Proper explanation of JavaScript execution order - javascript

I know there are lots of question on this subject, but all of them are tailored to a specific case. I'm asking this question and hoping for an answer that everyone can use when looking into this matter.
Say I have three functions that need to be executed in order, and they all do something async.
For two functions it's simple with a callback:
var fOne = function(callback) {
// do stuff
callback();
};
var fTwo = function() {
// do stuff
};
Calling:
fOne(fTwo);
Makes fTwo to be run after fOne is complete. How does one add fThree into this case so that it is run after fTwo, which is run after fOne?

Basically, if I understand your question right, you'll want to pass a lambda to fOne in order to be able to add arguments to fTwo, then have fTwo collect the callback and call it like fOne does.
var fOne = function(callback) {
// do stuff
callback();
};
var fTwo = function(callback) {
// do stuff
callback();
};
var fThree = function() {
// do stuff
};
fOne(function() { fTwo(fThree); }); // <- lambda

Since you're not using deferred and promises to run simultaneous asyncronous actions, but rather just starting one when the other has completed, adding a third goes pretty much the same way:
var fOne = function(callback) {
console.log('1')
callback();
};
var fTwo = function(callback) {
console.log('2')
callback();
};
var fThree = function() {
console.log('3')
};
fOne(function() {
fTwo(function() {
fThree();
});
});
FIDDLE

One more approach I came up with:
var stack = [fOne, fTwo, fThree];
stack.shift()(function() {
stack.shift()(stack[0]);
});
Where you define your functins as:
function fOne(callback) {
setTimeout(function() { // <-- example of async task
alert('fOne done');
callback && callback();
}, 1000);
}
http://jsfiddle.net/kRMr8/2/

Related

Using callbacks in window.onload

I think there is an easy solution for this, but for some reason I am not getting the expected results. My functions look like this:
var functionA = function(callback) {
loadData(fromURL1); // takes some time
loadData(fromURL2); // takes some time
callback(); // Should be called AFTER the loadData() functions are finished
}
var myCallBackFunction = function() {
// this function is called AFTER functionA() is finished
alert("All my loaded data from URL1 and URL2");
}
window.onload = function() {
functionA(myCallBackFunction);
}
Unfortunately, the callback() function above doesn't wait for loadData() to finish, and then just calls the alert with empty data.
I read a lot of online examples, but I think I am still missing something obvious.
If the loadData()s are async operations, you can do two things:
Using $.ajaxComplete():
var functionA = function(callback) {
loadData(fromURL1); // takes some time
loadData(fromURL2); // takes some time
$.ajaxComplete(function () {
callback(); // Should be called AFTER the loadData() functions are finished
});
}
Or chaining the functions:
var functionA = function(callback) {
loadData(fromURL1, function () {
loadData(fromURL2, function () {
callback(); // Should be called AFTER the loadData() functions are finished
}); // takes some time
}); // takes some time
}

How to make my JS callback work right?

I have the following problem:
I'm trying to implement a Callback in JavaScript. Now I just made it with a global variable which holds my callbacl function. Here is the example:
_callbackFkt = null;
requestCompleted = function(oControlEvent) {
console.log("Callback: " + _callbackFkt.toString());
};
myLibRequest = function(callback) {
// some code, which is attached to the requestComplete event when ready
_callbackFkt = callback;
};
Now I try to call the functions which use the callback:
myLibRequest(function () {
// callback function 1
});
myLibRequest(function () {
// callback function 2
});
myLibRequest(function () {
// callback function 3
});
the result in the console is:
Callback: function () {
// callback function 3
}
How can I define the callback to be bound to one function call and not global available? I want the result:
Callback: function () {
// callback function 1
}
Callback: function () {
// callback function 2
}
Callback: function () {
// callback function 3
}
There are several ways to do what you are trying to do, but your basic problem is that you want a list of event handlers, but you are only assigning a single value.
To modify what you are currently doing:
_callbackFkts = [];
myLibRequest = function(callback) {
// some code, which is attached to the requestComplete event when ready
_callbackFkts.push(callback);
};
Then, when you want to execute the callbacks:
_callbackFkts.forEach(function(callbackFkt) {
callbackFkt();
});
But, this global mechanism is a bit messy. You might consider some encapsulation (untested, but you get the idea):
function Events() {
this.callbacks = [];
}
Events.protototype.bind = function(callback) {
this.callbacks.push(callback);
};
Events.prototype.executeAll = function(params) {
this.callbacks.forEach(function(callback) {
callback.apply(this, params);
}
}
Then you can use it like this:
var events = new Events();
events.bind(function() {
//callback function 1
});
events.bind(function() {
//callback function 2
});
events.bind(function() {
//callback function 3
});
events.executeAll('with', 'parameters');
Finally, you might just use an off-the-shelf event library. There are lots. One quick google search finds this.
Having a global as the callback will only work if myLibRequest() contains only synchronous code (which I assume it doesn't).
Remove the global, and use the callback that is passed in as an argument.
Assuming you have some async call in there, and you call requestCompleted when it's done. Add an argument so requestCompleted receives the callback, instead of referenceing the global.
requestCompleted = function(oControlEvent, callback) {
console.log("Callback: " + callback.toString());
};
myLibRequest = function(callback) {
myAsyncFunction(function(){
// async complete
requestCompleted('event', callback);
});
};

