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How to store XMLHttpRequest response as variable [duplicate]

This question already has answers here:
How do I return the response from an asynchronous call?
(41 answers)
async/await implicitly returns promise?
(5 answers)
Closed 1 year ago.
In the below example from https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/onreadystatechange, I am trying to store the responseText in a variable that can be accessed outside of the onreadystatechange function.
const xhr = new XMLHttpRequest(),
method = "GET",
url = "https://developer.mozilla.org/";
xhr.open(method, url, true);
xhr.onreadystatechange = function () {
// In local files, status is 0 upon success in Mozilla Firefox
if(xhr.readyState === XMLHttpRequest.DONE) {
var status = xhr.status;
if (status === 0 || (status >= 200 && status < 400)) {
// The request has been completed successfully
console.log(xhr.responseText);
} else {
// Oh no! There has been an error with the request!
}
}
};
xhr.send();
At the moment, the only way I know to handle the return value from a function is to do the following
const variable = function()
But I can't figure out how to make that won't work due to the way the onreadystatechange event handler is initiated/called. I tried declaring a variable outside of the function scope, but it keeps returning 'undefined'.
Thanks for your help.
*Edit for async/await with fetch
I've been trying to implement the async/await fetch option but can't seem to get it to work right. I've been checking the output with alerts and the alert located outside the function always fires before the one inside the function. As a result the one outside is giving me "object Promise" and the one inside is the actual data. I'm returning the data but it's just not working.
const getData = async function (filePath) {
let myData
await fetch(filePath)
.then(response => response.text())
.then(data => myData = data)
alert(myData)
return (myData)
}
let theData = getData('./2021WeatherData.csv')
alert(theData)
So "alert(theData)" fires before "alert (myData)" and I can't understand why. Thanks so much.

This is a good question.
The XMLHttpRequest is code that runs asynchronous. And it can't run synchronously as the yellow warning on the Mozilla developer documentation explains.
So what does that mean?
(Pseudocode)
var result
console.log('start')
xhr.onreadystatechange = function() {
result = '5'
console.log('request done')
}
console.log('result', result)
This will output as you already noticed
start
result undefined
request done
and you are expecting:
start
request done
result 5
But this does not work, because the console.log('result', result) was
already executed before the variable was assigned.
Solutions
Using a callback
Whatever you want to do with the result, you need to do within the callback function. Like:
Console.log the value
Updating the browser dom (I added this as an example)
Store to the database on a node server
The callback function is the function you are assigning to the onreadystatechange.
(Pseudocode)
xhr.onreadystatechange = function() {
// This code is the callback function.
// You can do everything with the result, like updating the DOM,
// but you can't assign it to a variable outside.
}
Actual code
function logResultToTextBox(result) {
document.getElementById('#server-result-text-box').value = result
}
const xhr = new XMLHttpRequest(),
method = "GET",
url = "https://developer.mozilla.org/";
xhr.open(method, url, true);
xhr.onreadystatechange = function () {
// In local files, status is 0 upon success in Mozilla Firefox
if(xhr.readyState === XMLHttpRequest.DONE) {
var status = xhr.status;
if (status === 0 || (status >= 200 && status < 400)) {
// The request has been completed successfully
console.log(xhr.responseText);
// DO WHATEVER YOU NEED TO DO HERE WITH THE RESULT, BUT YOU CANT ASSIGN THE VALUE TO VARIABLE WHICH IS DECLARED OUTSIDE
logResultToTextBox(xhr.responseText);
} else {
// Oh no! There has been an error with the request!
}
}
};
xhr.send();
Wrap it using a promise
The promise is another way on how you can solve this. You wrap it into a promise function. Not that the promise has no return statement. It uses a resolver.
The actual work will then be done after you define the functions. It will be called in the then block.
Here you also can't assign a value to a variable that is declared outside.
Pseudocode:
Promise makeRequest(url) {
// doRequest to url and resolve result
}
makeRequest('http://example.com).then((result) => {
// do whatever you want with the result
})
Actual code:
function logResultToTextBox(result) {
document.getElementById('#server-result-text-box').value = result
}
function makeRequest (method, url) {
return new Promise(function (resolve, reject) {
var xhr = new XMLHttpRequest();
xhr.open(method, url);
xhr.onload = function () {
if (this.status >= 200 && this.status < 300) {
resolve(xhr.response);
} else {
reject({
status: this.status,
statusText: xhr.statusText
});
}
};
xhr.onerror = function () {
reject({
status: this.status,
statusText: xhr.statusText
});
};
xhr.send();
});
}
// Example:
makeRequest('GET', 'https://developer.mozilla.org/')
.then(function (result) {
console.log(result);
logResultToTextBox(result)
})
.catch(function (err) {
console.error('Augh, there was an error!', err.statusText);
});
Use async / await
For a few years, there is also a new syntax called async/await. This gives you the possibility to write code like it would a normal code.
Pseudocode
let myServerResult;
async function doAsyncCall(url) {
const serverResult = await callServer(url);
return serverResult
}
myServerResult = await doAsyncCall('http://example.com')
console.log(myServerResult)
In that case, I would use fetch instead of XMLHttpRequest because it is much simpler.
Actual code:
const response = await fetch('https://jsonplaceholder.typicode.com/posts/1');
const jsonResult = await response.json();
console.log(jsonResult)
(note that I am using jsonplaceholder.typicode.com as an API as the HTML output of the Mozilla documentation does not work with it. If you really need XMLHttpRequest then have a look into the answers on this question)
P. S. Don't use old synchronous code syntax
There is also an old-school way by using "Synchronous request". But this should not be used anymore, as it leads to a bad user experience because it makes the loading of the website slow.
https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/Synchronous_and_Asynchronous_Requests#synchronous_request
Answer after the question edit
You need to await the following line
instead of
let theData = getData('./2021WeatherData.csv')
you need to write
let theData = await getData('./2021WeatherData.csv')

