setting a property on a model in an ajax callback - javascript

I'm trying to set a property on a model in an ajax callback that I can use later, and I'm not sure if I can do this.
var self = this;
$.ajax {
self.views.someProperty = // something i get back from the server
}
then later do something with this.views.someProperty. Currently I get this.views.someProperty is undefined. I was wondering if I'm going about this correctly or not.

First off, you code snippet doesn't really make sense. $.ajax() calls a callback function when it's done with the result it retrieved and your code snippet doesn't show that proper form.
Assuming you are properly specifying a callback function, here are some other possible issues:
Timing - the result of the ajax call will not be available until AFTER the completion callback is called. This is some time later after $.ajax() is called. The result will not be available in code that executes right after $.ajax().
Does self.views exist already? If not, then setting self.views.someProperty would cause an error and would not work.
When you access this.views.someProperty later, you obviously need to make sure that this is the right value.
You can check the first item by putting a console.log("ajax call finished") in your ajax completion callback and a console.log("accessing someProperty") right before you try to access the value and then verify that the ajax completion is called before you try to access it.
You can check items 2 and 3 by examining the data in the debugger to make sure everything is what you intend.

Related

Variable scope with jQuery and javascript

So, I wrote the following function:
function getData() {
var data;
$(function () {
$.getJSON('https://ipinfo.io', function (ipinfo) {
data = ipinfo;
console.log(data);
})
})
console.log(data);
}
The problem with the above is the 2nd console.log doesn't retain the info from the assignment inside the jQuery and logs an undefined object. I'm not exactly sure what is wrong, but I believe it to be something quite minor. However, as much as I've searched online, I haven't found an answer for this particular problem.
One line: Javascript is Asynchronous.
While many struggle to figure out what it exactly means, a simple example could possibly explain you that.
You request some data from a URL.
When the data from second URL is received, you wish to set a variable with the received data.
You wish to use this outside the request function's callback (after making the request).
For a conventional programmer, it is very hard to grasp that the order of execution in case of JavaScript will not be 1,2 and then 3 but rather 1,3,2.
Why this happens is because of Javascript's event-loop mechanism where each asynchronous action is tied with an event and callbacks are called only when the event occurs. Meanwhile, the code outside the callback function executes without holding on for the event to actually occur.
In your case:
var data;
$(function () {
$.getJSON('https://ipinfo.io', function (ipinfo) {//async function's callback
data = ipinfo;
console.log(data);//first console output
})
})
console.log(data);//second console output
While the async function's callback is executed when the data is received from the $.getJSON function, javascript proceeds further without waiting for the callback to assign the value to the data variable, causing you to log undefined in the console (which is the value of the data variable when you call console.log.
I hope I was able to explain that.!

Modify global javascript variable inside a function

I'm trying clone a json object inside a function wanting to access the object outside the function. But when function is done, the object still seems to be undefined. Seems like it's not the same variable?
var jsonUserObj;
$.getJSON("user.json", function(content){
jsonUserObj = $.parseJSON(JSON.stringify(content));
console.log(content);
console.log(jsonUserObj); //looks fine!
});
console.log(jsonUserObj); //undefined
inside the callback function it contains all the data, but it does not remain outside of it. How to make it assessible globally?
$.getJSON is asynchronous so console.log at the end of your code runs before $.getJSON returns its result.
You should modify the variable inside the callback (where it looks fine) and then use the variable inside that function, this callback is the only place where you can guarantee your variable is set.
You could also use the synchronous version of $.ajax but that's really not recommended (and probably unnecessary).
You got a typo:
console.log(jsonUserObject);
It should be
console.log(jsonUserObj);
You need to declare var jsonUserObj outside a function.
Also it looks like you have a typeo, is it jsonUserObj or jsonUserObject?
Could it be a question of timing? If that getJSON method is firing asynchronously then it may not have returned it's value by the time you have fired the last line. Does that make sense?
$.getJSON performs an ajax call, which is asynchronous. The code after it will continue evaluating while it waits for a response. When the response comes back, the program flow will jump back into the success/error/complete handlers of the ajax call.
tldr: Anything you do with data from an ajax call must be in the success handler of that ajax call.