jQuery: using deferred properly in a local context (ie. no AJAX)

Apologies for what I'm sure is a repost; I really have looked quite widely for an answer to my question (that I also understood).
What I'm trying to learn to do is to arbitrarily chain functions such that they must complete before the next occurs, which, as I understand it, is the purpose of jQuery's deferred(). So in the below code, what I'm imagining should happen is:
the function contained within the load deferred objects executes; after which
the function contained in then() executes; after which
the function contained in done() executes.
Every tutorial in the universe uses a $.ajax() object after $.when(), which is useless if all one wants is control of execution sequence in a local context.
Here's what I've been trying:
var preloadDone = false,
var testDone = false,
var load = $.deferred(function() {
//cacheImages() is a plugin, works fine
$("img.image-loader.preload").cacheImages();
preloadDone = true;
});
var loader = $.when(load)
.then(function() {
if (preloadDone) {
console.log("then called in sequence");
} else {
console.log("then called out of sequence"); // wrong order, every time
}
XBS.cache.cbDone = true;
}).done(function() {
if (XBS.cache.cbDone) {
console.log("even done was called in right sequence!"); // proper order, every time
} else {
console.log("done() wasn't called in order..");
}
});
load.resolve(); // nothing happens
// load(); also tried this; nothing happens
So far as I can tell, this is identical to the example given in the jQuery $.when() documentation. Lil help?
Here is a demo on how to run many functions one after another but only after each funtion has completed. This is achieved by using an Async function.
Demo (Runs 3 functions one after the other. Where i have alert("starting *") that is were you want to put the work you like to do and in the done function you include the next function you want to run. )
http://jsfiddle.net/5xLbk91c/
//the Assync function. Pause is the time in miliseconds to pause between loops
var asyncFor = function(params) {
var defaults = {
total: 0,
limit: 1,
pause: 10,
context: this
},
options = $.extend(defaults, params),
def = $.Deferred(),
step = 0,
done = 0;
this.loop = function() {
if (done < options.total) {
step = 0;
for (; step < options.limit; step += 1, done += 1) {
def.notifyWith(options.context, [done]);
}
setTimeout.apply(this, [this.loop, options.pause]);
} else {
def.resolveWith(options.context);
}
};
setTimeout.apply(this, [this.loop, options.pause]);
return def;
};
function one() {
asyncFor({
total: 1, // run only once. If you want to loop then increase to desired total.
context: this
}).progress(function(step) {
alert("starting one")
}).done(function() {
alert("finished one")
two()
});
}
function two() {
asyncFor({
total: 1,
context: this
}).progress(function(step) {
alert("starting two")
}).done(function() {
alert("finished two")
three()
});
}
function three() {
asyncFor({
total: 1,
context: this
}).progress(function(step) {
alert("starting three")
}).done(function() {
alert("finished three and all done")
});
}
you may want to start your investigations by this change to your code:
var load = function() {
var deferred = $.Deferred();
$("img.image-loader.preload").cacheImages();
preloadDone = true;
return deferred;
};
Please also note you may pass array of promises to $.when().
Best regards

