Develop Reference

how to wait for a result of a http request (syncronous http request, http request as promise)

I have a typescript service that loads base data by htpp requests from a server. There are several requests for several data, arranged in order from undependent data to data depending on data, for which loading ist started bevor. Because the asynchronous http requests it is not guaranteed, that loading data by a request (e. g. customers in the following example) has finished, bevor loading the data that depends on it starts (e. g. devices in the following example) and i can not refer on loading devices to loaded customers. But i would like, that laoding the dependent data (devices) starts, after loading the data refereing to (customers) has finished. I think i need to use promises, but how to implement concrete in the following code situation?
export class BaseDataService
{
customers: Customer[] = new Array();
devices: Device[] = new Array();
// Loads the base data arranged in order from undependent data to data dependent data
loadBaseData() {
this.loadCustomers();
// this operation should be started after loading customers has finished (the response json
//is parsed to an array of customers at set to the instance variable of this service class)
this.loadDevices();
}
// Loads the customers and adds them to the collection of customers.
// Also puts the response json string of the customers to the local storage.
loadCustomers() {
console.log("Load customers");
var requestURL = 'https://myurl/kunden_json_user_extern.php';
var auth = window.localStorage.getItem('auth');
var requestparam = "auth="+auth;
var request = new XMLHttpRequest();
request.open('POST', requestURL);
request.setRequestHeader("content-type", "application/x-www-form-urlencoded");
request.send(requestparam);
request.onload = () => {
if (request.status === 200){
console.log("Load customers response");
// gets as json string with the customers array
var response = request.response;
// parses the json string of customers to an array of customers objects
this.customers = this.parseCustomers(response);
console.log("Load customers complete");
}
else if (request.status === 401){
alert("unautorized");
}
else {
// lade saved items aus local storage
alert("unerwarteter Fehler");
}
}
}
// Loads the devices and adds them to the collection of devices.
// Also puts the response json string of the devices to the local storage.
loadDevices() {
console.log("Load devices");
var requestURL = 'https://myurl/geraete_json_user_extern.php';
var auth = window.localStorage.getItem('auth');
var requestparam = "auth="+auth;
var request = new XMLHttpRequest();
request.open('POST', requestURL);
request.setRequestHeader("content-type", "application/x-www-form-urlencoded");
request.send(requestparam);
request.onload = () => {
if (request.status === 200){
console.log("Load devices response");
var response = request.response;
window.localStorage.setItem('devicesresponse', response);
this.devices = this.parseDevices(response);
console.log("Load devices complete");
}
else if (request.status === 401){
alert("unautorized");
}
else {
// lade saved items aus local storage
alert("unerwarteter Fehler");
}
}
}
// Parses a json string formatted like the response of customers to an array of customers
parseCustomers(customersjson: string)
{
let customerarray = JSON.parse(customersjson);
let customers: Customer[] = [];
for (let i = 0; i < customerarray.length; i++) {
let customer: Customer = new Customer();
customer.setId(customerarray[i]['id']);
customer.setName(customerarray[i]['name']);
customer.setPlace(customerarray[i]['ort']);
customer.setStreet(customerarray[i]['strasse']);
customers[i] = customer;
}
return customers;
}
// Parses a json string formatted like the response of devices to an array of devices
parseDevices(devicesjson: string)
{
let devicearray = JSON.parse(devicesjson);
let devices: Device[] = [];
for (let i = 0; i < devicearray.length; i++) {
let device = new Device();
device.setId(devicearray[i]['geraet_id']);
device.setSerialnumber(devicearray[i]['geraet_seriennummer']);
device.setDescription(devicearray[i]['geraet_bezeichnung']);
// here i would like to refer to a prevoiusly loaded customer
//device.setCustomer(this.baseDataService.getCustomer(devicearray[i]['geraet_kunde_id']);
device.setLocation(devicearray[i]['geraet_standort']);
devices[i] = device;
}
return devices;
}
}

