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why does xmlHttpRequest.Open only work when I input a string?

I'm trying to call a Restful service on my localhost. I am doing it this way because It's an asynchronous call. The appropriate Url plus the Uri-template to call my service is this:
"http://localhost:65016/Service1.svc/SN?lower=200&upper=300"
on the line where I try to open ( xhttp.open ), my client page only receives the proper data whenever I literally insert the url like this:
xhttp.open("GET", "http://localhost:65016/Service1.svc/SN?lower=200&upper=300" , true);
but I need the 200 and 300 numbers to be user input so I tried these two things:
I first tried grabbing the user input and simply concatenating it to the base URL in between the URi template like this:
<script>
function ServiceCall()
{
var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function ()
{
if (xhttp.readyState == 4 && xhttp.status == 200) {
var ans = document.getElementById("secretNum");
ans.innerHTML = xhttp.responseText;
}
}
var base_uri = "http://localhost:65016/Service1.svc/";
// grab the lower number
var ln = document.getElementById("LN").firstChild;
var LN = ln.nodeValue;
// grab upper number
var un = document.getElementById("UN").firstChild;
var UN = un.nodeValue;
//complete
var URL = base_uri + "SN?lower=" + LN + "&upper=" + UN;
xhttp.open("GET", URL, true);
xhttp.setRequestHeader("Content-type", "application/json");
xhttp.send();
}
</script>
Doesn't work. So i tried looking at the documentation for the xmlHttpRequest.open, and I saw that the parameter had to be a URL. so I tried using the URL(string) function and using the output as a parameter and that didn't work either.
Any help please?

Thank you. I it helped to look at the network request. I was simply using the wrong syntax to obtain the value inside of the html input tag.
var ln = document.getElementById("LN").value;
returns the real value inside of html input tag given by the user input.
I'm answering my own question because this is a homework assignment.
(Not that I was cheating. Answering this is far from solving the homework)

I am trying to implement an AJAX call. What am I doing wrong?

This is my HTML text:
<input type="text" class="resizedsearch" name="searchdb">
<button id="submit" onclick="ajaxCall()">Search!</button>
This is Javascript:
ajaxCall()
{
var xmlhttp = new XMLHttpRequest();
var url = "http://localhost:8080/CSE%205335%20Project%20One/userInfo.php";
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState == 4 && xmlhttp.status == 200)
{
myFunction(xmlhttp.responseText);
}
}
xmlhttp.open("GET", url, true);
xmlhttp.send('searchdb');
function myFunction(response)
{
var obj = JSON.parse(response);
document.getElementById("democity").innerHTML =
obj.city;
document.getElementById("demodes").innerHTML =
obj.description;
document.getElementById("latlon").innerHTML =
obj.latitude + "," + obj.longitude;
}
}
And this is where I am trying to display the response that I am receiving from the PHP file:
<b><font size="24" face="Cambria"><p id="democity"></p></font></b>
<font size="6" face="Cambria"><p id="demodes"></p></font>
</br>
The output of the PHP file is stored in $outp and it is in the JSON format.
Any help appreciated. Thank you.
!!UPDATE!!
function ajaxCall()
{
var xmlhttp = new XMLHttpRequest();
var url = "http://localhost:8080/CSE%205335%20Project%20One/userInfo.php";
xmlhttp.onreadystatechange=function()
{
xmlhttp.open("GET", url, true);
xmlhttp.send('searchdb');
if (xmlhttp.readyState == 4 && xmlhttp.status == 200)
{
myFunction(xmlhttp.responseText);
}
}
}
function myFunction(response)
{
var obj = JSON.parse(response);
document.getElementById("democity").innerHTML =
obj.city;
document.getElementById("demodes").innerHTML =
obj.description;
document.getElementById("latlon").innerHTML =
obj.latitude + "," + obj.longitude;
}
This is how the improvised code looks. Still not working.

