function get_request(url) {
var request = new getXMLObject();
request.onreadystatechange = function () {
if (request.readyState == 4) {
alert(request.responseText);
var data = eval('(' + request.responseText + ')');
alert(data);
return data;
}
}
request.open("GET", url, true);
//alert(document.getElementById('energy').innerHTML);
request.send();
}
function loadjobs() {
var url = "loadjobs.php?tab=1&id=1111";
//var data=
//alert(check());
alert(get_request(url));
//alert(data);
}
When i m getting data in json format...i am gettin NULL in alert(get_request(url));
while i m getting in alert(data);
Help me
This is because the request in asynchronous . The get_request(url) function does to return anything and hence the null ( although I think it should be undefined and not null ) .
The onreadystatechange function gets called later in the time , when the AJAX request has been completed and the data is returned from the server and hence the alert there works .
This is a misunderstanding of how AJAX works. Ajax is asynchronous. The onreadystatechange function will be called after loadjobs(). The "return path" you are specifying can never work. get_request() will never be able to return the fetched value.
You have two options. Either make the script synchronous - this can be done but is not recommended because it can freeze the browser.
Or, better, handle everything you need to do inside the onreadystatechange callback.
Well, it's an asynchronous call. You will receive the data of request your after get_request has already returned. That means your request.onreadystatechange = function () will be executed long after alert(get_request(url)); is already finished. This means get_request will not be able to return any data from the AJAX call. That's what you have the request.onreadystatechange callback function for, to execute code at an undefined later time when you received the response.
The problem is that Ajax requests work asynchronously. So you can't return the data right away. The way you should do it is to specify a callback function which will handle the response data.
function handleJSON( data ) {
// ...
// do whatever you want to do with the data
}
ajax( "url/file.php?param=value", handleJSON );
////////////////////////////////////////////////////////////////////////////////
function getXmlHttpObject() {
var xmlHttp;
try {
xmlHttp = new XMLHttpRequest();
} catch (e) {
try {
xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
} catch (e) {
xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
}
}
return xmlHttp;
}
function ajax(url, onSuccess, onError) {
var xmlHttp = getXmlHttpObject();
xmlHttp.onreadystatechange = function () {
if (this.readyState == 4) {
// onError
if (this.status != 200) {
if (typeof onError == 'function') {
onError(this.responseText);
}
}
// onSuccess
else if (typeof onSuccess == 'function') {
onSuccess(this.responseText);
}
}
};
xmlHttp.open("GET", url, true);
xmlHttp.send(null);
return xmlHttp;
}
Related
I am using a javascript function that calls another javascript function (zConvertEmplidtoRowid) that uses an ajax call that runs a query to return data in a variable (rowid). My problem is I don't know how to return the data to the original function.
Here is a snippet of the original function calling the ajax function (zConvertEmplidtoRowid)
var rowid = zConvertEmplidtoRowid(emplid);
//the alert should show what the query via ajax returned
alert(rowid);
zEmployeePortrait(emplid, ajaxlink);
}
And here is the ajax function...I imagine somewhere in here I need to place the return, but I've never used ajax before, so I'm not sure.
function zConvertEmplidtoRowid(emplid, ajaxlink, callback) {
if (typeof XMLHttpRequest == 'undefined') {
XMLHttpRequest = function() {
try { return new ActiveXObject("Msxml2.XMLHTTP.6.0"); } catch(e) {}
try { return new ActiveXObject("Msxml2.XMLHTTP.3.0"); } catch(e) {}
try { return new ActiveXObject("Msxml2.XMLHTTP"); } catch(e) {}
try { return new ActiveXObject("Microsoft.XMLHTTP"); } catch(e) {}
throw new Error('This browser does not support XMLHttpRequest or XMLHTTP.');
};
}
var request = new XMLHttpRequest();
request.onreadystatechange = function() {
if (request.readyState == 4 && request.status == 200) {
var rowid = request.responseText;
callback(rowid);
}
}
var ajax_link = ajax_link + "?emplid=" + emplid;
request.open('GET', ajax_link);
request.send();
}
As #epascarello pointed out, the ajax call is asynchronous and the code you have written is expecting the call to return in a synchronous way.
You have two options:
1) Make the ajax call synchronous (I highly recommend not to take this route).
