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jquery ajax request returns undefined

I am trying to have a link perform a really simple, request, but I can't seem to figure out why what I returned is "undefined".
Currently the link does a manual page request, but it's such a simple subscription thing, I want it to not refresh the page (hence the preventDefault). But if the JS doesn't work or it's blocked, the link should do a normal page request (best of both worlds?).
Here is the JS that captures the click on a link
$('#subscribe-link').click(function(e)
{
e.preventDefault();
var type = $(this).data('sub');
$.post('/includes/ajax/subscribe-article.php', { 'type':type },
function(data)
{
alert(data.result);
if (data.result == 'subscribed')
{
alert('subscribe');
}
else if (data.result == 'unsubscribed')
{
alert('unsubscribe');
}
});
});
And here is the PHP that feeds it:
if($_POST && isset($_SESSION['user_id']) && $_SESSION['user_id'] != 0)
{
if (isset($_POST['type']))
{
if ($_POST['type'] == 'subscribe')
{
echo json_encode(array("result" => "subscribed"));
return;
}
if ($_POST['type'] == 'unsubscribe')
{
echo json_encode(array("result" => "unsubscribed"));
return;
}
}
}
Now, I've checked what "data" returns by itself which is this:
{"result":"unsubscribed"}
Which is correct, I'm not sure what I'm missing this time.

As the variable data contains the JSON representation of your expected result, it is plainly a String. Yet you try and access information contained in that string as an object. For this to work, you first need to create an object from the string by decoding the returned JSON:
var myData = JSON.parse(data);
alert(myData.result);
...

You need to parse the result as JSON. IE, data_array=JSON.parse(data);

Retrieving a JSON.parse() string from a server

I will start by saying that I am learning how to program in jquery/javascript, and am running into an issue using JSON.parse(). I understand the format, and why people use it... but have not been able to get it to work in any of my code projects.
I have read in books/online on here in how to use it, but I think I read too much on it. I am now confused and second guessing what I know about it.
With that said, my jquery/javascript class I am taking is asking me to use it for an assignment, through AJAX using MAMP/localhost as the server.
The two codes below are for the section that I need to fill in the //TODO information. One is javascript (client-side), the other is php (server-side). I think that I've set the other //TODO information correctly, but I keep getting a token error for the JSON part.
I looked on here for a solution, but again, I think I've confused myself badly and need help. Appreciate any feedback, insight, or information.
-Javascript-
var calculateMpg = function () {
// These lines are commented out since the server will perform these checks
// if (!checkNumber("miles") || !checkNumber("gallons")) {
// return;
// }
var miles = $("#miles").val();
var gallons = $("#gallons").val();
console.log("ajax request issued.");
var result;
$.ajax({
url: "service.php?action=calculateMPG&miles="+miles+"&gallons="+gallons,
cache: false,
dataType: "text",
success: function(msg) {
console.log("ajax response received.");
// TODO: parse the JSON string returned from the server (see JSON.parse())
JSON.parse("result");
if (result.status === 'success') {
// TODO: get the mpg value returned from the server and display it to the user.
$("#mpg").val($_GET("result"));
console.log("JSON Working!");
}
else {
// TODO: get the name of the variable with the error. Hint: look at the 'fail' result from service.php
$_GET[fail(id)];
// TODO: report the error to the user using invalidNumber() function.
alert("{status: 'failure', variable: <variable name>}");
}
}
});
};
$(document).ready( function () {
$("#miles").blur(function () {
checkNumber("miles");
});
$("#gallons").blur(function() {
checkNumber("gallons");
});
$("#calculate").click(calculateMpg);
$("#miles").focus();
});
-PHP-
<?php
if ($_GET) {
if ($_GET['action'] == 'calculateMPG') {
$miles = htmlspecialchars($_GET['miles']);
$gallons = htmlspecialchars($_GET['gallons']);
// validate miles
if (strlen($miles) == 0) {
fail("miles");
}
$miles_chars = str_split($miles);
for ($i=0; $i< count($miles_chars); $i++) {
if ($miles_chars[$i] < "0" || $miles_chars[$i] > "9") {
//error_log("miles_chars check failed at: " + $i);
fail("miles");
}
}
// validate gallons
if (strlen($gallons) == 0) {
fail("gallons");
}
$gallons_chars = str_split($gallons);
for ($i=0; $i< count($gallons_chars); $i++) {
if ($gallons_chars[$i] < "0" || $gallons_chars[$i] > "9") {
fail("gallons");
}
}
// validate $miles and $gallons calling $fail along the way
$result = $miles/$gallons;
if ($result) {
success($result);
} else {
fail("mpg");
}
exit ;
}
}
function fail($variable) {
die(json_encode(array('status' => 'fail', 'variable' => $variable)));
}
function success($message) {
die(json_encode(array('status' => 'success', 'message' => $message)));
}
Edited Additional 1
I have made changes to the JSON information in regard to 'var result' (thanks to several of the responses here). I'm starting to understand JSON a bit better.
Another question I have (now) is how to isolate a part of the JSON message from the whole being transmitted?
A piece of the 'JSON.parse(msg)' returned DOES include the answer to the equation miles/gallons, but I don't know how to... extract it from the JSON.
The solution to the equation miles/gallons appears in the 'msg' output.
Thanks.
Edited Additional 2
This question has been solved! While perusing around stackoverflow for a solution to the question in my previous edited section, I found my answer here: JSON response parsing in Javascript to get key/value pair.
The answer is this: under the //TODO section asking for the mpg value, I put the following code - $("#mpg").val(result.message); - which says that in the JSON section of the variable result, take the part of the JSON marked 'message', the value being the equation solution.
Thank you to all who responded with their solutions to my problem. I appreciate the fast responses, the great suggestions, and the information in understanding JSON.
-ECP03