How to pass ajax call result to subsequent ajax calls

Im currently trying to utilize multiple ajax calls through ajax chaining, but unsure on the best approach since there are a few ways to do this via new frameworks, jquery and pure javascript.
I would prefer to do this with pure vanilla javascript given the native development on js has improved a lot over recent years, however, in the instance of multiple ajax calls, i believe there is still plenty to be improved upon, i believe one of the ways would be to use promises ? i do see many deal with this occurrence via jquery.
i would be very appreciative if fellow coders could give their example as to how they would code a modern approach ajax chain call dependent on ajax returned call values preceding it.
ok, so in short, i am attempting to pass the value of the first ajax call to the second ajax call, along with identifying the correct way of executing the second ajax call.
Below i have added code with comments:
// Establish functionality on window load:
window.onload = function() {
'use strict';
// get product id on load
var pid = document.getElementById('pid');
var colorlist = document.getElementById('colorlist');
var sizelist = document.getElementById('sizelist');
colorlist.onclick = function(e) {
if (typeof e == 'undefined') e = window.event;
var colorid = e.target.value
while (sizelist.firstChild) {
sizelist.removeChild(sizelist.firstChild);
}
// 2ND AJAX CALL
var xhr = getXMLHttpRequestObject();
xhr.open('GET', '/ajax/get_sizes.php?id=' + encodeURIComponent(pid.value) + '&colorid=' + encodeURIComponent(colorid), true);
// set header if sending to php
xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
xhr.send(null);
// Function to be called when the readyState changes:
xhr.onreadystatechange = function() {
// Check the readyState property:
if (xhr.readyState == 4) {
// Check the status code:
if ( (xhr.status >= 200 && xhr.status < 300)
|| (xhr.status == 304) ) {
var sizes = xhr.responseText;
var sizes = JSON.parse(sizes);
for (var i = 0, num = sizes.length; i < num; i++) {
var label = document.createElement('label');
label.setAttribute ("for", sizes[i].id);
label.classList.add("swatch");
label.innerHTML = sizes[i].size;
var radio = document.createElement('input');
radio.type = "radio";
radio.id = sizes[i].id;
radio.value = sizes[i].id;
radio.name = "sizes";
sizelist.appendChild(label);
sizelist.appendChild(radio);
} //END OF FOR LOOP
} else { // Status error!
document.getElementById('output').innerHTML = xhr.statusText;
}
} // End of readyState IF.
}; // End of onreadystatechange anonymous function.
}; // END OF COLORLIST ONCLICK
// 1ST AJAX CALL
var ajax = getXMLHttpRequestObject();
// Function to be called when the readyState changes:
ajax.onreadystatechange = function() {
// Check the readyState property:
if (ajax.readyState == 4) {
// Check the status code:
if ( (ajax.status >= 200 && ajax.status < 300)
|| (ajax.status == 304) ) {
var colors = ajax.responseText;
var colors = JSON.parse(colors);
for (var i = 0, num = colors.length; i < num; i++) {
var label = document.createElement('label');
label.setAttribute ("for", colors[i].id);
label.classList.add("swatch", colors[i].color);
label.innerHTML = colors[i].color;
var radio = document.createElement('input');
radio.type = "radio";
radio.id = colors[i].id;
radio.value = colors[i].id;
radio.name = "colors";
colorlist.appendChild(label);
colorlist.appendChild(radio);
} // END OF FOR LOOP
} //END OF STATUS CODE CHECK
else { // Status error!
document.getElementById('output').innerHTML = ajax.statusText;
}
} // End of onreadyState IF.
}; // End of onreadystatechange anonymous function.
ajax.open('GET', '/ajax/get_colors.php?id=' + encodeURIComponent(pid.value), true);
// set header if sending to php
ajax.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
ajax.send(null);
}; // End of onload anonymous function.
Regards David