Wait till a Function with animations is finished until running another Function

I'm having an issue with normal (non-ajax) functions that involve lots of animations within each of them. Currently I simply have a setTimeout between functions, but this isn't perfect since no browsers / computers are the same.
Additional Note: They both have separate animations/etc that collide.
I can't simply put one in the callback function of another
// multiple dom animations / etc
FunctionOne();
// What I -was- doing to wait till running the next function filled
// with animations, etc
setTimeout(function () {
FunctionTwo(); // other dom animations (some triggering on previous ones)
}, 1000);
Is there anyway in js/jQuery to have:
// Pseudo-code
-do FunctionOne()
-when finished :: run -> FunctionTwo()
I know about $.when() & $.done(), but those are for AJAX...
MY UPDATED SOLUTION
jQuery has an exposed variable (that for some reason isn't listed anywhere in the jQuery docs) called $.timers, which holds the array of animations currently taking place.
function animationsTest (callback) {
// Test if ANY/ALL page animations are currently active
var testAnimationInterval = setInterval(function () {
if (! $.timers.length) { // any page animations finished
clearInterval(testAnimationInterval);
callback();
}
}, 25);
};
Basic useage:
// run some function with animations etc
functionWithAnimations();
animationsTest(function () { // <-- this will run once all the above animations are finished
// your callback (things to do after all animations are done)
runNextAnimations();
});
You can use jQuery's $.Deferred
var FunctionOne = function () {
// create a deferred object
var r = $.Deferred();
// do whatever you want (e.g. ajax/animations other asyc tasks)
setTimeout(function () {
// and call `resolve` on the deferred object, once you're done
r.resolve();
}, 2500);
// return the deferred object
return r;
};
// define FunctionTwo as needed
var FunctionTwo = function () {
console.log('FunctionTwo');
};
// call FunctionOne and use the `done` method
// with `FunctionTwo` as it's parameter
FunctionOne().done(FunctionTwo);
you could also pack multiple deferreds together:
var FunctionOne = function () {
var
a = $.Deferred(),
b = $.Deferred();
// some fake asyc task
setTimeout(function () {
console.log('a done');
a.resolve();
}, Math.random() * 4000);
// some other fake asyc task
setTimeout(function () {
console.log('b done');
b.resolve();
}, Math.random() * 4000);
return $.Deferred(function (def) {
$.when(a, b).done(function () {
def.resolve();
});
});
};
http://jsfiddle.net/p22dK/
add the following to the end of the first function
return $.Deferred().resolve();
call both functions like so
functionOne().done(functionTwo);
Along with Yoshi's answer, I have found another very simple (callback type) solution for animations.
jQuery has an exposed variable (that for some reason isn't listed anywhere in the jQuery docs) called $.timers, which holds the array of animations currently taking place.
function animationsTest (callback) {
// Test if ANY/ALL page animations are currently active
var testAnimationInterval = setInterval(function () {
if (! $.timers.length) { // any page animations finished
clearInterval(testAnimationInterval);
callback();
}
}, 25);
};
Basic useage:
functionOne(); // one with animations
animationsTest(functionTwo);
Hope this helps some people out!
This answer uses promises, a JavaScript feature of the ECMAScript 6 standard. If your target platform does not support promises, polyfill it with PromiseJs.
You can get the Deferred object jQuery creates for the animation using .promise() on the animation call. Wrapping these Deferreds into ES6 Promises results in much cleaner code than using timers.
You can also use Deferreds directly, but this is generally discouraged because they do not follow the Promises/A+ specification.
The resulting code would look like this:
var p1 = Promise.resolve($('#Content').animate({ opacity: 0.5 }, { duration: 500, queue: false }).promise());
var p2 = Promise.resolve($('#Content').animate({ marginLeft: "-100px" }, { duration: 2000, queue: false }).promise());
Promise.all([p1, p2]).then(function () {
return $('#Content').animate({ width: 0 }, { duration: 500, queue: false }).promise();
});
Note that the function in Promise.all() returns the promise. This is where magic happens. If in a then call a promise is returned, the next then call will wait for that promise to be resolved before executing.
jQuery uses an animation queue for each element. So animations on the same element are executed synchronously. In this case you wouldn't have to use promises at all!
I have disabled the jQuery animation queue to demonstrate how it would work with promises.
Promise.all() takes an array of promises and creates a new Promise that finishes after all promises in the array finished.
Promise.race() also takes an array of promises, but finishes as soon as the first Promise finished.
Is this what you mean man: http://jsfiddle.net/LF75a/
You will have one function fire the next function and so on, i.e. add another function call and then add your functionONe at the bottom of it.
Please lemme know if I missed anything, hope it fits the cause :)
or this: Call a function after previous function is complete
Code:
function hulk()
{
// do some stuff...
}
function simpsons()
{
// do some stuff...
hulk();
}
function thor()
{
// do some stuff...
simpsons();
}
ECMAScript 6 UPDATE
This uses a new feature of JavaScript called Promises
functionOne().then(functionTwo);
You can do it via callback function.
$('a.button').click(function(){
if (condition == 'true'){
function1(someVariable, function() {
function2(someOtherVariable);
});
}
else {
doThis(someVariable);
}
});
function function1(param, callback) {
...do stuff
callback();
}
Here is a solution for n-calls (recursive function).
https://jsfiddle.net/mathew11/5f3mu0f4/7/
function myFunction(array){
var r = $.Deferred();
if(array.length == 0){
r.resolve();
return r;
}
var element = array.shift();
// async task
timer = setTimeout(function(){
$("a").text($("a").text()+ " " + element);
var resolving = function(){
r.resolve();
}
myFunction(array).done(resolving);
}, 500);
return r;
}
//Starting the function
var myArray = ["Hi", "that's", "just", "a", "test"];
var alerting = function (){window.alert("finished!")};
myFunction(myArray).done(alerting);
You can use the javascript Promise and async/await to implement a synchronized call of the functions.
Suppose you want to execute n number of functions in a synchronized manner that are stored in an array, here is my solution for that.
async function executeActionQueue(funArray) {
var length = funArray.length;
for(var i = 0; i < length; i++) {
await executeFun(funArray[i]);
}
};
function executeFun(fun) {
return new Promise((resolve, reject) => {
// Execute required function here
fun()
.then((data) => {
// do required with data
resolve(true);
})
.catch((error) => {
// handle error
resolve(true);
});
})
};
executeActionQueue(funArray);

