We Keep Coding

How to use XMLHttpRequest while telling site to do something?

First of all I have to say that I have NO EXPERIENCE in Ajax and I just need this one explanation in order for me to create a simple chrome extension.
There is not much I could find on internet even tho I believe this is very simple.
I need a part of code where I would "call" url from website and I need to adjust certain arguments in that url.
Request URL:http://URL_OF_THE_WEBSITE/v1/send?token=TOKEN_VALUE
Request Method:POST
Request Payload :
{amount: 1, user_id: 12345678}
amount: 1
user_id: 12345678
(this is something I get from Network panel- with url and token changed to real things - while calling url automatically from website, but I need to be able to call it manually too.)
So I have an idea of mixing AJAX(which I don't know) and JS in order for me to call this url.
I would use variables for both TOKEN_VALUE and amount&user_id, but I don't know how to even call that url and how to set "request payload" in order for site to do the thing I want it to do.
I would really appreciate if someone would be kind enough to help :)
Work I have done, but doesn't work:
var request=new XMLHttpRequest;
request.open("POST","https://URL_OF_THE_WEBSITE/v1/send?token=TOKEN_VALUE"),request.setRequestHeader("Content-Type","application/x-www-form-urlencoded;charset=UTF-8"),request.Payload("user_id=12345678&amount=5");
I basically tried to remake an example I found online, but it didn't work out, therefore I need someone to actually explain to me how this works and how can I adjust arguments that I need.

function callAjax() {
// the XMLHttpRequest returns the ajax object that has several cool methods, so you store it in the request variable
// #data contains the $_POST[amount],$_POST[user_id],$_POST[whatever] since we are using POST method, if you're using PHP as a server side language
var request = new XMLHttpRequest(),
url = 'place_here_the_url_only',
data = 'amount=1&user_id=12345678&whatever=dataYouWantToSendToServerFromBrowser',
token = document.querySelector('meta[name="csrf-token"]').content;
// when the server is done and it came back with the data you can handle it here
request.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
// do whatever you want!
console.log("The request and response was successful!");
}
};
// method post, your giving it the URL, true means asynchronous
request.open('POST', url, true);
// set the headers so that the server knows who is he talking to, I'm using laravel 5.5
request.setRequestHeader('Content-type', 'application/x-www-form-urlencoded; charset=UTF-8');
// Token needed
request.setRequestHeader('X-CSRF-TOKEN', token);
// then you send the data and wait for the server to return the response
request.send(data);
}
Ajax: Asynchronous JavaScript And XML
It is a mean of communication between the browser and the server hosting the website, it cannot call any other server.
Asynchronous means the website continues to function normally, until the request is returned from the server and the:
if (this.readyState == 4 && this.status == 200) { }
gets triggered

native javascript rest api call

I am trying to make api call to get spotify albums in native javascript without using any js frameworks. I am running into issues where I am unable to send Oauth token using native js. For spotify I have client id and client scret. I can either use that or the Oa
(function() {
var xhr = new XMLHttpRequest();
xhr.open("GET", "https://api.spotify.com/v1/albums", false);
xhr.send();
document.getElementById("results").innerHTML = xhr.responseText;
})();

function request(callback) {
var xobj = new XMLHttpRequest();
// true parameter denotes asynchronous
xobj.open('GET', YOUR_URL_HERE, true);
xobj.onreadystatechange = function () {
if (xobj.readyState == 4 && xobj.status == "200") {
// This marks that the response has been successfully retrieved from the server
// Utilize callback
callback(xobj.responseText);
}
};
xobj.send(null);
}
I would definitely recommend taking a look at the link Frobber provided. It's always better to understand why something does/doesn't work rather than just getting it to work. Here is a mock request to get you started. Hope this helps!

I think you need to read a basic tutorial on how to use XMLHttpRequest, which you can find here
One immediate problem with your code is that it's not using any callback to read the result that comes back from the server. This is all happening asynchronously, so what's occurring in your case is that you're send()ing the request, and then immediately setting innerHTML to a value that probably isn't even available from the server yet.
Check the tutorial for how to get that information back from the server when it's ready.
Note the use of the myFunction callback, and note the use of onreadystatechange. What's happening here is that send() is sending something to the server, in a separate execution thread. You need to register a callback function that will perform the data fetching and DOM update when the server reports back that the data is available, not immediately.

using javascript to detect whether the url exists before display in iframe

I'm using javascript to pass a dynamic url to iframe src. but sometimes the url does not exist, how could i detect the non-exist url beforehand, so that i can hide the iframe that with 404 error.