Welcome to SO and thank you for your question. I'll do my best to show you some examples of how you could execute your code in a way that might be preferable to you as a solution.
Callbacks
What is a callback?
Simply put: A callback is a function that is to be executed after another function has finished executing — hence the name ‘call back’.
Source of quote
In your code example you want to execute at least 2 HTTP request after one another. This would mean that a piece of code has to be executed twice. For this you can write a function around the XMLHTTPRequest piece to be able to execute it multiple times when writing it down only once.
The function below here has two parameters: url and callback. The url parameter is a string which will be injected in the second parameter of the xhr.open method. The callback parameter will be a function. This function will be called when the request has been succesfully finished.
function get(url, callback) {
var xhr = new XMLHTTPRequest();
xhr.onreadystatechange = function() {
if (xhr.readyState === 4 && xhr.status === 200) {
if ('function' === typeof callback) {
callback(xhr.responseText);
}
}
};
xhr.open('GET', url, true)
xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
xhr.send();
}
Here is a little example of how it would work. See that the callback function has a parameter called data1. That is the xhr.responseText that we got back from the XMLHTTPRequest. Inside the callback function call the get function again to make another request.
get('/ajax1.php', function(data1) {
// Do something with data1.
get('/ajax2.php', function(data2) {
// Do something with data2.
});
});
This is a fairly simple way to make a request after another is finished.
But what if we have 100 requests after each other?
Promisified XMLHTTPRequest
The Promise object represents the eventual completion (or failure) of an asynchronous operation, and its resulting value.
Source of quote
Enter Promises. The example below here is almost the same as the example above. Only this time we use a Promise. When calling get we immediately return a Promise. This promise will wait for itself to either resolve or reject. In this case we only use resolve for successful requests. Whenever the request has finished resolve is called and the Promise chain begins.
function get(url) {
return new Promise(resolve => {
var xhr = new XMLHTTPRequest();
xhr.onreadystatechange = function() {
if (xhr.readyState === 4 && xhr.status === 200) {
if ('function' === typeof callback) {
resolve(xhr.responseText);
}
}
};
xhr.open('GET', url, true)
xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
xhr.send();
});
}
So instead of using a callback function we use then. Inside then we do use a callback function which allows us to use the value that has been resolved in the returned Promise. then can be chained with more then's inifinitely until you run out of things to chain.
Inside the callback function call the next request.
get('/ajax1.php')
.then(data1 => {
// Do something with data1.
get('/ajax2.php')
.then(data2 => {
// Do something with data2.
});
});
Fetch
The Fetch API provides an interface for fetching resources (including across the network). It will seem familiar to anyone who has used XMLHttpRequest, but the new API provides a more powerful and flexible feature set.
Source of quote
Before we've created our own Promise version of an XMLHTTPRequest. But JavaScript has evolved an has gotten new tools to work with. fetch kind of looks like how our get function works, but has way more features and options to make it more powerful. And it also uses promises!
fetch('/ajax1.php')
.then(response1 => response1.text())
.then(data1 => {
// Do something with data1.
fetch('/ajax2.php')
.then(response2 => response2.text())
.then(data2 => {
// Do something with data2.
});
})
.catch(error => console.log(error));
Though the then syntax still makes us nest functions. Like before, what about when you have 100 callback functions to call? That will be a nesting mess!
Fetch + Async/Await
Now this is the way to tackle the nesting problem and make the syntax more like assigning a simple variable in JS. Async/Await is a part of modern JavaScript, so be wary of older browsers not supporting it. Check caniuse for the current support.
The Async/Await syntax works like the following. Create a function with the async keyword in front of it. This will indicate, like it implies, that async code will be performed here. It also makes the function with async before it automatically return a Promise.
Inside the async function use the await keyword whenever you call a function which returns a Promise, like fetch, or our own function get. This will return the resolved value without having to use then or a callback function.
The await keyword also makes the code actually wait before continueing to the next line of code. Now your JS looks nice and can be written with less nesting.
(async function() {
const response1 = await fetch('/ajax1.php');
const data1 = await response1.text();
// Do something with data1.
const response2 = await fetch('/ajax2.php');
const data2 = await response2.text();
// Do something with data1.
}());
I really hope this is helpful and helps you get where you need to be going.
If you have any questions regarding the above, please let me know!
Have a good one!