You can pass a callback to any of 'load' functions, so:
This code:
loadBaseData() {
this.loadCustomers();
// this operation should be started after loading customers has finished (the response json
//is parsed to an array of customers at set to the instance variable of this service class)
this.loadDevices();
}
Change to this code:
loadBaseData() {
this.loadCustomers(() => {
this.loadDevices();
});
}
And call this callback inside your request.onload:
// Loads the customers and adds them to the collection of customers.
// Also puts the response json string of the customers to the local storage.
loadCustomers(callback) { // <--- this is a callback passed to function
// ... many code here ...
request.onload = () => {
// ... many code here ...
console.log("Load customers complete");
if (typeof callback === 'function') callback(); // <--- here we call it
}
// ... many code here ...

You can make a synchronous ajax call like below. So it waits for the response before executing the next statements.
xhr.open("POST",url,false);

Make a while loop delay repeating until ajax calls in it are complete

Before I explain what I want to do, here's an MCV of what I'm coding
$("#button").submit(function(event) {
event.preventDefault();
var formData = new FormData(this);
var myString = $('#textarea').val();
var myRegexp = /src="blob:([^'"]+)"/gm;
match = myRegexp.exec(myString);
var inProgress = 0;
while (match != null) {
var xhr = new XMLHttpRequest();
addr = match[1];
xhr.open('GET', 'blob:' + addr, true);
xhr.responseType = 'blob';
xhr.onload = function(e) {
if (this.status == 200) {
var myBlob = this.response;
var data = new FormData();
data.append('file', myBlob);
$.ajax({
url: "uploader.php",
type: 'POST',
data: data,
contentType: false,
processData: false,
beforeSend: function() {
inProgress++;
},
success: function(data) {
myString = myString.replace("blob:" + addr, data);
},
error: function() {
// error
},
complete: function() {
--inProgress;
}
});
} else {
// error
}
};
xhr.send();
match = myRegexp.exec(myString);
}
if (!inProgress) {
formData.set('textarea', myString);
submitForm(formData);
}
});
So, I have a text area and it contains an unknown number of BLOB objects. I first try to find these BLOB objects using regexp and then I upload them to the server using a PHP file called uploader.php. Once the file is uploaded, it will return the URL of the uploaded file and I want to replace the BLOB URL by the URL of the uploaded file in the text area and then submit the final content of the textarea to the server for further processing.
It turns out that when I run the code, only the last instance of the regexp is replaced by its uploaded URL. The others remain as they were. I suspect that this is because the while loop does not wait for the ajax requests to be complete. I had a similar problem when trying to submit the form and I solved it by following the suggestions in this answer but I don't know how to fix this issue this time.
Any idea is appreciated
Update: I tried to make ajax work synchronously but my browser said that it was deprecated and it didn't work.

It seems you don't really need it to be synchronous (and I can't see a case when it's better to make an async action look synchronous), but rather only need it to be sequential.
It is possible to make async actions sequential by the use of callbacks (which are rewritable as Promise and in turn rewritable as async/await methods but I'll keep it simple):
// myString is made global for simplicity
var myString;
function uploadBlob(myBlob, addr, callback) {
var data = new FormData();
data.append('file', myBlob);
$.ajax({
url: "uploader.php",
type: 'POST',
data: data,
contentType: false,
processData: false,
success: function(data) {
// file uploaded OK, replace the blob expr by the uploaded file url(data)
myString = myString.replace("blob:" + addr, data);
callback();
},
error: function() {
// error, the uploaded most likely failed, we leave myString alone
// or alternatively replace the blob expr by empty string
// because maybe we dont want to post blob in the final form submit
// uncomment if needed
// myString = myString.replace("blob:" + addr, "");
callback();
}
});
}
function getBlobAndUpload(addr, callback) {
var xhr = new XMLHttpRequest();
xhr.open('GET', 'blob:' + addr, true);
xhr.responseType = 'blob';
xhr.onload = function(e) {
if (this.status == 200) {
var myBlob = this.response;
uploadBlob(myBlob, addr, callback);
} else {
// error, but callback anyway to continue processing
callback();
}
};
xhr.send();
}
function processAddresses(addresses, callback, current) {
var index = current || 0;
// all addresses processed?
if (index >= addresses.length) {
// yes no more address, call the callback function
callback();
} else {
var addr = addresses[index];
// once the get/upload is done the next address will be processed
getBlobAndUpload(addr, function() {
processAddresses(addresses, callback, index + 1);
});
}
}
$("#button").submit(function(event) {
event.preventDefault();
var formData = new FormData(this);
var addresses = [];
// initialize both localString and myString to the content of the textArea
// localString will be used to extract addresses,
// while myString will be mutated during the upload process
var localString = myString = $('#textarea').val();
var myRegexp = /src="blob:([^'"]+)"/gm;
match = myRegexp.exec(localString);
// collect all addresses first
while (match != null) {
addr = match[1];
addresses.push(addr);
match = myRegexp.exec(localString);
}
// initiate sequential processing of all addresses, and
// pass the callback function triggering the form submit
processAddresses(addresses, function() {
// all the successfully uploaded blob exprs in my string should
// be now replaced by the remote file url now (see commented part
// in upload blob error for a variation of the feature
formData.set('textarea', myString);
submitForm(formData);
});
});

