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How to access the correct `this` inside a callback
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Closed 7 years ago.
im trying to assign a variable inside the ajax success callback.
i know that in order to assign the value i need to use a call back function. but i want that function to be inside the class. is it possible to implement something like this?
function callMgmt_class() {
this.room_id = 'Error';
this.assignRoomVar = function(room_id) {
alert();
this.room_id = room_id;
}
this.getRoomID = function() {
$.ajax({
url: "/get_room_id.php",
dataType: "json",
success: function(data) {
this.assignRoomVar(data.room_id);
}
})
}
}
is there some way to use bind?
i tried:
success: function(data) {
(function() { this.assignRoomVar(data.room_id); }).bind(this);
}
so i dont get any errors but the function does not seem to be called.
Your use of this is flawed. 'this' inside the callback will use the context of the callback whereas what you need is the context of your class. You can do that by caching the 'this' context in a variable and using it instead.
this.getRoomID = function() {
var me = this;
$.ajax({
url: "/get_room_id.php",
dataType: "json",
success: function(data) {
me.assignRoomVar(data.room_id);
}
})
}
this is not the this you are looking for.
The this keyword is tricky and takes on different references depending on the context. When you are accessing this in the ajax success function, it is not referring to the callMgmt_class object anymore (I think it would reference the ajax function).
The best way to get around this is to assign this to another variable when you are sure of its value. So at the beggining of your function is the best bet
this.getRoomID = function() {
var that= this;
//Now use that instead of this when you want to refer to the callMgmt_class object
$.ajax({
url: "/get_room_id.php",
dataType: "json",
success: function(data) {
that.assignRoomVar(data.room_id);
}
})
}
Addition:
I think using bind or apply are other alternatives, but I am not very familiar, so I'll leave that for someone else to answer.
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I have a situation where I want to have a global variable like a whose value I'll be fetching from jQuery Ajax from some server and then want the value to be available on call from other function. To visualize:
var a = [];
$(document).ready(function () {
$.ajax({
type: 'GET',
url: 'some url',
success: function (response) {
a = response.somearray;
},
error: function () {
alert("Not successfull");
}
});
});
function something() { // this function will be called onclick of a button in html
console.log(a); // here I want the value that came from the server. But currently, it's printing empty array
}
What I have tried are as follow:
Removing var keyword from (a) to make it global.
Assigning no value (here empty array) to (a) declaration above (that turned out to be a blunderous mistake though).
Removing (a) declaration completely from the top.
The thing to take care about is that something() function is being called from an HTML button on complete page load and after ajax call. So value is already available.
Here is a working example of what you want, check the console logs:
https://codepen.io/antoniandre/pen/NebpQj?editors=0010
var a = [];
$(document).ready(function () {
$.getJSON("https://api.flickr.com/services/feeds/photos_public.gne?jsoncallback=?", {
tags: "snowboarding",
tagmode: "any",
format: "json"
})
.done(function (response) {
a = response.items;
// something(); Don't need this.
});
});
function something() { // this function will be called onclick of a button in html
console.log(a); // here I want the value that came from the server. But currently, it's printing empty array
}
EDIT:
HTML:
<button onclick="something()">hello</button>
Scope was lost, since ajax callback is asynchronous function. I think, it should be
var a = [];
$(document).ready(function () {
var me = this;
$.ajax({
type: 'GET',
url: 'some url',
success: function (response) {
me.a = response.somearray;
},
error: function () {
alert("Not successfull");
});
});
I have a function which returns data to getDates(), after processing i want the data in a global variable but it's not working.
var bookedDates=[];
function getDates(result) {
var bookedDates1=result.split(",");
var bookedDates2=[];
for(i=1;i<bookedDates1.length;i++){
bookedDates2.push(bookedDates1[i]);
}
bookedDates=bookedDates2;
}
alert(bookedDates);
$.ajax({
type: "POST",
url: "getbookeddates.php",
cache: false,
success: function(result){
getDates(result);
//dates booked
}
bookDates is blank in this case but it should be an array with ["21","22"].
It looks like you have done a very silly mistake.
You are declaring the function getDates which is taking a parameter result in it.
But you are not calling the function So the inside of the function will not get executed until you call it.
Try to add a calling line of a function.
getDates(result);
before the alert line.
I have this:
var myapp;
function Menu($scope){
$.ajax({
url: "/user.php",
type: "POST",
success: function(data){
}
});
}
How do I use the data out of the function scope?
You're having trouble because the ajax operation is asynchronous. This means that, after $.ajax is called, you can't control when exactly the response will arrive. In the mean time, your code keeps running normally, and if you try to use the response before it arrives, it will be empty.
You say in the comments that a framework calls your Menu function, so I'm assuming you can't control what parameters are passed to it. In this case, you should only use the data inside the success callback:
function Menu($scope){
$.ajax({
url: "/user.php",
type: "POST",
success: function(data){
// USE data HERE
}
});
}
In case you can modify how Menu is called, you can pass a callback function to it, and let that manipulate the results:
function Menu($scope, ajaxCallback){
$.ajax({
url: "/user.php",
type: "POST",
success: ajaxCallback
});
}
// Define the callback where you will use the data
function processData(data) {
// USE data HERE
}
// Call Menu passing the callback
Menu(whateverScopeIs, processData);
You can create a variable at the scope you need, and save the data to that variable. Or you can call a function and pass that data to it.
For example:
function alertData(data){
alert(data); // I can use the data here now!