How can I create an Asynchronous function in Javascript?

Check out this code :
Link
<span>Moving</span>
$('#link').click(function () {
console.log("Enter");
$('#link').animate({ width: 200 }, 2000, function() {
console.log("finished");
});
console.log("Exit");
});
As you can see in the console, the "animate" function is asynchronous, and it "fork"s the flow of the event handler block code. In fact :
$('#link').click(function () {
console.log("Enter");
asyncFunct();
console.log("Exit");
});
function asyncFunct() {
console.log("finished");
}
follow the flow of the block code!
If I wish to create my function asyncFunct() { } with this behaviour, how can I do it with javascript/jquery? I think there is a strategy without the use of setTimeout()
​
You cannot make a truly custom asynchronous function. You'll eventually have to leverage on a technology provided natively, such as:
setInterval
setTimeout
requestAnimationFrame
XMLHttpRequest
WebSocket
Worker
Some HTML5 APIs such as the File API, Web Database API
Technologies that support onload
... many others
In fact, for the animation jQuery uses setInterval.
You can use a timer:
setTimeout( yourFn, 0 );
(where yourFn is a reference to your function)
or, with Lodash:
_.defer( yourFn );
Defers invoking the func until the current call stack has cleared. Any additional arguments are provided to func when it's invoked.
here you have simple solution (other write about it)
http://www.benlesh.com/2012/05/calling-javascript-function.html
And here you have above ready solution:
function async(your_function, callback) {
setTimeout(function() {
your_function();
if (callback) {callback();}
}, 0);
}
TEST 1 (may output '1 x 2 3' or '1 2 x 3' or '1 2 3 x'):
console.log(1);
async(function() {console.log('x')}, null);
console.log(2);
console.log(3);
TEST 2 (will always output 'x 1'):
async(function() {console.log('x');}, function() {console.log(1);});
This function is executed with timeout 0 - it will simulate asynchronous task
Here is a function that takes in another function and outputs a version that runs async.
var async = function (func) {
return function () {
var args = arguments;
setTimeout(function () {
func.apply(this, args);
}, 0);
};
};
It is used as a simple way to make an async function:
var anyncFunction = async(function (callback) {
doSomething();
callback();
});
This is different from #fider's answer because the function itself has its own structure (no callback added on, it's already in the function) and also because it creates a new function that can be used.
Edit: I totally misunderstood the question. In the browser, I would use setTimeout. If it was important that it ran in another thread, I would use Web Workers.
Late, but to show an easy solution using promises after their introduction in ES6, it handles asynchronous calls a lot easier:
You set the asynchronous code in a new promise:
var asyncFunct = new Promise(function(resolve, reject) {
$('#link').animate({ width: 200 }, 2000, function() {
console.log("finished");
resolve();
});
});
Note to set resolve() when async call finishes.
Then you add the code that you want to run after async call finishes inside .then() of the promise:
asyncFunct.then((result) => {
console.log("Exit");
});
Here is a snippet of it:
$('#link').click(function () {
console.log("Enter");
var asyncFunct = new Promise(function(resolve, reject) {
$('#link').animate({ width: 200 }, 2000, function() {
console.log("finished");
resolve();
});
});
asyncFunct.then((result) => {
console.log("Exit");
});
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
Link
<span>Moving</span>
or JSFiddle
This page walks you through the basics of creating an async javascript function.
Since ES2017, asynchronous javacript functions are much easier to write. You should also read more on Promises.
If you want to use Parameters and regulate the maximum number of async functions you can use a simple async worker I've build:
var BackgroundWorker = function(maxTasks) {
this.maxTasks = maxTasks || 100;
this.runningTasks = 0;