Due to my low reputation I couldn't comment on Derek 朕會功夫's answer.
I've tried that code as it is and it didn't work well. There are three issues on Derek 朕會功夫's code.
The first is that the time to async send the request and change its property 'status' is slower than to execute the next expression - if(request.status === "404"). So the request.status will eventually, due to internet band, remain on status 0 (zero), and it won't achieve the code right below if. To fix that is easy: change 'true' to 'false' on method open of the ajax request. This will cause a brief (or not so) block on your code (due to synchronous call), but will change the status of the request before reaching the test on if.
The second is that the status is an integer. Using '===' javascript comparison operator you're trying to compare if the left side object is identical to one on the right side. To make this work there are two ways:
Remove the quotes that surrounds 404, making it an integer;
Use the javascript's operator '==' so you will be testing if the two objects are similar.
The third is that the object XMLHttpRequest only works on newer browsers (Firefox, Chrome and IE7+). If you want that snippet to work on all browsers you have to do in the way W3Schools suggests: w3schools ajax
The code that really worked for me was:
var request;
if(window.XMLHttpRequest)
request = new XMLHttpRequest();
else
request = new ActiveXObject("Microsoft.XMLHTTP");
request.open('GET', 'http://www.mozilla.org', false);
request.send(); // there will be a 'pause' here until the response to come.
// the object request will be actually modified
if (request.status === 404) {
alert("The page you are trying to reach is not available.");
}

Use a XHR and see if it responds you a 404 or not.
var request = new XMLHttpRequest();
request.open('GET', 'http://www.mozilla.org', true);
request.onreadystatechange = function(){
if (request.readyState === 4){
if (request.status === 404) {
alert("Oh no, it does not exist!");
}
}
};
request.send();
But notice that it will only work on the same origin. For another host, you will have to use a server-side language to do that, which you will have to figure it out by yourself.

I found this worked in my scenario.
The jqXHR.success(), jqXHR.error(), and jqXHR.complete() callback methods introduced in jQuery 1.5 are deprecated as of jQuery 1.8. To prepare your code for their eventual removal, use jqXHR.done(), jqXHR.fail(), and jqXHR.always() instead.
$.get("urlToCheck.com").done(function () {
alert("success");
}).fail(function () {
alert("failed.");
});

I created this method, it is ideal because it aborts the connection without downloading it in its entirety, ideal for checking if videos or large images exist, decreasing the response time and the need to download the entire file
// if-url-exist.js v1
function ifUrlExist(url, callback) {
let request = new XMLHttpRequest;
request.open('GET', url, true);
request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');
request.setRequestHeader('Accept', '*/*');
request.onprogress = function(event) {
let status = event.target.status;
let statusFirstNumber = (status).toString()[0];
switch (statusFirstNumber) {
case '2':
request.abort();
return callback(true);
default:
request.abort();
return callback(false);
};
};
request.send('');
};
Example of use:
ifUrlExist(url, function(exists) {
console.log(exists);
});

You could test the url via AJAX and read the status code - that is if the URL is in the same domain.
If it's a remote domain, you could have a server script on your own domain check out a remote URL.

Using async/await, this worked for me for opening a new tab; I needed to detect a 404 for the same reason as the OP:
openHelp : async function(iPossiblyBogusURL) {
const defaultURL = `http://guaranteedToWork.xyz`;
const response = await fetch(iPossiblyBogusURL);
if (response.status == 200) {
window.open(iPossiblyBogusURL, `_blank`);
} else if (response.status === 404) {
window.open(defaultURL, `_blank`);
}
},

You can try and do a simple GET on the page, if you get a 200 back it means the page exists. Try this (using jQuery), the function is the success callback function on a successful page load. Note this will only work on sites within your domain to prevent XSS. Other domains will have to be handled server side
$.get(
yourURL,
function(data, textStatus, jqXHR) {
//load the iframe here...
}
);

There is no need to make a separate HTTP request to check beforehand.
You could switch the logic around: only display the iframe if it has been loaded successfully. For this purpose, you can attach an onload event listener to the iframe.
See this related question for details: Capture iframe load complete event

parameter "true" in xmlHttpRequest .open() method

From the reference I read in MDN, it says
If TRUE (the default), the execution of the JavaScript function will continue while the response of the server has not yet arrived.
This is the A in AJAX.
I have been using AJAX but then I was a little confused when I read that. I think the problem may be that I am not understanding AJAX concept clearly. I know of course AJAX does not refresh the page which means the connection to the server and the response are completely done in the background.
But what I can imagine happening according to that reference is that if I have a code like this in my JavaScript:
//true, therefore process the function while server retrieves url
var xmlResponse;
var url = "http://example.com/file.xml";
xml_req.open("GET", url, true);
xml_req.onreadystatechange = function() {
if(xml_req.readyState == 4 && xml_req.status == 200) {
if(xml_req.responseText != null)
xmlResponse = xml_req.responseXML; //server response may not yet arrive
else {
alert("failed");
return false;
}
};
xml_req.send(null);
Doesn't that mean xmlResponse could be undefined in the sense that the server is still retrieving the data? Could somebody explain what really is the flow of the execution in AJAX technology? Thanks in advance.

