Javascript : callback is not a function - javascript

I am using vanilla javascript to making AJAX call to my APIs.
Below is my code which i am calling on click of a simple button:
getUserByUserId : function (callback){
var userid = localStorage.getItem('userid');
var userApiUrl = "http://174.129.30.174:8080/users/"+userid;
var xmlhttp = micropaywall.getAjaxInstance();
xmlhttp.onreadystatechange = function()
{
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
var response = JSON.parse(xmlhttp.responseText);
callback(xmlhttp.responseText);
console.log(response);
console.log(xmlhttp.responseText);
}
};
xmlhttp.open("GET", userApiUrl, true);
xmlhttp.send(null);
}
But i get "callback is not a function" at line
callback(xmlhttp.responseText);
Below is my callback function :
getUserByUserIdCallback : function(response){
if(response != "")
{
var res = JSON.parse(response);
var arrAppid = res.appIds;
var userid = res._id;
localStorage.setItem('appid', arrAppid[0]);
var appid = localStorage.getItem('appid');
micropaywall.getUserAccountDetails(userid, appid);
}
}
Please correct me where I am making mistake.

If you don't need to change the "callback" function then you can just call it directly:
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
var response = JSON.parse(xmlhttp.responseText);
getUserByUserIdCallback(xmlhttp.responseText);
console.log(response);
console.log(xmlhttp.responseText);
}
You would only need to pass in a callback function if you want different behavior when calling the getUserByUserId function.

Related

I tried to create AJAX function but it shows noting in output

I tried to create AJAX function but it shows noting in output
var ajaxObj = function(url, meth, data = "") {
var x = new XMLHttpRequest();
x.onreadystatechange = function() {
if (x.readyState == 4 && x.status == 200) {
this.responseAjax = this.responseText;
}
}
x.open(meth, url, true);
x.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
x.send(data);
}
function showHint(str) {
var xhttp = new ajaxObj("gethint.php?q=" + str, "GET");
if (str.length == 0) {
document.getElementById("txtHint").innerHTML = "";
return;
}
document.getElementById("txtHint").innerHTML = xhttp.responseAjax;
}
<!DOCTYPE html>
<html>
<body>
<h3> Start typing a name in the input field below :</h3>
<form action="">
First Name : <input type="text" id="txt1" onkeyup="showHint(this.value)">
</form>
<p>Suggestions:
<sapn id="txtHint"></sapn>
</p>
</body>
</html>
I tried to get suggested Names from gethint.php file when user starts typing into text box.
But it seems that responseAjax gets value after showHint() call please help me.
You need to handle the AJAX request asynchronously. In your showHint function when you call
document.getElementById("txtHint").innerHTML = xhttp.responseAjax;
the AJAX call has not yet returned, so the xhttp.responseAjax object is not yet defined. You need to wait to handle the response once it arrives. You can pass a callback function in to the ajaxObj definition, and the object will call that function when it gets its response.
var ajaxObj = function(url, meth, callback, data = "") {
var x = new XMLHttpRequest();
x.onreadystatechange = function() {
if (x.readyState == 4 && x.status == 200) {
// we're ready to handle the response data now
callback(x.responseText);
}
}
x.open(meth, url, true);
x.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
x.send(data);
}
// Callback function to be invoked when AJAX is complete
function fillTxtHint(responseHtml)
{
document.getElementById("txtHint").innerHTML = responseHtml;
}
function showHint(str) {
if (str.length == 0) {
document.getElementById("txtHint").innerHTML = "";
}
// Pass in the callback function to be invoked when AJAX has returned
ajaxObj("gethint.php?q=" + str, "GET", fillTxtHint);
}
this inside onreadystatechange's handler is an instance of XMLHttpRequest so the line this.responseAjax = this.responseText is adding a field to the XMLHttpRequest object and setting its value to another field in the same object. This is completely redundant. In showHint, xhttp is an instance of ajaxObj and there is no responseAjax field ever defined for this object. You can directly set innerHTML of the element that shows suggestion inside the handler of onreadystatechange like the following:
function getSuggestions (meth, data) {
meth = meth.toUpperCase();
var params = "q=" + data;
var url = (meth == "GET") ? "gethint.php?" + params : "gethint.php";
var elem = document.getElementById("txtHint");
if (data.length == 0) {
elem.innerHTML = "";
return;
}
var x = new XMLHttpRequest();
x.onreadystatechange = function() {
if (x.readyState == 4 && x.status == 200) {
elem.innerHTML = this.responseText;
}
}
x.open(meth, url, true);
x.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
if (meth == "GET") {
x.send();
} else {
x.send(params);
}
}
and showHint becomes:
function showHint(str) {
getSuggestions ("GET", str);
}

