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
Related
I am trying to update the Html inside the GET function. When I print it right after request.onload, the content is updated, however, outside the function it remains the same:
function locationData(zip_code)
{
var request = new XMLHttpRequest();
request.open("GET", url, true);
request.send();
request.onload = () =>
{
if (request.status == 200)
{
data = JSON.parse(request.response);
document.getElementById("forecast").innerHTML = "Upadted content";
console.log(document.getElementById("forecast").innerText); // **Updated content**
}
else {(console.log("error"));}
}
console.log(document.getElementById("forecast").innerText); //**Old content**
}
console.log(document.getElementById("forecast").innerText); //**Old content**
window.onload = function() {
console.log(document.getElementById("forecast"));
}
I suspect that async function doesn't stop on time when Updated content is requested. That is why I'm trying window.onload, but still not working.
Any help is appreciated.
This is all as expected, maybe if you visualize like this:
locationData() {
// now (== when locationData is called)
// now
// now
request.onload = () => {
// future
// future
// future
}
// now
// now
// now
}
You simply have to give up on using data that only will be available in the future. If you want to do something with data in the future, then you must put that logic in the future as well.
And you alread update (....innerHTML = something) in the future, so your code probably works as expected, except for the console.logs of course.
Just be aware that if you log an object in the browser console, you will see it's current state, and not "when" it was logged. It's a pitfall, but convenient when you know how it works. you can console.log(JSON.parse(JSON.stringify(obj))) to make sure you see the object as it was when it was logged.
I am new to jQuery and I am trying to build something with AJAX. I tried to use the normal, basic XMLHttpRequest class and then $.ajax({...}) but none of them work.
function getNormal_XHR_Request(){
let request = new XMLHttpRequest();
// I also tried to make a variable here, assign it to the request.response and return it in the end
let returnValue = null;
request.addEventListener(
"load",
(event) => {
if(request.readyState === 4 && request.status === 200){
console.log(request); // this will work
console.log(request.response); // this will work too
// Assign the returnValue to the request.response
returnValue = JSON.parse(request.response); // JSON.parse(request.response) doesn't return undefined, it returns my object that I want to use, but even if I equal the returnValue to it, it will still return undefined
return JSON.parse(request.response);
}
},
false // dispatch event in the bubbling phase not in the capturing phase
)
request.open("GET", "scripts/users.json");
request.setRequestHeader("Accept", "text/json");
request.send();
return returnValue
}
This is the normal XMLHttpRequest() that I used. The problem is, that if I want to say for example
x = getNormal_XHR_Request()
so that I would have the JSON file in my x variable, it doesn't work and it returns undefined.
And the exact same things happens with $.ajax({...})
class XHR_REQUEST{
constructor(path){
this.path = path;
}
getRequest_XHR(requestHeaders){
this.requestHeaders = requestHeaders;
var self = this;
let returnValue = [];
$.ajax({
url : self.path,
dataType : "json",
headers : self.requestHeaders,
})
.done((data) => {
returnValue = data;
})
.fail((jqXHR, errorMessage, error) => {
console.log(jqXHR);
console.log(errorMessage);
console.log(error);
});
return returnValue;
}
};
It works the same as the normal function that I used above, everything that I do, returns undefined, even if request.response gives me the correct answer. If I try to console.log(request.response), I will get the object, if I try to return it and console.log() the result, it will give me back undefined.
Why ?
The result you are getting in both examples is completely normal. The reason you are getting an undefined value is because both requests are happening asynchronously. The correct place where to return something or take some action with the results you get are:
1) request.addEventListener('load', event => {} .... within the event here you can perform the action on a succesfull response.
2) with the $.ajax() call you do it within the done() promise handler.
Those are the right places to take proper action on the results. You have to modify your mindset to start using the responses from the asynchronous callbacks.
The problem there is that you are returning the result, even before the callback has some value filled in the variable.
There are new constructions this days to make asynchronous calls behave synchronously withe async-await constructions. But besides this I think you have to modify or adjust your code to react to the async results in a way it works.
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
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);
Not sure if my question is subjective/objective but as a JavaScript newbie i'm encountering this problem quite a lot. So here I go.
