Develop Reference

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.

Is it possible to return wether or not an image has loaded in javascript

I'm (attempting) working on a function right now that will return whether or not the user has a gravatar. Javascript is limited to cross-site checks so this already feels kinda hacky. I'm writing this in django which I could use and write an API for... but I'd rather see if there is a way to do this client side instead of server side, but maybe that's the wrong mentality.
This is being written in an angular directive; but this is what I've came up with so far.
$scope.hasAvatar = function(user) {
if (user === undefined)
return false;
var url = 'http://www.gravatar.com/avatar/' + md5(user.email);
var image = new Image();
image.onload = function() {
return true;
}
image.onerror = function() {
return false;
}
image.src = url + "?d=404";
}
I'm still learning javascript. I know that because of the nature of javascript it doesn't wait to call the onload or onerror, it simply completes the function, which defaults to false.
Am I barking up the wrong tree trying to solve it this way? or is it possible to do with callbacks or promises? I really appreciate the help.

You should do this with a promise since image loading is async:
$scope.hasAvatar = function(user) {
var dfd = $q.defer();
if (user === undefined)
return $q.when(false);
var url = 'http://www.gravatar.com/avatar/' + md5(user.email);
var image = new Image();
image.onload = function() {
dfd.resolve(true);
}
image.onerror = function() {
dfd.resolve(false);
}
image.src = url + "?d=404";
return dfd.promise;
}
Then you call it like this
$scope.hasAvatar(user).then(function(hasAvatar) {
// logic here. perhaps bind to showAvatar
$scope.showAvatar = hasAvatar;
});
$scope.showAvatar = false;

How to change emscripten browser input method from window.prompt to something more sensible?

I have a C++ function which once called consumes input from stdin. Exporting this function to javascript using emscripten causes calls to window.prompt.
Interacting with browser prompt is really tedious task. First of all you can paste only one line at time. Secondly the only way to indicate EOF is by pressing 'cancel'. Last but not least the only way (in case of my function) to make it stop asking user for input by window.prompt is by checking the checkbox preventing more prompts to pop up.
For me the best input method would be reading some blob. I know I can hack library.js but I see some problems:
Reading blob is asynchronous.
To read a blob, first you have to open a file user has to select first.
I don't really know how to prevent my function from reading this blob forever - there is no checkbox like with window.prompt and I'm not sure if spotting EOF will stop it if it didn't in window.prompt case (only checking a checkbox helped).
The best solution would be some kind of callback but I would like to see sime hints from more experienced users.

A way would be to use the Emscripten Filesystem API, for example by calling FS.init in the Module preRun function, passing a custom function as the standard input.
var Module = {
preRun: function() {
function stdin() {
// Return ASCII code of character, or null if no input
}
var stdout = null; // Keep as default
var stderr = null; // Keep as default
FS.init(stdin, stdout, stderr);
}
};
The function is quite low-level: is must deal with one character at a time. To read some data from a blob, you could do something like:
var data = new Int8Array([1,2,3,4,5]);
var blob = new Blob([array], {type: 'application/octet-binary'});
var reader = new FileReader();
var result;
reader.addEventListener("loadend", function() {
result = new Int8Array(reader.result);
});
var i = 0;
var Module = {
preRun: function() {
function stdin() {
if (if < result.byteLength {
var code = result[i];
++i;
return code;
} else {
return null;
}
}
var stdout = null; // Keep as default
var stderr = null; // Keep as default
FS.init(stdin, stdout, stderr);
}
};
Note (as you have hinted), due to the asynchronous nature of the reader, there could be a race condition: the reader must have loaded before you can expect the data at the standard input. You might need to implement some mechanism to avoid this in a real case. Depending on your exact requirements, you could make it so the Emscripten program doesn't actually call main() until you have the data:
var fileRead = false;
var initialised = false;
var result;
var array = new Int8Array([1,2,3,4,5]);
var blob = new Blob([array], {type: 'application/octet-binary'});
var reader = new FileReader();
reader.addEventListener("loadend", function() {
result = new Int8Array(reader.result);
fileRead = true;
runIfCan();
});
reader.readAsArrayBuffer(blob);
var i = 0;
var Module = {
preRun: function() {
function stdin() {
if (i < result.byteLength)
{
var code = result[i];
++i;
return code;
} else{
return null;
}
}
var stdout = null;
var stderr = null;
FS.init(stdin, stdout, stderr);
initialised = true;
runIfCan();
},
noInitialRun: true
};
function runIfCan() {
if (fileRead && initialised) {
// Module.run() doesn't seem to work here
Module.callMain();
}
}
Note: this is a version of my answer at Providing stdin to an emscripten HTML program? , but with focus on the standard input, and adding parts about passing data from a Blob.