return value from onreadystatechange with callback [duplicate]

This question already has answers here:
How do I return the response from an asynchronous call?
(41 answers)
Closed 6 years ago.
I'm trying to return a value from an onreadystatechange AJAX call... I found this page : stackoverflow link. I though I had it working but realised that it made no difference to add or remove the fn function. The following code works :
username_is_available();
function username_is_available() {
var username = document.getElementById('username').value;
get_data('username', username, function(returned_value) {
if (returned_value == 'true') {
document.getElementById('username_err').innerHTML = 'Taken';
} else {
document.getElementById('username_err').innerHTML = 'Available';
};
});
}
function get_data(data_type, data, fn) {
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
fn(xmlhttp.responseText);
}
};
xmlhttp.open("GET", "availability.php?" + data_type + "=" + data, true);
xmlhttp.send();
}
It all works fine but that's not my goal, I would like a function username_is_available() that returns true if the username is indeed available.
Instead, here I an action happens (innerHTML is changed). And if I try and do a return in the anonymous function I get the same result as if I had returned it from directly inside the onreadystatechange : var unasigned
Unfortunately, since the process to determine if a username is taken is asynchronous, there is no way to simply return a value of true or false from the function call. What you can do is set up something similar to what you have now (callbacks) using language features specifically designed for this exact purpose.
A Promise is one of these features.
Usage would look something roughly like this:
function username_is_available(username) {
return new Promise(resolve => {
get_data("username", username, resolve);
});
}
function get_data(data_type, data, fn) {
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
fn(xmlhttp.responseText == "true");
}
};
xmlhttp.open("GET", "availability.php?" + data_type + "=" + data, true);
xmlhttp.send();
}
// Usage:
username_is_available("Ivan").then(available => {
let text = available ? "Available" : "Taken";
document.getElementById("username_err").innerHTML = text;
});
This relies on availablity.php returning true and false as text, which is converted to a Boolean before resolve is called.
In the future, when ES7+ async and await directives are available, using the promise will be as simple as this (note the await keyword):
let available = await username_is_available("Ivan");
let text = available ? "Available" : "Taken";
document.getElementById("username_err").innerHTML = text;
Edit: If you can't use ES6 or promises, it's back to good ol' callbacks!
function username_is_available(username, callback) {
get_data("username", username, callback);
}
function get_data(data_type, data, fn) {
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
fn(xmlhttp.responseText == "true");
}
};
xmlhttp.open("GET", "availability.php?" + data_type + "=" + data, true);
xmlhttp.send();
}
// Usage:
username_is_available("Ivan", function(available) {
var text = available ? "Available" : "Taken";
document.getElementById("username_err").innerHTML = text;
});
I've been playing around with observers as an alternative to promises. I set up a an example in plnkr to show how it can work. I think in your case it would look like this:
function Producer() {
this.listeners = [];
}
Producer.prototype.add = function(listener) {
this.listeners.push(listener);
};
Producer.prototype.remove = function(listener) {
var index = this.listeners.indexOf(listener);
this.listeners.splice(index, 1);
};
Producer.prototype.notify = function(message) {
this.listeners.forEach(function(listener) {
listener.update(message);
});
};
var notifier = new Producer;
function get_data(data_type, data) {
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function () {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
notifier.notify(xmlhttp.responseText);
}
};
xmlhttp.open("GET", "availability.php?" + data_type + "=" + data, true);
xmlhttp.send();
}
var username_is_available = {
update: function(returned_value) {
var username = document.getElementById('username').value;
if (returned_value == 'true') {
document.getElementById('username_err').innerHTML = 'Taken';
} else {
document.getElementById('username_err').innerHTML = 'Available';
};
}
}
notifier.add(username_is_available);
get_data("username", username);
Note the Producer code is reusable, you would make a new instance for other ajax/observable requests.

