Develop Reference

Why a code continues when the function doesnt return value yet. (in xmlhttprequest, calling php file.) Is it related to Sync/Async functions?

Came here again with lame questions as I am in process of learning/coding.
I would like to change a property of a disable value on button: During function performing its job, button should be disabled, once function finishes and return the values, button should be enabled again.
In function which creates a buttons I am calling update() function which loading php file via XMLHttpRequest. Then running the php code and return values on page. I want to have button disabled during this time. But everytime I call the function the button will not change. Or if changed it was so fast that I didnt even saw it.
here is a code:
global_button = document.createElement("button");
// let btn1 = document.createElement("button");
global_button.innerHTML = "UPDATE";
global_button.id = "update";
global_button.disabled = false;
document.body.appendChild(global_button);
document.getElementsByClassName("two")[0].append(global_button);
global_button.addEventListener("click", function () {
console.log("After CLICKED");
global_button.disabled = true;
update();
global_button.disabled = false;
console.log("AFTER FUNCTION RETURN VALUES");
update function:
var xmlhttp;
function loadFile(file, func){
xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = func;
xmlhttp.open("GET", file, true);
xmlhttp.send();
}
function update(){
loadFile("update.php", function(){
if (xmlhttp.readyState == 4 && xmlhttp.status == 200){
document.getElementById("content").innerHTML = xmlhttp.responseText;
}
});
}
When I checked the console, it shows both console logs immediately: "After CLICKED" and "AFTER FUNCTION RETURN VALUES" messages. And couples seconds later, result of the function appear. But button wont change whatsoever.
I am suspecting the sync/async functions ? I read something for the .open method and vale true/false, but nothing changed if I switched from true to false. Also thinking if I should put it on the loop or something which will check the button clicked ? But I thought that listener would do the job.
Can anybody check and give me an advice ? or correct my thinking if it's wrong?
many thanks all of you. :)

The problem is indeed due to the asynchronous nature of the send method of XMLHttpRequest - and therefore of your update, which calls it.
When you call update(), which itself calls this:
loadFile("update.php", function(){
if (xmlhttp.readyState == 4 && xmlhttp.status == 200){
document.getElementById("content").innerHTML = xmlhttp.responseText;
}
});
all that happens is that you set up an XMLHttpRequest object and use its send method to send a request, telling it to call this function:
function(){
if (xmlhttp.readyState == 4 && xmlhttp.status == 200){
document.getElementById("content").innerHTML = xmlhttp.responseText;
}
});
as a "callback" when the readyState changes. (And in particular, when the request is complete and a response received.) But calling update does not wait for that state change to happen and block your code from running - hence the next lines of code, which set the disabled state of the button to false and log to the console - are executed straight away. So the button gets disabled but then instantly un-disabled, and therefore you never see it disabled. (In fact the browser will never even "paint" the screen with a disabled button, since it doesn't get a chance to do this while your code is running, so even if you could in theory do a freeze-frame here you would never see a disabled button.)
To fix it, you have to work with the asynchronous code you're using. Anything you want to happen after the state change has to take place in the callback function you pass it. So you can simply fix your problem by changing the update definition to this:
function update(){
loadFile("update.php", function(){
if (xmlhttp.readyState == 4 && xmlhttp.status == 200){
document.getElementById("content").innerHTML = xmlhttp.responseText;
global_button.disabled = false;
console.log("AFTER FUNCTION RETURN VALUES");
}
});
}
and delete those two lines of code from the place you've currently got them, after the update call.
Although note that this will only work if global_button is in scope inside update, which it might not be depending on how your code is structured (it probably shouldn't be to be honest). And even if it does, it's not good to hardcode your update to always undisable the button afterwards, with no guarantee the button will even be disabled first.
It's therefore better to define update to itself take a callback function:
function update(callback){
loadFile("update.php", function(){
if (xmlhttp.readyState == 4 && xmlhttp.status == 200){
document.getElementById("content").innerHTML = xmlhttp.responseText;
callback();
}
});
}
and then call it like this in your main code:
global_button.disabled = true;
update(function() {
global_button.disabled = false;
console.log("AFTER FUNCTION RETURN VALUES");
});
because this separates the concerns of update itself ("make this request and set the content inner HTML to the response"), from whatever you might want to do afterwards, which could be different each time.
Finally, I can't not mention that this callback-based asynchronous code is very old-fashioned now. XMLHTTPRequest itself is quite a cumbersome API. I highly recommend you look into its modern equivalent, fetch, which is based on Promises - which while not without their mental gotchas are a much more understandable way to write asynchronous code. In particular with async and await you can write code that looks much like what you originally had: putting await update(); would actually do what you are waiting, and have the rest of your code wait for update to complete. But you can't just make that change to your original code because that only works if update returns a Promise, which in turns would mean completely rewriting your loadFile to use a more modern, Promise-based approach.