Possible Javascript scope issue

I'm sort of a noob with this so please forgive me :)
I can't get this one part of the function to update the variable. Could anyone possibly take a look a see what I'm doing wrong?
http://pastie.org/private/zfnv8v2astglabluo89ta
From line 142 thru 172 I'm not getting any results in the end. I've tested inside that function to make sure it is actually returning data, but the "body" variable is passing back up after line 172. So if I look at my generated HTML on the page, it simply looks the function skips from 140 to 174.
Thanks for any feedback!!
Your $.get is asynchronous. That means it will finish sometime AFTER the rest of the code, thus you won't see it's effect on the body variable inside that function. Instead, it's success callback function will be called long after this function has already finished.
To chain multiple asynchronous ajax calls like you have here, you can't just use normal sequential programming because asynchronous ajax calls aren't sequential. The network request is sent, then your javascript continues executing and SOMETIME LATER when the response arrives, the success handler is called and is executed.
To run sequential ajax calls like you have, you have to nest the work inside the success handler so that the ONLY code that uses the response is actually in the success handler. In pseudo-code, it looks like this:
$.get(..., function(data) {
// operate on the results only in here
// a second ajax function that uses the data from the first
// or adds onto the data from the first
$.get(..., function(data) {
// now finally, you have all the data
// so you can continue on with your logic here
});
// DO NOT PUT ANYTHING HERE that uses the responses from the ajax calls
// because that data will not yet be available here
});
You cannot do what you're doing which is like this:
var myVariable;
$.get(..., function(data) {
// add something to myVariable
});
$.get(..., function(data) {
// add something to myVariable
});
$.get(..., function(data) {
// add something to myVariable
});
// do something with myVariable
None of those ajax calls will have completed before the end of your function. You have to follow a design pattern like in my first example.
For more advanced tools, one can always use jQuery deferreds which are just a different way of defining code to run after an ajax call is done. It looks a little more like sequential programming even though it's really just scheduling code to run the same way my first code example does.
Function 8 will be invoke after line 174-180. You must put code from 174-180 line to the end of function

Ajax event handler does not update variables?

I'm having issues getting a variable declared in an .click function to be updated in a Get function within the click function. I've gathered that even though the variable has the same name, within the Get function it is really declaring it anew.
I've tried to find examples helping me, but it appears to me that the Get method is such a specialized function that the examples didn't seem to apply.
I would like the value of 'SettingContentToEdit' to get updated with information retrieved with the Get function.
Below is the code.
Thank you for your help!
$(".cellSetting").click(function () {
var clickedClass = $(this).attr("class");
var clickedItemID = $(this).attr("id")
var SettingContentToEdit = "not changed";
var JSONSend = {
'ItemName': clickedItemID, //send the item name so the model knows which one to pull up!
'ItemSetting': clickedClass
};
$.get(
'/Home/getItem',
JSONSend,
function (data) {
// $('#dialog').html(data.ItemSettings[data.SettingToEdit]);
SettingContentToEdit = data.ItemSettings[data.SettingToEdit];
alert(SettingContentToEdit); //returns the correct info
}
);
alert(SettingContentToEdit); //returns "not changed"
});
Your issue is that your ajax call is asyncronous. The success handler for the get() function is called some time after your click handler and the alert() has already completed. You can refer to the local variables in your success handler, but the code that follows the success handler executes BEFORE the success handler does.
This is because your get() operate is asynchronous. Calling it just STARTS the networking operation and then your javascript execution continues (while the networking operation works in the background). After starting the networking operation, your alert() is called. Then, some time LATER, the Ajax call completes and the success handler is executed.
This is a very common mistake and has indeed been asked and answered hundreds (if not thousands) of times here on SO (I've personally probably answered 20-30) so you are not alone in missing this understanding at first.
If you want to use the results of the ajax operation, then you need to put the code that does that either in your success handler or put it in a function that you call from your success handler. You cannot put it after the get() function because it will execute too soon before the results are known.
AJAX is asynchronous. If you check SettingContentToEdit a second or so later than you are doing, you'd see the value has updated.
Either put your code inside the get function (where you have your alert showing the correct value) or make the request synchronous (you'll have to look up the jQuery docs because I don't use jQuery).

Chaining asynchronous methods in JavaScript

I'd like to do something like this:
var res = myAjax.post();
myAjax is basically my own ajax wrapper (asynchroneous). I don't necessary need the chaining, but i'd like to be able to do the call like shown above. I'd like a callback to be handled by the object itself. I've read about asynchroneous method queues, but, honestly, couldn't fully understand it. I know i could use a synchroneous ajax call, but it should be possible to do it asynchroneously as well if i understand it correctly. Any help is very appreciated. Thank you.
I've done a demo here that might be something along the lines of what you mean...
In that demo, there are two types of objects: MyAjax which has the 'post' method as above, and a MyRes object with a getData method that will return null until the async call has successfully completed. After it has completed, it will return the data fetched from the ajax call.
In the demo, I have a function checking for the response every second, but the post method also handles a callback which allows you to be notified when the ajax call has completed successfully.
I don't know why you'd want to do it this way, but it is doable.

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