I wrote a more detailed article here, but this is the basic idea.
Setting it to true means you are making an asynchronous request. That means the code does not pause until the http request is complete. A synchronous call locks up the browser so nothing else runs. That can cause problems, so people prefer asynchronous.
The XHR object updates us on what it is doing. It gives us the updates with the onreadystatechange event. We register a function with it so we can keep track of its status. The onreadystatechange gets called 4 times. Each with a different state
0 = uninitialized
1 = loading
2 = loaded
3 = interactive
4 = complete
The data is available to us when the readystate is 4.
Now in the code you posted, it is checking for the complete state and it makes sure that the status is 200 [ok]
if(xml_req.readyState == 4 && xml_req.status == 200){
The value for xmlResponse will be undefined if you try to use it somewhere else in the code before it is returned. An example
ml_req.send(null);
alert(xmlResponse );
One of the very first articles on the XMLHttpRequest article might be a good read for you. Apple Article on xmlhttpreq

The important thing to understand is that your onreadystatechange handler is not executed immediately. And it is executed more than once. It may be easier to conceptualize, if you break the pieces out into individual functions:
function makeRequest(url)
{
var xhr = new XMLHttpRequest();
xhr.open("GET", url, true);
xhr.onreadystatechange = receiveResponse;
xhr.send();
}
function receiveResponse(e)
{
if (this.readyState == 4)
{
// xhr.readyState == 4, so we've received the complete server response
if (this.status == 200)
{
// xhr.status == 200, so the response is good
var response = this.responseXML;
...
}
}
}
First, makeRequest is called and then exits. Then, as soon as we hear anything back from the server, receiveResponse is called. Each time, we check to see if the response is fully received, and only then do we continue to process that response.

What am I missing in the XMLHttpRequest?

I'm completely new to the javascript and ajax world but trying to learn.
Right now I'm testing the XMLHttpRequest and I can't make work even the simplest example. This is the code I'm trying to run
<script type="text/javascript">
function test() {
xhr = new XMLHttpRequest();
xhr.onreadystatechange = function() {
if (xhr.readyState == 4 && xhr.status == 200){
var container = document.getElementById('line');
container.innerHTML = xhr.responseText;
} else {
alert(xhr.status);
}
}
xhr.open('GET', 'http://www.google.com', true);
xhr.send(null);
}
</script>
And I always get the alert with the status 0. I've read tons of webs about this and I don't know what am I missing. I will appreciate any help, thanks!

You are running into the Same Origin Policy.
Unless your code is actually running on www.google.com (which is unlikely), this is going to error.
Also, and while this isn't causing you a problem at the moment, it is poor practice and can lead to race conditions, you are using globals all over the place.
Make the xhr variable local to the function
var xhr = new XMLHttpRequest();
And refer to it with this inside the onreadstatechange method.
if (this.readyState == 4 && this.status == 200){
// etc etc

Following from David's answer:
You have to use a relative path to stay within the same origin policy. Otherwise most browsers will simply return an empty responseText and status == 0.
As one possible workaround, you could set up a very simple reverse proxy (with mod_proxy if you are using Apache). This would allow you to use relative paths in your AJAX request, while the HTTP server would be acting as a proxy to any "remote" location.
The fundamental configuration directive to set up a reverse proxy in mod_proxy is the ProxyPass. You would typically use it as follows:
ProxyPass /ajax/ http://google.com/
In this case, the browser would be requesting /ajax/search?q=stack+overflow but the server would serve this by acting as a proxy to http://google.com/search?q=stack+overflow.

In addition to the same origin policy issue, your alert is in an illogical place. When you use XMLHttpRequest, the function assigned to xhr.onreadystatechange will be called whenever readyState changes. readyState should change (in theory) from 0 (initialized) to 1 (sent) to 2 (loading) to 3 (interactive) to 4 (finished).
What your code does is check the readyState and see if the request is finished (if (xhr.readyState == 4)), and if not, alert the HTTP status code. Since the request hasn't been sent yet (or has just been sent), there shouldn't be an HTTP status yet.
Ideally, you should move the alert inside the if block, so it lets you know when it finishes.