Continuous DB calls with AJAX in straight JS

This is my first question here so I will start by saying: Hello to everyone!
I am trying to make a live data "presentation" in the form of a fancy table using straight JavaScript and Ajax, backend is PHP. In order to refresh the table I need to make requests at every, lets say 3 seconds, 4 is ok to. The database is RedisDB. I made the php script to fetch the data and its ok. I made a single JS file to request and handle/process the data and it's sort of ok.
The presentation looks great and the algorithm that is written in JS works excellent, it's about 600 lines, some simple if-else's and switches other a little more complex.
It now gets nasty. I can't get the freaking thing to do this continuously, I tried ways with setTimeout() and setInterval(), I made timers and infinite loops with sleep-like functions and so on, but it just can't survive that next request after the initial when the page loads.
The code for Ajax request is pretty straightforward for pure JS. I get my JSON string data parse it with jsonParse() and then do my processing of the data.
This all works, it's the next request that fails, no matter how it's initiated (user action or something else).
AJAX code:
window.onload = function(){
getEventdataFromDB();
}
function getEventdataFromDB(){
var xmlhttp;
if(window.XMLHttpRequest){
xmlhttp = new XMLHttpRequest();
}else{
xmlhttp = new ActiveXObject('Microsoft.XMLHTTP');
}
xmlhttp.onreadystatechange = function(){
if(xmlhttp.readyState == 4 && xmlhttp.status == 200){
var dbData = xmlhttp.responseText;
if(dbData != ''){
processEvents(dbData); //function for data processing
}
}
}
xmlhttp.open('GET','getEvents.php?dataRequest=true',true);
xmlhttp.send();
}
I know that it's a bit of a sin these days not to follow the flow and use jQuery and other frameworks but I'm just not big on the latest and greatest simplification of stuff that works just fine, to begin with.
What is the best way to do this, how should I proceed, what to use. I did some researched and things point to chaining callbacks and Promise objects.
There is obviously no way that I am the first one to do this so what is the way forward here.

The only way to ensure the previous request is over before starting the next one is so start the next call from the onreadystatechanged event of the previous one. Because ajax runs asynchronously, there's no guarantee whether any other code you run will execute before or after the ajax call finishes.
So you need to re-organise your script a bit:
var xmlhttp;
window.onload = function(){
setupAjaxObject();
getEventdataFromDB();
}
function setupAjaxObject()
{
if(window.XMLHttpRequest){
xmlhttp = new XMLHttpRequest();
}else{
xmlhttp = new ActiveXObject('Microsoft.XMLHTTP');
}
xmlhttp.onreadystatechange = function(){
if(xmlhttp.readyState == 4){
if(xmlhttp.status == 200){
var dbData = xmlhttp.responseText;
if(dbData != ''){
processEvents(dbData); //function for data processing
}
}
getEventdataFromDB(); //run the next request
}
}
}
function getEventdataFromDB(){
xmlhttp.open('GET','getEvents.php?dataRequest=true',true);
xmlhttp.send();
}
If you wanted a little delay between the requests, you could wrap the call to the next request inside a timeout.

XMLHttprequest.send(null) is crashing my code

I'm currently writing a search function using JavaScript.
However, when I attempt to test my creation, I find that it stops about halfway through for no discernible reason.
Below is my code:
document.getElementById("test").innerHTML = "";
var Connect = new XMLHttpRequest();
Connect.open("GET", "xmlTest.xml", false);
document.getElementById("test").innerHTML = "1";
Connect.send(null);
document.getElementById("test").innerHTML = "2";
var docX = Connect.responseXML;
var linjer = docX.getElementsByTagName("linjer");
The first line is there to clear a potential error message from earlier in the code. Then I attempt to open up an XML file, as I need to read from it.
As you can see, I've entered two debug statements there; they will print 1 or 2 depending on how far I get in the code.
Using this, I've found that it stops exactly on the Connect.send(null); statement (as 1 gets printed, but 2 never does), but I can't figure out why. Google says that it might be that chrome can't access local files, but when I found a way to allow Chrome to do this, it still did not work.
What am I doing wrong?

This might be a synchronous issue that requires a response that your code simply is not getting.
Try using an async call instead:
Connect.open("GET", "xmlTest.xml", true);
Also make sure to setup proper callbacks since you'll be using async here now instead of synchronous code, like so:
// Global variable scope
var docX;
var linjer;
// Define your get function
getDoc = function(url, cbFunc) {
var Connect = new XMLHttpRequest();
// Perform actions after request is sent
// You'll insert your callback here
Connect.onreadystatechange = function() {
// 4 means request finished and response is ready
if ( Connect.readyState == 4 ) {
// Here is where you do the callback
cbFunc(Connect.responseXML);
}
};
// 'true' param means async, it is also the default
Connect.open('GET', url, true);
Connect.send();
}
// Define your callback function
callbackFunction = function(responseXML) {
// XML file can now be stored in the global variable
window.docX = responseXML;
window.linjer = window.docX.getElementsByTagName("linjer");
}
// And here is the call you make to do this
getDoc("xmlTest.xml", callbackFunction);
For better understanding of all of this, do some research on scope, closures, callbacks, and async.