So. I said in comments, that you could use async/await, and gave links. Now I am going to try to teach you how to work with promises and XMLHttpRequest.
So first thing. I would use my own 'library' (not really a library, just 3 new command) called PromiseReq which has XMLHttpsRequest that returns Promises.
You would need two functions from it:
sendToServer(config, data) and getServerFile(config). They do what their names implies.(sendToServer is not so good at the time, but I will improve it sometime later). They just use Promises as returns. They work in very easy way. Code # Github
BUT It was designed for my uses only, so it is not very flexible (although I hope I will improve it sometime).
So we need to learn how to use Promises.
Firstly you need to know what Promise is and why do we use it.
Then you can create one like this:
let pmq = new Promise((res,rej)=>{
// PROMISE BODY HERE
});
Here first warning. Promises made that way don't support return as a way to resolve Promise! You have to use res()!
Some functions just return them (such as fetch()) and we can handle them right after calling function.
Now pmq will be our promise.
You can use pmq.then(callback) to handle what will happen if somewhere in promise body is res() call and pmq.catch(callback) to handle what happens when rej() is called. Remember, that .catch(cb) and .then(cb) returns a Promise, so you can safely chain more than one .then() and at the end add .catch() and it will handle rejection from every one of .then()s.
For example:
pmq = fetch("file.txt");
pmq.then(e=>console.log(e.json())).then(console.log).catch(console.error);
There is a big note there.
The order in which this events will fire.
If for example rP() waits 1s than logs "A" then resolves, this code:
let a = rP();
a.then(_=>console.log("B")).catch(console.error);
console.log("C");
will result in:
C
A
B
Becuase of this there is async/await needed to do this.
To do so you have to make an async function with keyword async.
let fn = async ()=>{}
Here is second thing. Async functions ALWAYS return Promise. And that is the second way you can create a promise. You just don't use res(), rej() only return and throw.
Now we can call inside fn():
let a = await rP().then(_=>console.log("B")).catch(console.error);
console.log("C");
and we will get our
A
B
C
Now. How to use it with XMLHttpRequest?
You need to create new Promise with simple XMLHttpRequest inside:
let xmlp = (type, path,data) => return new Promise((res,req)=>{
let xhr = new XMLHttpsRequest();
xhr.open(type, path, true); // true implies that is it asynchronous call
//...
xhr.send(data);
});
And now when to resolve?
XMLHttpRequest has useful event properties and events. The one that is best for our case is onreadystatechange.
You can use it like so:
xhr.onreadystatechange = _=>{
if(xhr.readyState === 4 && xhr.status === 200) // Everything went smoothly
res(xhr.responseText);
else if(shr.readyState === 4 && xhr.status !== 200) // Something went wrong!
rej(xhr.status);
}
And then to get data you can either
Async/Await
// INSIDE ASYNC FUNCTION
let resData = await xmpl("GET", "path.txt", null).catch(console.error);
.then()
let resData;
xmpl("GET", "path.txt", null).then(r=>{
resData = r;
// REST OF WHOLE FUNCTION TO ENSURE THAT resData HAS BEEN SET
})
.catch(console.error);
You can also send data with xmpl().
xmpl("POST", "observer.php", "Data to send to observer.php!")
.then(whatToDoAfterSendFN);
/*
to get such data in PHP you have to use
$post = file_get_contents('php://input');
*/
I know that this answer is a bit messy and stuff, but I didn't have any idea how to write it :P Sorry.