JavaScript var outside function

How do I retrieve returndata variable outside fn() function?
function fn() {
var xmlhttp = new XMLHttpRequest();
var vars = "request=something";
xmlhttp.open("POST", "script.php", true);
xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xmlhttp.onreadystatechange = function() {
if(xmlhttp.readyState == 4 && xmlhttp.status == 200) {
var returndata = xmlhttp.responseText;
}
}
xmlhttp.send(vars);
}
You need to define global variable before function and then store the result into this variable. The way you do it now, is definition of local variable.
var returndata;
function fn() {
var xmlhttp = new XMLHttpRequest();
var vars = "request=something";
xmlhttp.open("POST", "script.php", true);
xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xmlhttp.onreadystatechange = function() {
if(xmlhttp.readyState == 4 && xmlhttp.status == 200) {
returndata = xmlhttp.responseText;
}
}
xmlhttp.send(vars);
}
AJAX requests are asynchronous. You cannot have the pizza before it is baked. In real life you call the pizza company. They bake it and you wait. AJAX is the same. So setting the returndata won't do it all by itself.
function fn() {
var xmlhttp = new XMLHttpRequest();
var vars = "request=something";
xmlhttp.open("POST", "script.php", true);
xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xmlhttp.onreadystatechange = function() {
if(xmlhttp.readyState == 4 && xmlhttp.status == 200) {
var returndata = xmlhttp.responseText;
}
}
xmlhttp.send(vars);
}
The readystate function isn't there for nothing. It waits until the request has been processed. From there on you can go on. Every function/script that is depended on the returned data should be called upon from that function.
Still you can do this:
var returndata; //this will now be a global variable.
function fn() {
var xmlhttp = new XMLHttpRequest();
var vars = "request=something";
xmlhttp.open("POST", "script.php", true);
xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xmlhttp.onreadystatechange = function() {
if(xmlhttp.readyState == 4 && xmlhttp.status == 200) {
var returndata = xmlhttp.responseText;
doSomeThing(); //fire depended value.
}
}
xmlhttp.send(vars);
}
function doSomething()
{
if(returndata)
{
//do Something
}
else
{
alert("Data isn't loaded yet");
}
}