AJAX call with common parameters gives always readyState = 1

I'm writing a ColdFusion application that fills with some HTML content some divs once the corresponding button is clicked.
What happens is that the readyState never goes up from the initial state of 1.
The fact that makes me crazy is that I used the same AJAX code in other modules that work fine.
I tried manually the code in my applet "___AJAX_load_translator.cfm" to see if works correctly (inputting a complete url with parameters and query string) and it works.
I put many alerts in these javascript functions to trace if the url was created correctly, the parameters were formatted correctly and so on. Everything seems fine. This is driving me crazy. The result is the same on FireFox and IE.
function getHTTPObject(){
if (window.ActiveXObject) return new ActiveXObject("Microsoft.XMLHTTP");
else
if (window.XMLHttpRequest) return new XMLHttpRequest();
else {
alert("No AJAX support.");
return null;
}
}
function setOutput(divID){
if(httpObject.readyState == 4 && httpObject.status == 200){
document.getElementById(divID).innerHTML = httpObject.responseText;
} // else alert(httpObject.readyState + ' ' + httpObject.status);
}
function loadeditor(divID,CP,PP){
<CFOUTPUT>var CF_TOKENS = "CFID=#CFID#&CFTOKEN=#CFTOKEN#";</CFOUTPUT>
var operativeurl= "___AJAX_load_translator.cfm?"+CF_TOKENS+"&CP="+CP+"&PP="+PP;
httpObject = getHTTPObject();
if (httpObject != null) {
httpObject.open("POST", operativeurl, true);
httpObject.onreadystatechange = setOutput(divID);
httpObject.send(null);
}
}
I noticed that, putting an alert into the setOutput function, it displays a sudden readystate of 1. Then the browser statusbar shows the status of wait for a call to the server, that disappears quite immediately. It seems that the call is really done in that moment, and probably it is imho.
But it seems to me that after that readyness of the call (state 1) there is no more proceeding. It seems somehow blocked. Or, the function setOutput is deactivated. Maybe a second change to a state of 4 happens and this state is not registered by the callback ? In this case, why the DIV is not updated with the new content ?
Thanks for any help.

httpObject.onreadystatechange = setOutput(divID);
^^^^^^^
You're calling/executing your setouput function right then and there, and whatever the function returns becomes on the onreadystatechange callback "pointer".
Remove the (divID) portion, so you assign the function itself, not whatever it returns:
httpObject.onreadystatechange = setOutput;

onreadystatechange in a loop, readystate changes too late

I issue in a loop a total of eight xmlhttprequests to a Google map server and process the json objects the server returns to retrieve the zip codes. The code works fine if the xmlhttprequests are synchronous. Since I'm supposed to use asynchronous requests, I'm trying to convert the code to asynchronous. It doesn't work.
I use two alerts to monitor myZip. When the code is run, the second alert, right above the return, runs eight times and shows myZip as null or undefined and that is what is returned. Then the first alert runs eight times and gives the desired zip code, too late, unfortunately. It seems to me the readystate doesn't change until too late.
How should I modify the code so it will return the zip code, not null? Any help will be greatly appreciated.
var url = "http://maps.googleapis.com/maps/api/geocode/json?address="+address+city+state+"&sensor=false";
req.open("GET", url,true);
var myZip;
req.onreadystatechange = function()
{
if(req.readyState == 4 && req.status == 200) {
(function(data){
var myObj = eval( '(' + data + ')' );
if(myObj.status=="OK"){
for(i=0; i <myObj.results[0].address_components.length; i++){
if(myObj.results[0].address_components[i].types=="postal_code"){
myZip=myObj.results[0].address_components[i].long_name;
alert('zip is '+myZip);
}
}
}
else
{
alert("Error: returned status code "+req.status+" "+req.statusText);
}
})(req.responseText);
}
}
req.send();
alert(myZip);
return myZip;

You should either use a callback instead of return statement, or you should run this as Stratified JavaScript: http://stratifiedjs.org.
Then you can write it in a synchronous fashion, like you kind of did, even though it will NOT block your browser.

I guess you use the same object(req) a couple of times and will be overwritten on each loop.
So use separate objects for every requests or start a new request if the last one is finished.
How exactly you can do this I cannot say without knowing what req is.