}
var foo;
$.ajax({
url: "/user.php",
type: "POST",
success: function(data){
alertData(data);
}
});
}
Create a global variable, assign data to it, and then use the global variable.
e.g.
var foo;
function bar(baz) {
foo = baz;
}
bar("test");
alert(foo);
I'm having serious problems with the function below:
function requestUploadedSearch()
{
var cookie = JSON.parse(readCookie("user_search_cookie"));
$.ajax({
dataType: "script",
async: false,
data: {
context: "search-get",
code: removeNull(cookie, cookie !== null, "code")
},
success: function(data)
{
var keywords = search_return["keywords"];
return keywords; // here is the damn problem.
}
});
}
Seams that simply nothing comes out of the function except for the undefined value and no errors are shown in the debugger.
And I'm seriously almost throwing my laptop on the wall.
If anyone could help me doing this, please answer!
Thanks in advance.
1st off: Where is the search_return variable? Why are you ignoring data?
I have a feeling this is what you want to do:
function requestUploadedSearch()
{
var cookie = JSON.parse(readCookie("user_search_cookie"));
var keywords;
$.ajax({
dataType: "json",
async: false,
data: {
context: "search-get",
code: removeNull(cookie, cookie !== null, "code")
},
success: function(data)
{
keywords = data["keywords"];
}
});
return keywords;
}
The issue is that since the Ajax call will complete at an arbitrary time in the future, you cannot simply return a value from its success handler.
One issue is that you're not actually doing anything with the data returned by the server, which seems puzzling.
The nutshell version is that you need to implement the functionality as part of the success callback. That can be done in-line, or you can create the callback function outside of the Ajax call itself and use it as the value for the success property:
function onSuccess(data) {
// Do something with the data here
}
...
$.ajax({ // etc.
success: onSuccess
};
You may also use jQuery's $.when function.
The problem is the scope you're trying to return your keywords from. The success function is called by jQuery, and you don't have any control over what jQuery does with that return value. You could do return $.ajax(... but you wouldn't get what you expect, since according to the documentation: "As of jQuery 1.5, the $.ajax() method returns the jqXHR object, which is a superset of the XMLHTTPRequest object" (http://api.jquery.com/Types/#jqXHR).
What you should do instead is set up a callback function like:
function doSomethingWithKeywords(keywords) {
// do stuff
};
and in the success function call that function:
doSomethingWithKeywords(keywords);
EDIT: Hogan's is a good solution, since your call isn't asynchronous.
The problem you are having is the return you are passing is not the return of the function -- it is the return of the event of success. Often closures (implied passing of a local variable to a function) are used to solve this problem in JavaScript.
NB I still don't think your function will work because I don't see where search_return["keywords"] is defined. But at least you won't have to worry about the closure issue. Once your success function is correct the main function will return it.
Like this:
function requestUploadedSearch()
{
var cookie = JSON.parse(readCookie("user_search_cookie"));
var returnClosure;
$.ajax({
dataType: "script",
async: false,
data: {
context: "search-get",
code: removeNull(cookie, cookie !== null, "code")
},
success: function(data)
{
// returnClosure = data["keywords"];
returnClosure = search_return["keywords"];
}
});
return returnClosure;
}
I have this ajax call to a doop.php.
function doop(){
var old = $(this).siblings('.old').html();
var new = $(this).siblings('.new').val();
$.ajax({
url: 'doop.php',
type: 'POST',
data: 'before=' + old + '&after=' + new,
success: function(resp) {
if(resp == 1) {
$(this).siblings('.old').html(new);
}
}
});
return false;
}
My problem is that the $(this).siblings('.old').html(new); line isn't doing what it's supposed to do.
thanks..
all helpful comments/answers are voted up.
Update: it appears that half of the problem was the scope (thanks for the answers that helped me clarify that), but the other half is that I'm trying to use ajax in a synchronous manner. I've created a new post
You should use the context setting as in http://api.jquery.com/jQuery.ajax/
function doop(){
var old = $(this).siblings('.old').html();
var newValue = $(this).siblings('.new').val();
$.ajax({
url: 'doop.php',
type: 'POST',
context: this,
data: 'before=' + old + '&after=' + newValue,
success: function(resp) {
if(resp == 1) {
$(this).siblings('.old').html(newValue);
}
}
});
return false;
}
"this" will be transfer to the success scope and will act as expected.
First of all new is a reserved word. You need to rename that variable.
To answer your question, Yes, you need to save this in a variable outside the success callback, and reference it inside your success handler code:
var that = this;
$.ajax({
// ...
success: function(resp) {
if(resp == 1) {
$(that).siblings('.old').html($new);
}
}
})
This is called a closure.
this is bound to the object to which the executing function was applied. That could be some AJAX response object, or the global object (window), or something else (depending on the implementation of $.ajax.
Do I need to capture $(this) into a variable before entering the $.ajax call, and then pass it as a parameter to the $.ajax call? or do I need to pass it to the anonymous success function? If that's going to solve the problem, where do I pass it to the $.ajax?
You do indeed need a way to capture the value of this before defining the success function. Creating a closure is the way to do this. You need to define a separate variable (e.g. self):
function doop() {
var old = $(this).siblings('.old').html();
var new = $(this).siblings('.new').val();
var self = this;
$.ajax({
url: 'doop.php',
type: 'POST',
data: 'before=' + old + '&after=' + new,
success: function(resp) {
if(resp == 1) {
$(self).siblings('.old').html(new);
}
}
});
return false;
}
The success function will retain the value of self when invoked, and should behave as you expected.