From what I understand you could try the following:
Implement selecting a file in Javascript and access it via Javascript Blob interface.
Allocate some memory in Emscripten
var buf = Module._malloc( blob.size );
Write the content of your Blob into the returned memory location from Javascript.
Module.HEAPU8.set( new Uint8Array(blob), buf );
Pass that memory location to a second Emscripten compiled function, which then processes the file content and
Deallocate allocated memory.
Module._free( buf );
Best to read the wiki first.

url callback function not working

How can I use the callback function properly cause it's not working.
I want to retrieve my json values from the server to the client side.
For starters I tried this to see if its working.
sample code
index.html
<button onClick = "Json()">Click Here</button>
my.js
function Json()
{
var url = "http://www.sample.com/test.js?callback=displayUser";
var script = document.createElement('script');
script.src = url;
document.body.appendChild(script);
}
function displayUser(json)
{
alert("working");
}
test.js
var obj = { "a": "hey", "b": "what?"}
but does not return any alert, so I guess it does not proceed to my displayUser function.

If you want displayUser() called in test.js, you have to have code in test.js that will call it. It won't get called just because you put it in the URL. You would need to code parse it out of the URL and then call it.
Or, alternatively, you can hook up a notification when the script has finished loading and just call displayUser() yourself from outside of test.js after test.js is fully loaded.
To detect when the external script file has loaded successfully and just call displayUser() yourself, see this reference.
If you want to get the callback value from the test.js script file itself and execute it from within test.js, this is the only way I know of to do it:
window.displayUser = function() {
// do whatever
}
var scripts = document.getElementsByTagName("script");
for (var i = 0; i < scripts.length; i++) {
var url = scripts[i].src;
// extract filename and callback parameter from the URL
var matches = url.match(/\/([^\/]+)\?.*callback=(.*?)(&|$)/);
if (matches && matches[1] == "test.js") {
// call the callback function
window[matches[2]]();
}
}

The script that is returned by "http://www.sample.com/test.js?callback=displayUser" actually has to be valid javascript and contain a call to displayUser.
test.js should have a call to dispayUser().

making parallel ajax requests

i have javascript code that does these things in a loop
create a div element,append it to the dom and get its reference
pass this reference to a function that makes an ajax post request
set the response of the ajax request to the innerHTML of the passed element reference
here is the code
window.onload = function () {
var categories = document.getElementById('categories').children;
for (i = 0; i < categories.length; i++) {
var link = categories[i].children[1].children[0].attributes['href'].nodeValue;
var div = document.createElement('div');
div.className = "books";
div.style.display = "none";
categories[i].appendChild(div);
getLinks(link, div);
}
}
function getLinks(url, div) {
xhr = new XMLHttpRequest();
xhr.open('POST', 'ebook_catg.php', true);
xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
url = encodeURIComponent(url)
var post = "url=" + url;
xhr.node=div; //in response to Marc B's suggestion
xhr.onreadystatechange = function () {
if (xhr.readyState == 4) {
xhr.node.innerHTML = xhr.responseText;
xhr.node.style.display = "block";
}
}
xhr.send(post);
}
now when i check this in firebug i can see that the div element is created and appended to the categories element and its display is set to hidden. also the ajax post requests are being sent and the response is being received as expected. But the innerHTML property of div is not set and neither its display is set to block.
This means that the function getLinks loses the div reference.
when i type console.log(div) in the firefox console it says ReferenceError: div is not defined.
can somebody explain whats going on here?
in response to Franks's comment i changed readystate to readyState and i am able to attach the response of the last ajax request to the dom. so that makes it obvious that the div reference is being lost.