2) Pass a callback function as a parameter to the function making the ajax call and then invoke the callback function once the call returns.
e.g:
function zConvertEmplidtoRowid(emplid, ajaxlink, callback) { //Added a callback function parameter
if (typeof XMLHttpRequest == 'undefined') {
XMLHttpRequest = function() {
try { return new ActiveXObject("Msxml2.XMLHTTP.6.0"); } catch(e) {}
try { return new ActiveXObject("Msxml2.XMLHTTP.3.0"); } catch(e) {}
try { return new ActiveXObject("Msxml2.XMLHTTP"); } catch(e) {}
try { return new ActiveXObject("Microsoft.XMLHTTP"); } catch(e) {}
throw new Error('This browser does not support XMLHttpRequest or XMLHTTP.');
};
}
var request = new XMLHttpRequest();
request.onreadystatechange = function() {
if (request.readyState == 4 && request.status == 200) {
var rowid = request.responseText;
//now you invoke the callback passing the rowid as argument
callback(rowid);
}
}
var ajax_link = ajax_link + "?emplid=" + emplid;
request.open('GET', ajax_link);
request.send();
}
zConvertEmplidtoRowid(emplid, ajaxlink, function(rowId) {
alert(rowId);
zEmployeePortrait(emplid, ajaxlink);
});
As epascarello has implied in his comment, you need to make the javascript call synchronously in order to get a return value...
I tend to use jquery to assist with the call, but a quick google suggests you can do it your way by changing:
request.open('GET', ajax_link);
to:
request.open('GET', ajax_link, false);
and the response is then accessible through:
request.responseText
taken from here: https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/Synchronous_and_Asynchronous_Requests
I've got a problem with this Ajax code, is returning 0 everytime I access 'readyState'. Don't know what the source of the problem is yet, any help would be appreciated:
var xhr = null;
function performAjax(inputUrl){
// instantiate XMLHttpRequest object
try{
xhr = new XMLHttpRequest();
alert("XMLHttpRequest");
}
catch(e){
xhr = new ActiveXObject("Microsoft.XMLHTTP");
}
// handle old browsers
if( xhr == null ) {
alert("Ajax not supported by your browser");
return;
}
// get the URL
var url = inputUrl;
alert(inputUrl);
// get Ajax answer
xhr.onreadystatechange = handler();
//alert(xhr.readyState);
xhr.open("POST", url, true);
xhr.send(null);
}
function handler() {
alert("Handler: " + xhr.readyState + " Status: " + xhr.status);
// handle only loaded requests
if(xhr.readyState == 4) { // state 4: that data has been received
alert("here");
if(xhr.status == 200) {
alert(xhr.reponseText);
}
else alert("Error with Ajax");
}
}
You're assigning the handler function incorrectly:
xhr.onreadystatechange = handler; // <--- THERE SHOULD BE NO PARENTHESES
When you include the parentheses, you're asking that the function be called. Without them, you're merely referring to the function, which is what you want.
I have this problem.
I have a function for example called functionA() that needs the results from another function called functionB().
var globalVar="";
function functionA(){
//...
functionB();
//here i have to use the global variable (that is empty because functionB isn't finished)
}
function functionB(){
//ajax request
globalVar=ajaxRequest.responseText;
}
How can I do to let the functionB finish befor continue with the execution of functionA?
Thanks!
This is the code:
var ingredientiEsistenti="";
function ShowInserisciCommerciale() {
getElementiEsistenti();
JSON.parse(ingredientiEsistenti);
}
function getElementiEsistenti(){
// prendo gli ingredienti esistenti.
var url = "http://127.0.0.1:8080/Tesi/Ingredienti";
var xmlHttp = new XMLHttpRequest();
xmlHttp.open("POST", url, false);
xmlHttp.send(null);
xmlHttp.setRequestHeader("Content-type",
"application/x-www-form-urlencoded");
xmlHttp.onreadystatechange = function() {
if (xmlHttp.readyState == 4) // COMPLETED
{
if (xmlHttp.status == 200) // SUCCESSFUL
{
ingredientiEsistenti = xmlHttp.responseText;
} else {
alert("An error occurred while communicating with login server.");
}
}
};
}
You've got one of many options, that don't require an evil global variable:
Move the code you want to see executed to the onreadystatechange callback of the ajax request, that way, it won't get executed until you received a response
Redefine functionA, so that it takes a parameter that allows you to skip the first bit:
Make the request synchronous, not recommended, though
use a timeout/interval to check the readystate of the request manually (brute-force, not recommended either)
Perhaps there is some worker trickery that could do the trick, too, in your particular case
function functionA(skipDown)
{
skipDown = skipDown || false;
if (skipDown === false)
{
//doStuff
return functionB();//<-- call functionA(true); from the readystatechange callback
}
//this code will only be called if skipDown was passed
}
It is impossible to have a sleep/wait in JavaScript when the call is asynchronous. You need to use a callback pattern to make this action occur.
It is possible to make an XMLHttpRequest synchronous, but that can lead to other problems. It can hang the browser as it blocks all other actions from happening. So if you want to show a loading animation, it most likely will not execute.
You can make your AJAX request synchronous. https://developer.mozilla.org/en-US/docs/DOM/XMLHttpRequest/Synchronous_and_Asynchronous_Requests
var request = new XMLHttpRequest();
// Last parameter makes it not asnychronous
request.open('GET', 'http://www.mozilla.org/', false);
request.send(null);
// Won't get here until the network call finishes
if (request.status === 200) {
console.log(request.responseText);
}
However, that will block the UI while waiting for the server to respond, which is almost never what you want. In that case, you should use a callback to process results.