Example by FactoryAidan is not going to work as it violates Same Origin Policy (unless you'll run the code in browser console on Google page). Try replacing http://www.google.com with your local address. I tested the code with a little modification and it works, or at least gives alert, so the function is called. Here's it is:
function ajaxCall(){
var xmlhttp = new XMLHttpRequest();
var url = "http://localhost:8080"; /* but make sure the url is accessible and of same origin */
xmlhttp.onload=function(){
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
myFunction(xmlhttp.responseText);
}
}
xmlhttp.open("GET", url, true);
xmlhttp.send('searchdb');
}
function myFunction(response){
alert('I made it here');
}

Your code update after my first answer looks like it was done in haste and I think makes the question a little harder. The .open() and .send() methods ended up inside your .onreadystatechange function definition but they need to be outside. Your first one didn't have those placement issues. The code I wrote below has your exact building blocks but with no placement issues so you should be able to follow along with how it matches your example code. I also tested it and it sucessfully sends and receives data back and successfully calls the myFunction() callback function.
Nonetheless, I took your code and rewrote it a bit. If you get an alert('') message when you run it, that means that your xml request worked perfectly. If you don't see an alert('') message. It means your xml request is returning a http 404 error, which means your request URL is bad. Try changing your request URL to something you know won't give you a 404 error, like 'http://www.google.com'. If it works and you get the alert message, then the only problem is that your localhost:8080 url is a bad url.
Also, in your myFunction callback function, javascript treats line-breaks as the end of a line of code. So you must write assignments that use an '=' sign on the same line with no line-breaks. Due to this javascrit principle, you also don't need a semicolon ';' at the end of a single line like you would in PHP script.
Finally, a big cause of errors can be the JSON.parse() call. The data received MUST be a valid json string. So if the URL you call returns anything other than pure json... your myFunction() callback function will break on the JSON.parse() command.
Lastly, if there is an error in your myFunction() callback function, your browser inspector will not report it in a useful way and will instead throw an error that points to your xmlhttp.onreadystatechange=function(){} as being the culprit because that is where the browser thinks the error resides (being the calling function), even though the real error is in your myFunction() callback function. Using my edit of your ajaxCall(){...} code and with a valid url, you can be positive that the ajax call works and any errors you have are in your myFunction() callback function.
Lastly again, You have to be careful in your callback function because there are so many things that could break it. For example, document.getElementById() will cause an error if no html element exists on your web-page with the id you provided. Also, if the JSON you received back from the ajax call is missing any properties you mentioned like (city or latitude) it is likely that the innerHTML will be set to 'undefined'. But some browsers may treat the missing json properties as an error instead of just saying they are 'undefined' when you try to call them.
function ajaxCall(){
var xmlhttp = new XMLHttpRequest();
var url = "http://www.google.com";
xmlhttp.onreadystatechange=function(){
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
myFunction(xmlhttp.responseText);
}
}
xmlhttp.open("GET", url, true);
xmlhttp.send('searchdb');
}
function myFunction(response){
alert('I made it here')
/*
var obj = JSON.parse(response);
document.getElementById("democity").innerHTML = obj.city
document.getElementById("demodes").innerHTML = obj.description
document.getElementById("latlon").innerHTML = obj.latitude + "," + obj.longitude
*/
}

Reading first line of a text file in javascript

Let's say I have a text file on my web server under /today/changelog-en.txt which stores information about updates to my website. Each section starts with a version number, then a list of the changes.
Because of this, the first line of the file always contains the latest version number, which I'd like to read out using plain JavaScript (no jQuery). Is this possible, and if yes, how?

This should be simple enough using XHR. Something like this would work fine for you:
var XHR = new XMLHttpRequest();
XHR.open("GET", "/today/changelog-en.txt", true);
XHR.send();
XHR.onload = function (){
console.log( XHR.responseText.slice(0, XHR.responseText.indexOf("\n")) );
};

So seeing as the txt file is externally available ie: corresponds to a URL, we can do an XHR/AJAX request to get the data. Note without jQuery, so we'll be writing slightly more verbose vanilla JavaScript.
var xmlHttp;
function GetData( url, callback ) {
xmlHttp = new XMLHttpRequest();
xmlHttp.onreadystatechange = callback;
xmlHttp.open( "GET", url, true );
xmlHttp.send( null );
}
GetData( "/today/changelog-en.txt" , function() {
if ( xmlHttp.readyState == 4 && xmlHttp.status == 200 {
var result = xmlHttp.responseText;
var allLines = result.split("\n");
// do what you want with the result
// ie: split lines and show the first line
var lineOne = allLines[0];
} else {
// handle the error
}
});

How can I retrieve multiple JSON files using JavaScript?