What is the vanilla JS version of Jquery's $.getJSON

I need to build a project to get into a JS bootcamp I am applying for. They tell me I may only use vanilla JS, specifically that frameworks and Jquery are not permitted. Up to this point when I wanted to retrieve a JSON file from an api I would say
$.getJSON(url, functionToPassJsonFileTo)
for JSON calls and
$.getJSON(url + "&callback?", functionToPassJsonPFileTo)
for JSONP calls. I just started programming this month so please bear in mind I don't know the difference between JSON or JSONP or how they relate to this thing called ajax. Please explain how I would get what the 2 lines above achieve in Vanilla Javascript. Thank you.
So to clarify,
function jsonp(uri){
return new Promise(function(resolve, reject){
var id = '_' + Math.round(10000 * Math.random())
var callbackName = 'jsonp_callback_' + id
window[callbackName] = function(data){
delete window[callbackName]
var ele = document.getElementById(id)
ele.parentNode.removeChild(ele)
resolve(data)
}
var src = uri + '&callback=' + callbackName
var script = document.createElement('script')
script.src = src
script.id = id
script.addEventListener('error', reject)
(document.getElementsByTagName('head')[0] || document.body || document.documentElement).appendChild(script)
})
}
would be the JSONP equivalent?

Here is the Vanilla JS version for $.getJSON :
var request = new XMLHttpRequest();
request.open('GET', '/my/url', true);
request.onload = function() {
if (request.status >= 200 && request.status < 400) {
// Success!
var data = JSON.parse(request.responseText);
} else {
// We reached our target server, but it returned an error
}
};
request.onerror = function() {
// There was a connection error of some sort
};
request.send();
Ref: http://youmightnotneedjquery.com/
For JSONP SO already has the answer here
With $.getJSON you can load JSON-encoded data from the server using
a GET HTTP request.

ES6 has Fetch API which provides a global fetch() method that provides an easy, logical way to fetch resources asynchronously across the network.
It is easier than XMLHttpRequest.
fetch(url) // Call the fetch function passing the url of the API as a parameter
.then(res => res.json())
.then(function (res) {
console.log(res)
// Your code for handling the data you get from the API
})
.catch(function() {
// This is where you run code if the server returns any errors
});

Here is a vanilla JS version of Ajax
var $ajax = (function(){
var that = {};
that.send = function(url, options) {
var on_success = options.onSuccess || function(){},
on_error = options.onError || function(){},
on_timeout = options.onTimeout || function(){},
timeout = options.timeout || 10000; // ms
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
//console.log('responseText:' + xmlhttp.responseText);
try {
var data = JSON.parse(xmlhttp.responseText);
} catch(err) {
console.log(err.message + " in " + xmlhttp.responseText);
return;
}
on_success(data);
}else{
if(xmlhttp.readyState == 4){
on_error();
}
}
};
xmlhttp.timeout = timeout;
xmlhttp.ontimeout = function () {
on_timeout();
}
xmlhttp.open("GET", url, true);
xmlhttp.send();
}
return that;
})();
Example:
$ajax.send("someUrl.com", {
onSuccess: function(data){
console.log("success",data);
},
onError: function(){
console.log("Error");
},
onTimeout: function(){
console.log("Timeout");
},
timeout: 10000
});