Thats because you are using a public (global) variable div that keeps getting overwritten.
Try this in your for loop:
for (i = 0; i < categories.length; i++) {
var link = categories[i].children[1].children[0].attributes['href'].nodeValue;
var div = document.createElement('div'); //USE var!
div.className = "books";
div.style.display = "none";
categories[i].appendChild(div);
getLinks(link, div);
}

Remember that the response handlers innards aren't "fixated" when the callback is defined, so the 'current' value of the div var doesn't get embedded into the function's definition. It'll only be resolved when the function actually executes, by which time it might have been set to some completely other div, or been reset to null as the parent function's scope has been destroyed.
You could store the div value as a data attribute on the xhr object, which you can then retrieve from within the callback:
xhr.data('thediv', div);
xhr.onreadystatechange = function () {
if (xhr.readystate == 4) {
div = xhr.data('thediv');
etc....

Ok, you've got a few globals going on that you don't want. Rule of thumb: unless you need to access a variable outside of a function, place var in front of it. Otherwise you'll have data clobbering itself all over the place:
// changed the name to `d` because div seems to already be a global var.
function getLinks(url, d) {
// make xhr a local variable so it won't get re-written.
var request = new XMLHttpRequest();
request.open('POST', 'ebook_catg.php', true);
request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
url = encodeURIComponent(url)
var post = "url=" + url;
request.onreadystatechange = function () {
// when the request was global, this would be false until the last
// request completed
if (request.readyState == 4) {
// since d only exists as a parameter to getLinks, this should
// already be bound when the onreadystatechange is created.
d.innerHTML = request.responseText;
d.style.display = "block";
}
}
request.send(post);
}
So, why did I just do such strange, strange things? Well, it looks like div was being assigned as a global variable and while JS should always look to function parameter name for binding, we want to eliminate all possible problems. So I changed the name of that variable. Then I set xhr to reflect a local variable with the var keyword. I also changed the name to request. Once again, it shouldn't matter -- var means that the variable will be bound to that scope, but the change is harmless and since I don't know what else you have, I decided to remove ambiguities. If it does not help JS, it will at least help the reader.
NOTE:
The important part of the above answer is var in front of request.

here i am answering my question.The following code works,i mean the response from each post is appended to the corresponding div element.
var xhr=new Array();
window.onload=function() {
var categories=document.getElementById('categories').children;
for(i=0;i<categories.length;i++)
{
var link=categories[i].children[1].children[0].attributes['href'].nodeValue;
var div=document.createElement('div');
div.className="books";
div.style.display="none";
categories[i].appendChild(div);
getLinks(link,div,i);
}
}
function getLinks(url,div,i)
{
xhr[i]=new XMLHttpRequest();
xhr[i].open('POST','ebook_catg.php',true);
xhr[i].setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
url=encodeURIComponent(url)
var post="url="+url;
xhr[i].node=div;
xhr[i].onreadystatechange=function() {
if(xhr[i].readyState==4)
{
xhr[i].node.innerHTML=xhr[i].responseText;
xhr[i].node.style.display="block";
}
}
xhr[i].send(post);
}
i am not marking it as accepted because i still dont understand why i need to use an array of xhr since a local xhr object should be enough because each time the onreadystate function executes it has the reference of the xhr object. Now since javascript functions are also objects therefore every instance of onreadystate function should have its own reference of xhr object and therefore i shouldnt need to create an array of xhrs.
please correct me if i am wrong here