get data from XMLHttprequest

I want to get data in json format.
I have typed this code but it doesn't return anything.
where is the problem in my code?!!
<script language="JavaScript">
var xmlhttp = new XMLHttpRequest();
var url = "http://codeforces.com/api/contest.list?gym=true";
xmlhttp.onreadystatechange = myfunction;
xmlhttp.open("GET", url, true);
xmlhttp.send(null);
function myfunction() {
if (XMLHttp.readyState == 0) {
window.alert("Uninitialized");
}
if (XMLHttp.readyState == 1) {
window.alert("loading");
}
if (XMLHttp.readyState == 2) {
window.alert("loaded");
}
if (XMLHttp.readyState == 3) {
window.alert("waiting");
}
if (XMLHttp.readyState == 4) {
window.alert("completed");
var y = JSON.parse(xmlhttp.responseText);
document.getElementById("id01").innerHTML =y[1].id;
}
}
</script>
in the html code, i have a div with id = "id01"
remember that javascript is case sensitive.
edit it to:
var xmlhttp = new XMLHttpRequest();
var url = "http://codeforces.com/api/contest.list?gym=true";
xmlhttp.onreadystatechange = myfunction;
xmlhttp.open("GET", url, true);
xmlhttp.send(null);
function myfunction() {
if (xmlhttp.readyState == 0) {
window.alert("Uninitialized");
}
if (xmlhttp.readyState == 1) {
window.alert("loading");
}
if (xmlhttp.readyState == 2) {
window.alert("loaded");
}
if (xmlhttp.readyState == 3) {
window.alert("waiting");
}
if (xmlhttp.readyState == 4) {
window.alert("completed");
var y = JSON.parse(xmlhttp.responseText);
document.getElementById("id01").innerHTML =y[1].id;
}
}
try this:
xmlhttp.onload = function() {
if (xmlhttp.status >= 200 && xmlhttp.status < 400) {
// Success!
var data = JSON.parse(xmlhttp.responseText);
} else {
// We reached our target server, but it returned an error
}
};
disclaimer: i took this code from http://youmightnotneedjquery.com/#json
Just use fetch. It is the modern XMLHttpRequest.
const url = "http://codeforces.com/api/contest.list?gym=true";
fetch(url)
.then(
response => response.json() // .text(), etc.
// same as function(response) {return response.json();}
).then(
jsonString => {
const json = JSON.parse(jsonString);
document.getElementById("id01").innerHTML = json[1].id;
}
);
More Info:
Mozilla Documentation
Can I Use (75% Aug 2017)
Matt Walsh Tutorial

How will I pass a refrence of a variable, to the second funciton once the first function has called the second function inside of the first function?

In the change_text() function I am trying to pass it the xmhttp.responsetext variable.. but how will I do this? As I see no way yet on how to pass it?
<script type="text/javascript">
function ajax(url, func) {
this.url = url;
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
if(xmlhttp.readyState == 2 && xmlhttp.status == 200) {
this.func = func;
}
}
xmlhttp.open();
xmlhttp.send()
}
function change_text() {
target = document.getElementById("x");
target.innerHTML = xmlhttp.responseText;
}
ajax("url.php", change_text);
</script>
Actually, to do this you don't want to use this at all, since you are not creating any instances of an object and it doesn't act like a constructor.
You can just:
function ajax(url, func) {
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
if(xmlhttp.readyState == 4 && xmlhttp.status == 200) {
// note: loaded is 4 ^
func(xmlhttp.responseText);
}
}
xmlhttp.open('GET', url, true);
// ^~~ you also forgot to pass parameters here
xmlhttp.send('');
}
That would make the ajax function work.
For your change_text function, it's not in the same function as ajax, so it does not have access to the xmlhttp variable. However, the ajax function passed the responseText to it, so just make your function receive it:
function change_text(responseText) {
var target = document.getElementById("x");
// ^~~ don't forget to declare local variables with `var`.
target.innerHTML = responseText;
}
For a working example, see the jsFiddle.
Change this.func = func; to this.func = function () { func(xmlhttp.responseText); };
<script type="text/javascript">
function ajax(url, func) {
this.url = url;
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
if(xmlhttp.readyState == 2 && xmlhttp.status == 200) {
func(xmlhttp);
}
}
xmlhttp.open();
xmlhttp.send()
}
function change_text(xmlhttp) {
target = document.getElementById("x");
target.innerHTML = xmlhttp.responseText;
}
ajax("url.php", change_text);
</script>
You have to call your callback with parameters. Also note that this.url = url just sets window.url = url which is probably a bug.
You probably want xmlhttp.open("POST", url) instead.
Mike Samuel is almost correct, but change_text is missing an argument, try this:
<script type="text/javascript">
function ajax(url, func) {
this.url = url;
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
//did you mean: func(xmlhttp.responseText);
if(xmlhttp.readyState == 2 && xmlhttp.status == 200) {
this.func = function() {
func(xmlhttp.responseText);
};
}
}
xmlhttp.open();
xmlhttp.send()
}
function change_text(responseText) {
target = document.getElementById("x");
target.innerHTML = responseText;
}
ajax("url.php", change_text);
</script>

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