You can use Promise chain which could be like the example mentioned below:
.then(function(result) {
return doSomethingElse(result);
})
.then(function(newResult) {
return doThirdThing(newResult);
})
.then(function(finalResult) {
console.log('Got the final result: ' + finalResult);
})
.catch(failureCallback);enter code here
You can the documentation mentioned below:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises

make page wait for the to be read from cache before loading up

I have js function which is called in header section of my page. This Js function basically calls an API with async ajax and when it have the result back it caches the data in local storage(Reason calling the api in header is to save some time when page loads). In body (which is an angular app)of my html page I am suppose to read the returned value of API and construct page DOM. Issue is that sometime the value is not in cache yet (API returning later) and I am not sure how to make reading the value from local storage wait until there is a value there. Any suggestion as how I can make this work? Probably I need to change reading from local storage to some sort of promise which I don't know how.
Here is what I have in header:
<script>
var xhttp = new XMLHttpRequest();
this.xhttp.open("GET", "/api/orgData", true);
this.xhttps.send();
getOrgData();
async function getOrgData() {
var x = await resolveAfterOrgResponse();
localStorage.setItem('OrgData', (x));
console.log(x);
}
function resolveAfterOrgResponse() {
return new Promise(resolve => {
this.xhttp.onreadystatechange = function () {
if (xhttp.readyState == 4 && xhttp.status == 200) {
console.log(xhttp.responseText);
resolve(xhttps.responseText);
}
};
});
}
</script>
In the body of the app, this is what I have:
config = JSON.parse(window.localStorage.getItem('orgData'));
Thanks in advance

Loading configuration async in JavaScript, how best to wait for response?

