method chaining...
if doSome is not returning this, being the object on which doSome was executed, rest assured it is returning some object with a then method...
as #patrick points out, there is no then() for standard js
doSome("task")must be returning a promise object , and that promise always have a then function .So your code is just like this
promise.then(function(env) {
// logic
});
and you know this is just an ordinary call to member function .
In this case then() is a class method of the object returned by doSome() method.
The ".then()" function is wideley used for promised objects in Asynchoronus programming For Windows 8 Store Apps.
As far as i understood it works some way like a callback.
Find Details in this Documentantion
http://msdn.microsoft.com/en-us/library/windows/apps/hh700330.aspx
Of Cause it could also be the name for any other defined function.
I am about 8 years late, well...anyways, I don't really know what then() does but maybe MDN might have an answer. Actually, I might actually understand it a little more.
This will show you all the information (hopefully), you need. Unless someone already posted this link.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/then
The format is promise.prototype.then()
The promise and prototype are kind of like variables but not like variables in javascript, I mean like other things go there like navigator.getBattery().then() where this one actually exists but is barely used on the web, this one shows statuses about the battery of the device, more information and more on MDN if you are curious.