Here's an example using a callback without relying on a global variable. You should always run away from those
function ShowInserisciCommerciale( ) {
getElementiEsistenti(function(responseText) {
JSON.parse(responseText);
});
}
function getElementiEsistenti(successCallback){
var url = "http://127.0.0.1:8080/Tesi/Ingredienti";
var xmlHttp = new XMLHttpRequest();
xmlHttp.open("POST", url, false);
xmlHttp.send(null);
xmlHttp.setRequestHeader("Content-type",
"application/x-www-form-urlencoded");
xmlHttp.onreadystatechange = function() {
if (xmlHttp.readyState == 4) // COMPLETED
{
if (xmlHttp.status == 200) // SUCCESSFUL
{
successCallback(xmlHttp.responseText);
} else {
alert("An error occurred while communicating with login server.");
}
}
};
}
I have a function which attempts to capture a return value from a calling function in the following manner:
var select = xhrRetrieve(projID);
Here is an example of the xhrRetrieve function:
function xhrRetrieve(projID) {
var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function() {
if(xhr.readyState == 4) {
if(xhr.status == 200) {
var obj = $.parseJSON(xhr.responseText);
return obj.select.toString();
}
}
}
var url = "ajax.cgi";
var data = "action=retrieve-opp&proj-id=" + projID;
xhr.open("POST",url);
xhr.setRequestHeader("Content-Type","application/x-www-urlencoded");
xhr.send(data);
}
I am using jQuery in conjunction with straight JavaScript. Whenever I attempt to get the value of obj.select using:
var select = xhrRetrieve(projID);
Select always comes back undefined.
What am I doing wrong?
The function doesn't return anything
The moment you call your function, the (not currently present) return value is being assigned to select. At the same moment, your ajax request is being fired, which takes time to complete; the callback function will not be called until the ajax request has completed (and succeeded).
This should work:
function doStuffWithTheAjaxResponse(select) {
// do stuff
}
function xhrRetrieve(projID) {
var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function() {
if(xhr.readyState == 4) {
if(xhr.status == 200) {
var obj = $.parseJSON(xhr.responseText);
doStuffWithTheAjaxResponse(obj.select.toString());
}
}
}
var url = "ajax.cgi";
var data = "action=retrieve-opp&proj-id=" + projID;
xhr.open("POST",url);
xhr.setRequestHeader("Content-Type","application/x-www-urlencoded");
xhr.send(data);
}
Since the request is asynchronous the function will return before your code in onreadestatechange fires. You can switch to synchronous and get the value before the function returns:
function xhrRetrieve(projID) {
var returnVal;
var xhr = new XMLHttpRequest();
var url = "ajax.cgi";
var data = "action=retrieve-opp&proj-id=" + projID;
//3rd param is false to switch to synchronous
xhr.open("POST",url, false);
xhr.setRequestHeader("Content-Type","application/x-www-urlencoded");
xhr.send(data);
if(xhr.readyState == 4) {
if(xhr.status == 200) {
var obj = $.parseJSON(xhr.responseText);
return obj.select.toString();
}
}
}
The function xhrRetrieve doesn't have a return value. What do you expect to happen?
You have two functions there. The inner function returns a value, but not the outer one. The inner function is an event handler so the return value doesn't go anywhere. Your XMLHttpRequest is asynchronous, so you won't get a return value right away. See this post for a more detailed explanation: parameter "true" in xmlHttpRequest .open() method
I'm tying to do what seems like a simple ajax but can't get it to work. Here's my code:
var xmlHttpRequest;
function processRequest(){
alert("process request called with " + xmlHttpRequest);
if(xmlHttpRequest.readyState==4){
alert("status = " + xmlHttpRequest.status);
if(xmlHttpRequest.status == 200){
}
} else {
alert("process request no luck readyState = " + xmlHttpRequest.readyState);
}
alert("process request exiting");
}
function updateCount(customerID, productID) {
xmlHttpRequest = init();
function init(){
if (window.XMLHttpRequest) {
return new XMLHttpRequest();
} else if (window.ActiveXObject) {
return new ActiveXObject("Microsoft.XMLHTTP");
} else {
alert("Your browser does not support AJAX!");
}
}
xmlHttpRequest.open("GET", url, true);
xmlHttpRequest.onreadystatechange = processRequest();
}
Like I said in the subject line, readyState is always 1. What am I doing wrong?
Thanks!
Eddy
You are calling processRequest before you start your request.
xmlHttpRequest.onreadystatechange = processRequest();
is wrong and has to be
xmlHttpRequest.onreadystatechange = processRequest;
This will store a reference to your method instead of calling it directly.
As soon as the ready state changes, the xmlHttpRequest object trys to call this reference.
Add xmlHttpRequest.send(); after xmlHttpRequest.onreadystatechange = processRequest;.