I'm used to write C#, so my JavaScript structure looks like C#. And just that, that gives problems I think ;-)
Let's give a simple example where I met my problem again today:
MyLibrary.fn.InitAddEntityForm = function () {
$('a#btnAddEntity').click(function () {
//post data and receive object with guid and isPersisted boolean
var persistedObject = MyLibrary.fn.CheckAndSendAddEntityForm("name", "avatarurl.png");
console.log("test");
//check if persisted and go to next step
if (persistedObject.isPersisted) {
MyLibrary.fn.InitAddAnotherEntityForm(persistedObject.gdEntityId);
} else {
alert("Oops, something went wrong. Please call 911");
}
});
};
//////*****/////
//SOME FUNCTION THAT SENDS MY FORM AND RETURNS AN OBJECT WITH TRUE VALUE AND POSTED ENTITY ID
/////*****//////
MyLibrary.fn.CheckAndSendAddForm = function (txtName, ImageUrl) {
var postUrl = "/admin/add";
var persistedObject = new Object();
$.post(
postUrl,
{ Name: txtName, ImageUrl: txtImageUrl},
function (data) {
if (data.Status == 200) {
console.log("Post status:" + data.Message);
persistedObject.isPersisted = true;
persistedObject.gdEntityId = data.Data;
} else if (data.Status == 500) {
console.log("Failed to post entitiy");
} else {
console.log("Fault with Javascript");
}
}, "json"
);
return persistedObject;
};
Okay, thats it. Everything looks okay right? Browser says no.
I tried to debug it using firebug, looping over my code line by line, and that way the browser does what I want: Execute a new function to show the next panel in my wizard.
After placing a lot of Console.logs() in my code I figured out that this must be something about timing in JavaScript. In C# the code executes line by line, but apparently JavaScript doesn't.
By placing that Console.log("test") I noticed that "test" appeared in my console before "Post status: Success!".
So here's my question, how should I write my JavaScript code so I have control over the way the browser executes my code?
Should I really replace the code below to the end of my CheckAndSendAddEntityForm()?
//check if persisted and go to next step
if (persistedObject.isPersisted) {
MyLibrary.fn.InitAddAnotherEntityForm(persistedObject.gdEntityId);
} else {
alert("fout");
}
Is this how I have to write JavaScript: One big domino effect or am I just doing something wrong?
$.post is a shortcut for an AJAX call, AJAX is by definition asynchronous, which means it won't wait on a response before continuing processing. If you switch it to a regular AJAX() method, there is an async option you can set to false, which will make it behave as you are expecting.
Alternatively you can also define a function to execute on successful return of the AJAX request, in which you can call the next step in your process chain.
The AJAX call is asychronous; that means that the callback method exposes by $.post will be executed when the request completes, but your javascript will continue executing as soon as the invoke to $.post finishes. If you want to do something after the ajax call is done, you need to provide a callback method and do something else, ex:
MyLibrary.fn.CheckAndSendAddForm = function (txtName, ImageUrl, callback) {
var postUrl = "/admin/add";
var persistedObject = new Object();
$.post(
postUrl,
{ Name: txtName, ImageUrl: txtImageUrl},
function (data) {
if (data.Status == 200) {
console.log("Post status:" + data.Message);
persistedObject.isPersisted = true;
persistedObject.gdEntityId = data.Data;
} else if (data.Status == 500) {
console.log("Failed to post entitiy");
} else {
console.log("Fault with Javascript");
}
callback(); // This is where you return flow to your caller
}, "json"
);
};
Then you invoke like so:
var persistedObject = MyLibrary.fn.CheckAndSendAddEntityForm("name", "avatarurl.png", function()
{
console.log("test");
//check if persisted and go to next step
if (persistedObject.isPersisted) {
MyLibrary.fn.InitAddAnotherEntityForm(persistedObject .gdPronoId);
} else {
alert("Oops, something went wrong. Please call 911");
}
});
JavaScript is single-threaded. If you have asynchronous functionality, a simple boolean semaphore variable will help not to allow invocations of a function while some processes are running.
If you want to execute asynchronous tasks one by one (like a domino line), you will need to use callback functions.
What you're encountering is the "asynchronous" bit of AJAX. If you want to physically (as in the line line by line in the Javascript file) you can use the .success,.pipe or .done jQuery methods to add a callback to process the data further. Don't embed your callbacks if you can help it, or you will get a "domino effect" as you call it.