I appreciate the vanilla js equivalent of a $.getJSON above
but I come to exactly the same point. I actually was trying of getting rid of jquery which I do not master in any way .
What I'm finally strugglin with in BOTH cases is the async nature of the JSON request.
What I'm trying to achieve is to extract a variable from the async call
function shorten(url){
var request = new XMLHttpRequest();
bitly="http://api.bitly.com/v3/shorten?&apiKey=mykey&login=mylogin&longURL=";
request.open('GET', bitly+url, true);
request.onload = function() {
if (request.status >= 200 && request.status < 400) {
var data = JSON.parse(request.responseText).data.url;
alert ("1:"+data); //alerts fine from within
// return data is helpless
}
};
request.onerror = function() {
// There was a connection error of some sort
return url;
};
request.send();
}
now that the function is defined & works a treat
shorten("anyvalidURL"); // alerts fine from within "1: [bit.ly url]"
but how do I assign the data value (from async call) to be able to use it in my javascript after the function was called
like e.g
document.write("My tiny is : "+data);

ajax - request data from multiple files on one page?

I am trying to run multiple Ajax functions on load in a single page which will get data from two different php pages. Both the Ajax functions will then print the retrieved data onto the page from which the ajax function was called. The problem I encountered was that the last function call which I make from the Ajax overrides the first function call, and so only the second function result is showed.
The code for one of the Ajax function (since both of the are very similar to each other):
function favorite_track_request(str){
switch(str){
case 'next_track':
var feed = 'require_fav_track_info';
var offset = track_currentOffset + 5;
if(offset > max_track_range){
offset -= 5;
}
break;
case 'prev_track':
var feed = 'require_fav_track_info';
var offset = track_currentOffset - 5;
if(offset < 0){
offset = 0;
}
break;
default:
var feed = 'require_fav_track_info';
var offset = 0;
}
request = new ajaxRequest()
request.open("GET", "scripts/"+feed+".php?offset="+offset, true)
request.onreadystatechange = function(){
if(this.readyState == 4){
if(this.status == 200){
if(this.responseText != null){
if(request.responseText){
document.getElementById('fav_tracks').innerHTML = request.responseText;
}
}else alert("No data recieved");
}else {
alert("Ajax error: "+this.statusText);
}
}
}
request.send(null);
track_currentOffset = offset;
}
This ajax would then print to <div id="fav_tracks"></div>, however this gets overridden because another call (similar to the Ajax above) is made and that overrides the previous one. Is there any way to stop this?

I built a data handler "class" to manage just such a thing. You are right, the one overrides the other. I haven't investigated it, but it's probabably because your are re-assigning the onEvent that AJAX uses.
Below is the class I built (I know, it's not JQuery... it works). What it does is uses timeouts to "know" when to fire the second and third async request. There probably is a JQuery function that does the same thing.
You would call this by using the below for each AJAX call (giving each call a unique var name):
dataHandler = new DataHandler("[name of datafile to call]");
dataHandler.query['[myQueryName]'] = 'myValue' //this is an Object used to build a query string, if needed, so use as many data pairs as you need
dataHandler.asynchronous(myOnReadyStateChangeFN);//put the fn you want to use for readystatechange as a reference... do not includ the ()
Here's the "class":
function DataHandler(dataFile){
this.dataFile = dataFile;
dataInProgress = false;
this.query = new Object();
this.asynchronous = function(fn){
var thisFunction = this.asynchronous
var rand = Math.floor(Math.random()*100001);
var query, timeOutFunctionString;
if(this.dataInProgress){
timeOutFunctionString = callingObjectName+".asynchronous("+fn+")";
this.thisTimeout = setTimeout(timeOutFunctionString,500);
}else{
dataInProgress = true;
this.assignRequestObject.xmlHttp.onreadystatechange = function () {
fn();
dataInProgress = false;
return;
};
}
query = this.dataFile + '?r=' + rand;
for (var key in this.query) query = query + '&' + key + '=' + this.query[key];
//console.info("DataHandler.asynchronous\nquery = "+query+'\nthis.dataFile = ' + this.dataFile);
this.assignRequestObject.xmlHttp.open('GET', query, true);
this.assignRequestObject.xmlHttp.send(null);
};
this.AssignRequestObject = function() {
try { this.xmlHttp = new XMLHttpRequest() } catch (e) {
try { this.xmlHttp = new ActiveXObject("Msxml2.XMLHTTP") } catch (e) {
try { this.xmlHttp = new ActiveXObject("Microsoft.XMLHTTP") } catch (e) {
alert("Your browser does not support AJAX!");
return false
}
}
}
};
this.assignRequestObject = new this.AssignRequestObject();
};

How do I make Ajax calls at intervals without overlap?