I have a web application that receives json from a server. I was using this code:
var http_request = new XMLHttpRequest();
var url = "url where I have the json"
http_request.onreadystatechange = handle_json;
http_request.open("GET", url, true);
http_request.send(null);
var obj;
function handle_json() {
if (http_request.readyState == 4) {
if (http_request.status == 200) {
var json_data = http_request.responseText;
obj = eval("(" + json_data + ")");
processData(obj);
} else {
alert("A problem ocurred");
}
http_request = null;
}
}
But now I want to receive json from two url's and show the information. How can I do this using JavaScript? I know eval is not the appropiate thing to do but this is just a prototype.
Thank you so much! :)

As others have mentioned, you simply need to make 2 requests. In order to re-use the code you have already written, you could define a function to get json that takes a url argument. Something like this:
function getJson(url, callback){
function handle_json() {
if (http_request.readyState == 4) {
if (http_request.status == 200) {
var json_data = http_request.responseText;
var parser = (JSON && typeof JSON.parse == 'function') ? JSON.parse : eval;
var obj = parser("(" + json_data + ")");
callback(obj);
} else {
alert("A problem ocurred");
}
http_request = null;
}
}
var http_request = new XMLHttpRequest();
http_request.onreadystatechange = handle_json;
http_request.open("GET", url, true);
http_request.send(null);
}
I replaced the call to eval with some logic that will call JSON.parse if it is present, otherwise it will use eval. Using this function would allow you to make multiple requests by calling it multiple times, like so:
getJson("some url", processData);
getJson("some other url", processData");
If you wanted to process data from different urls in different ways, just define another function similar to processData and pass it along instead, like getJson("some crazy url", processCrazyData);
Using a framework like jQuery would reduce the amount of code that you have to write, but this solution should get it done using basic javascript.

The easiest way would be to put it into a function.
function getJson(url) {
//Remove the var url="string" line
//Rest of code
}
function handleJson() {
//Other code
}
Alternatively, you could use jQuery, in which case your code would be:
$.getJSON('url goes in here',function(data){
processData(data);
});
And just use that whenever you want to grab a page.

How to get JSON object in Servlet from JSP?

In JSP page I have written:
var sel = document.getElementById("Wimax");
var ip = sel.options[sel.selectedIndex].value;
var param;
var url = 'ConfigurationServlet?ActionID=Configuration_Physical_Get';
httpRequest = new ActiveXObject("Microsoft.XMLHTTP");
httpRequest.open("POST", url, true);
httpRequest.onreadystatechange = handler(){
if (httpRequest.readyState == 4) {
if (httpRequest.status == 200) {
param = 'ip='+ip;
param += 'mmv='+mmv;
param += "tab="+tab;
}};
httpRequest.send(param);
I want this param variable in my ConfigurationServlet. Can anyone tell me how to get this json object in servlet?
Update: I changed my statements and now it is showing status code as 200.
var index = document.getElementById("Wimax").selectedIndex;
var ip = document.getElementById("Wimax").options[index].text;
httpReq = GetXmlHttpObject();
alert(httpReq);
var param = "ip=" + ip;
param += "&mmv=" + mmv;
param += "&tab=" + tab;
alert("param "+param);
var url="http://localhost:8080/WiMaxNM/ConfigurationServlet?ActionID=Configuration_Physical_Get";
url = url+"?"+param;
httpReq.open("GET",url,true);
alert("httpReq "+httpReq);
httpReq.onreadystatechange = handler;
httpReq.send(null);
But new problem has occured. Control is not at all going to the servlet action ID as specified in url. Please tell me what is wrong here.

The code in the handler will only be invoked AFTER the request is been sent. You need to populate param before this. You would also need to concatentate separate parameters by &.
Thus, e.g.
// ...
httpRequest.onreadystatechange = handler() {
// Write code here which should be executed when the request state has changed.
if (httpRequest.readyState == 4) {
// Write code here which should be executed when the request is completed.
if (httpRequest.status == 200) {
// Write code here which should be executed when the request is succesful.
}
}
};
param = 'ip=' + ip;
param += '&mmv=' + mmv;
param += "&tab=" + tab;
httpRequest.send(param);
Then you can access them in the servlet the usual HttpServletRequest#getParameter() way.
That said, the Ajax code you posted there will only work in Microsoft Internet Explorer, not in all the four other major webbrowsers the world is aware of. In other words, your Javascript code won't work for about half of the people in the world.
I suggest to have a look at jQuery to lessen all the verbose work and bridge the crossbrowser compatibility pains. All your code could be easily replaced by
var params = {
ip: $("Wimax").val();
mmv: mmv,
tab: tab
};
$.post('ConfigurationServlet?ActionID=Configuration_Physical_Get', params);
And still work in all webbrowsers!
Update: as per your update, the final URL is plain wrong. The ? denotes a start of the query string. You already have one in your URL. You should use & to chain parameters in the query string. I.e.
url = url + "&" + param;