This is what I am trying to do:
Request a JSON object from remote server script
WAIT until the JavaScripts gets all the response data
print a value from the response object
I'm trying to use setTimeout in the get config function to recall itself after 1 second if the value is undefined, when it's no longer undefined do something with the value.
Alternatively it seems I could just create some method that loops for some number of seconds, but i'd like to avoid that if there is a better way to accomplish what i'm trying to do? My current code just seems to recurse without any delay which breaks my runtime
Thanks for any ideas, heres the code:
function runApplication() {
var initialiser = new Initialiser();
var config = new Config();
initialiser.fetchConfigurations(config);
config.alertValue('setting1');
}
function Initialiser() {
debug.log("Started");
}
Initialiser.prototype.fetchConfigurations = function(config) {
var req = new XMLHttpRequest();
var url = CONFIGURATION_SERVER_URL;
req.onreadystatechange = function() {
if (req.readyState == 4 && req.status == 200) {
var configObject = eval('(' + req.responseText + ')');
config.setConfig(configObject);
} else {
debug.log("Downloading config data...please wait...");
}
}
req.open("GET", url, true);
req.send(null);
}
function Config() {
this.config
}
Config.prototype.setConfig = function(configObject) {
this.config = configObject;
}
Config.prototype.getValue = function(setting) {
if(this.config === undefined) {
setTimeout(this.getValue(setting), 1000);
} else {
return this.config[setting];
}
}
Config.prototype.alertValue = function(setting) {
if(this.config === undefined) {
setTimeout(this.alertValue(setting), 1000);
} else {
alert(this.config[setting]);
}
}

From what I'm looking at, you should just add an extra step to .setConfig, which handles the next piece.
Config.prototype.setConfig = function (data) {
this.config = data;
this.doSomethingAfterTheDataExists();
};
There are lots of ways to do this...
...writing a moderator/observer/pub-sub implementation is dirt-simple, and could be done in relatively few lines for the benefit given...
Writing a Promise system would be even more powerful, but would require more work (because it's like the above implementations, with an added layer of abstraction, to make a really clean and straightforward interface for async).
The other alternative to having .setConfig publish a message, or fire another method, would be to call another method, after .setConfig in the AJAX call.
xhr.onreadystatechange = function () {
// .......
config.setConfig(/* ... */);
config.doSomethingAfterTheDataExists();
};
If you're making multiple AJAX calls, with multiple assignments to multiple properties, and you want to wait until every single call is done, then you really should look into implementing a Promise system.
It will save your sanity and would be well-worth the ~60-100 lines which you could minify into oblivion.
If you're just waiting for that one response to come back, then keep in mind, what happens after the onreadystatechange says that it's finished and the data's there (your success check) is synchronous.
The waiting is the async part.
Once it's delivered, everything (including the stuff inside of that check), goes back to being procedural.

Just use jQuery and getJSON with success/done callback.

You could use a synchronous call instead of asynchronous call for XMLHttpRequest. But this may not be the best option if your your users need to interact with the web page while you wait for a response.
...
req.open("GET", url, false);
request.send(null);

Loading remote data, caching, and continuing in javascript

Basic use case. I have a global variable where I store remotely pulled data. If there is no data then I want to load it initially, wait for it load, and then continue processing. I don't really want to use a synchronous process if I don't have to.
Consider something like this where _companies is a global variable...
if (_companies === undefined || _companies.length == 0) {
loadExternalData();
}
// do something with the data in _companies
I feel like I'm missing something obvious. I understand that I can call async = false but that seems like a cludge. I could also put all the code in the block in a function, make an if..else and then call the function from loadExternalData() as well as in my else statement but again that seems like a cludge. It seems like I should be able to wrap that entire thing in a callback but I don't know how to do that.

Have a look at the code below, including the comments. The code has the same structure as the functions in your question. If anything is unclear, add a comment.
var companies; //Cache
function loadExternalData(callback){
var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function(){ //Set readystatechange event
if(xhr.readyState == 4 && xhr.status == 200){ //Request finished: 200 OK
callback(xhr.responseText); //Call callback func with data
}
}
xhr.open("get", "http://example.com", true); //True = async
xhr.send(null);
}
function parseData(data){
if(data) {//If data is defined, set companies
_companies = data;
}
if (typeof _companies == "undefined" || _companies.length == 0) {
loadExternalData(parseData); //Pass callback function
return; //No data, return function
}
//When the function reaches this point, _companies exist.
//Normal function behavior
}
See also: MDN: Using XMLHttpRequest