Uncaught SyntaxError: Unexpected token u in json - json

Question

I have read online that the unexpected token issue can come from using JSON.parse().
I am getting this error Uncaught SyntaxError: Unexpected token u
What I am doing wrong?
My code look like this
var t=null;
var xmlhttp=new XMLHttpRequest();
xmlhttp.onreadystatechange=function(responseText)
{
if (xmlhttp.readyState == 4 &&xmlhttp.status==200)
var obj = JSON.parse(xmlhttp.responseText);
var str=JSON.stringify(obj);
var newArr = JSON.parse(str);
var len=newArr.length;
$.mobile.pageContainer.pagecontainer( "change","sales_home.html");
$(document).on('pageshow', "#temp", function (event, data) {
while (len > 0) {
len--;
}
});
}
xmlhttp.onerror=function(E)
{
alert("error"+ E);
}
xmlhttp.open("GET","url",true);
xmlhttp.send();

Answer 1

The problem becomes apparent if you indent the code consistently and correctly:
var t = null;
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function(responseText) {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200)
var obj = JSON.parse(xmlhttp.responseText);
var str = JSON.stringify(obj); // <====
var newArr = JSON.parse(str); // <====
var len = newArr.length;
$.mobile.pageContainer.pagecontainer("change", "sales_home.html");
$(document).on('pageshow', "#temp", function(event, data) {
while (len > 0) {
len--;
}
});
}
xmlhttp.onerror = function(E) {
alert("error" + E);
}
xmlhttp.open("GET", "url", true);
xmlhttp.send();
Note how the code does
var str = JSON.stringify(obj);
var newArr = JSON.parse(str);
no matter what the value of readyState and status are. That causes the error, because obj will be undefined, so JSON.stringify(obj) will return undefined, so JSON.parse will coerce that to the string "undefined", which it then cannot parse, failing on the first character, u.
You probably want to add a block:
var t = null;
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function(responseText) {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) { // Block starts here
var obj = JSON.parse(xmlhttp.responseText);
var str = JSON.stringify(obj);
var newArr = JSON.parse(str);
var len = newArr.length;
$.mobile.pageContainer.pagecontainer("change", "sales_home.html");
$(document).on('pageshow', "#temp", function(event, data) {
while (len > 0) {
len--;
}
});
} // Block ends here
}
xmlhttp.onerror = function(E) {
alert("error" + E);
}
xmlhttp.open("GET", "url", true);
xmlhttp.send();
Not quite following why you're parsing, then stringifying, then parsing again though... Or why you have an empty while loop...