I'm looking to setup a web page that samples data via AJAX calls from an embedded web-server. How would I set up the code so that one request doesn't overlap another?
I should mention I have very little JavaScript experience and also a compelling reason not to use external libraries of any size bigger than maybe 10 or so kilobytes.

You may want to consider the option of relaunching your AJAX request ONLY after a successful response from the previous AJAX call.
function autoUpdate()
{
var ajaxConnection = new Ext.data.Connection();
ajaxConnection.request(
{
method: 'GET',
url: '/web-service/',
success: function(response)
{
// Add your logic here for a successful AJAX response.
// ...
// ...
// Relaunch the autoUpdate() function in 5 seconds.
setTimeout(autoUpdate, 5000);
}
}
}
This example uses ExtJS, but you could very easily use just XMLHttpRequest.
NOTE: If you must have an exact interval of x seconds, you would have to keep track of the time passed from when the AJAX request was launched up to the setTimeout() call, and then subtract this timespan from the delay. Otherwise, the interval time in the above example will vary with the network latency and with the time to processes the web service logic.

I suggest you use a small toolkit like jx.js (source). You can find it here: http://www.openjs.com/scripts/jx/ (less than 1k minified)
To setup a request:
jx.load('somepage.php', function(data){
alert(data); // Do what you want with the 'data' variable.
});
To set it up on an interval you can use setInterval and a variable to store whether or not a request is currently occuring - if it is, we simple do nothing:
var activeRequest = false;
setInterval(function(){
if (!activeRequest) {
// Only runs if no request is currently occuring:
jx.load('somepage.php', function(data){
activeRequest = false;
alert(data); // Do what you want with the 'data' variable.
});
}
activeRequest = true;
}, 5000); // Every five seconds

AJAX, despite the name, need not be asynchronous.
Here is the asynchronous method...
var req;
function ajax(method,url,payload,action)
{
if (window.XMLHttpRequest)
{
req = new XMLHttpRequest();
req.onreadystatechange = action;
req.open(method, url, true);
req.send(payload);
}
else if (window.ActiveXObject)
{
req = new ActiveXObject("Microsoft.XMLHTTP");
if (req)
{
req.onreadystatechange = action;
req.open(method, url, true);
req.send(payload);
}
else
{
alert("Could not create ActiveXObject(Microsoft.XMLHTTP)");
}
}
}
...but here is a synchronous equivalent...
function sjax(method,url,payload,action)
{
if (window.XMLHttpRequest)
{
req = new XMLHttpRequest();
req.open(method, url, false);
req.send(payload);
action();
}
else if (window.ActiveXObject)
{
req = new ActiveXObject("Microsoft.XMLHTTP");
if (req)
{
req.onreadystatechange = action;
req.open(method, url, false);
req.send(payload);
}
else
{
alert("Could not create ActiveXObject(Microsoft.XMLHTTP)");
}
}
}
... and here is a typical action ...
function insertHtml(target)
{
var pageTarget = arguments[0];
if (req.readyState == 4) // 4 == "loaded"
{
if (req.status == 200) // 200 == "Ok"
{
if (req.responseText.indexOf("error") >= 0)
{
alert("Please report the following error...");
pretty = req.responseText.substring(req.responseText.indexOf("error"),1200);
pretty = pretty.substring(0,pretty.indexOf("\""));
alert(pretty + "\n\n" + req.responseText.substring(0,1200));
}
else
{
div = document.getElementById(pageTarget);
div.innerHTML = req.responseText;
dimOff();
}
}
else
{
alert("Could not retreive URL:\n" + req.statusText);
}
}
}