this.taskQueue = [];
};
/* runs an async task */
BackgroundWorker.prototype.runTask = function(task, delay, params) {
var self = this;
if(self.runningTasks >= self.maxTasks) {
self.taskQueue.push({ task: task, delay: delay, params: params});
} else {
self.runningTasks += 1;
var runnable = function(params) {
try {
task(params);
} catch(err) {
console.log(err);
}
self.taskCompleted();
}
// this approach uses current standards:
setTimeout(runnable, delay, params);
}
}
BackgroundWorker.prototype.taskCompleted = function() {
this.runningTasks -= 1;
// are any tasks waiting in queue?
if(this.taskQueue.length > 0) {
// it seems so! let's run it x)
var taskInfo = this.taskQueue.splice(0, 1)[0];
this.runTask(taskInfo.task, taskInfo.delay, taskInfo.params);
}
}
You can use it like this:
var myFunction = function() {
...
}
var myFunctionB = function() {
...
}
var myParams = { name: "John" };
var bgworker = new BackgroundWorker();
bgworker.runTask(myFunction, 0, myParams);
bgworker.runTask(myFunctionB, 0, null);
Function.prototype.applyAsync = function(params, cb){
var function_context = this;
setTimeout(function(){
var val = function_context.apply(undefined, params);
if(cb) cb(val);
}, 0);
}
// usage
var double = function(n){return 2*n;};
var display = function(){console.log(arguments); return undefined;};
double.applyAsync([3], display);
Although not fundamentally different than the other solutions, I think my solution does a few additional nice things:
it allows for parameters to the functions
it passes the output of the function to the callback
it is added to Function.prototype allowing a nicer way to call it
Also, the similarity to the built-in function Function.prototype.apply seems appropriate to me.
Next to the great answer by #pimvdb, and just in case you where wondering, async.js does not offer truly asynchronous functions either. Here is a (very) stripped down version of the library's main method:
function asyncify(func) { // signature: func(array)
return function (array, callback) {
var result;
try {
result = func.apply(this, array);
} catch (e) {
return callback(e);
}
/* code ommited in case func returns a promise */
callback(null, result);
};
}
So the function protects from errors and gracefully hands it to the callback to handle, but the code is as synchronous as any other JS function.
Unfortunately, JavaScript doesn't provide an async functionality. It works only in a single one thread. But the most of the modern browsers provide Workers, that are second scripts which gets executed in background and can return a result.
So, I reached a solution I think it's useful to asynchronously run a function, which creates a worker for each async call.
The code below contains the function async to call in background.
Function.prototype.async = function(callback) {
let blob = new Blob([ "self.addEventListener('message', function(e) { self.postMessage({ result: (" + this + ").apply(null, e.data) }); }, false);" ], { type: "text/javascript" });
let worker = new Worker(window.URL.createObjectURL(blob));
worker.addEventListener("message", function(e) {
this(e.data.result);
}.bind(callback), false);
return function() {
this.postMessage(Array.from(arguments));
}.bind(worker);
};
This is an example for usage:
(function(x) {
for (let i = 0; i < 999999999; i++) {}
return x * 2;
}).async(function(result) {
alert(result);
})(10);
This executes a function which iterate a for with a huge number to take time as demonstration of asynchronicity, and then gets the double of the passed number.
The async method provides a function which calls the wanted function in background, and in that which is provided as parameter of async callbacks the return in its unique parameter.
So in the callback function I alert the result.
MDN has a good example on the use of setTimeout preserving "this".
Like the following:
function doSomething() {
// use 'this' to handle the selected element here
}
$(".someSelector").each(function() {
setTimeout(doSomething.bind(this), 0);
});

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