JSON.parse() requires that you send it a valid JSON string.
"result" is not a valid JSON string. In your success function you have defined a parameter msg - what does this contain? Try console.log(msg) at the beginning of your success function and look at the console output.

You have two options:
Option 1: -- Parse the string returned.
Change JSON.parse("result"); to:
var result = JSON.parse( msg );
Option 2: -- Request JSON instead of plain text - no need to parse
Use $.getJSON() which is shorthand for:
$.ajax({
dataType: "json",
url: url,
data: data,
success: success
});

Instead of parsing the JSON yourself, jQuery already provides you with a convenient function that will parse JSON:
var path = "service.php?action=calculateMPG&miles="+miles+"&gallons="+gallons;
$.getJSON(path, function (data) {
if (data.status == 'success') {
console.log('Success! Message:', data.message);
} else {
console.log('Failed :( Variable:', data.variable);
}
});
For your original code, what you would need to do is call JSON.parse(msg) in your success callback, which would return a JavaScript object with the values you sent from your PHP script. By specifying dataType: 'json' in the $.ajax call, jQuery does this for you. The $.getJSON method does this and some other things for you.

You need to use the result returned by the success function:
var result = JSON.parse(msg);
Then, you could do stuff like result.status.
When you put JSON.parse("result") you're saying "parse the string 'result'," which doesn't make any sense. However, if you say JSON.parse(msg) you're saying "Parse the variable that was returned from the ajax action," which makes sense.

JSON.parse() is used to convert your json data to object, then you can manipulate it easly.JSON.parse(msg); instead of JSON.parse("result").
For example:
var json = '{"value1": "img", "value2":"img2"}'
var obj = JSON.parse(json);
for ( k in obj ) {
console.log(obj[k])
}

This is totally wrong: JSON.parse("result");. .parse() expects a JSON string, e.g. the string that came back from you ajax request. You're not providing that string. you're providing the word result, which is NOT valid JSON.
JSON is essentially the right-hand side of an assignment expression.e.g.
var foo = 'bar';
^^^^^---this is json
var baz = 42;
^^---also json
var qux = ['a', 'b', 'c'];
^^^^^^^^^^^^^^^---even more json
var x = 1+2;
^^^---**NOT** json... it's an expression.
What you're doing is basically:
var x = parse;
^^^^^^---unknown/undefined variable: not JSON, it's an expression

Undefined parameter when returning data from jQuery.post to PHP

I am trying to return data from a database and populate a text field after the user enters an ID in the first text box. Currently I had the code working as long as the user did not enter a space in the ID number. Now I am attempting to allow that use case. My PHP code returns a json encoded array with three fields: first_name, last_name, and full_name.
When I use console.log(data) to view the data being returned I receive the following:
{"first_name":"Test","last_name":"Test","full_name":"Test Test"}
However in my code, I try to write data.full_name in a .val() nothing is populated, and when use the console.log I get an error saying "undefined".
Here is the whole jQuery Code:
$("#ID").blur(function () {
var params = {};
params.ID = encodeURIComponent($(this).val());
$.post("getter.php", params, function ( data ) {
if (!data) {
$("input[name=username]").val("User Not Found");
} else {
$("input[name=username]").val(data.full_name);
$("input[name=username]").attr("readonly", true);
}
});
});
Any help you can offer would be much appreciated.

Force jQuery to read the returned data as json:
$("#ID").blur(function () {
var params = {};
params.ID = encodeURIComponent($(this).val());
$.post("getter.php", params, function ( data ) {
if (!data) {
$("input[name=username]").val("User Not Found");
} else {
$("input[name=username]").val(data.full_name);
$("input[name=username]").attr("readonly", true);
}
}, "json"); // <- json forced
});
and then make sure your returned data is in proper json format (for example with json_encode in php)

Use trim() to remove spaces.
Then you can check if the parameter value is_numeric(), and if false, set a default value.

How to put a check on Jquery Data that either the data is in form of JSON or a simple HTML?

I want to check for the JSON Data.
That if I got a Data in form of JSON
Do this code
Else
This Code.
How can i do this ?
Example Variable for Data in Jquery:
data

Use JSON.parse() to check it.
Try:
try
{
var json = JSON.parse(your string);
}
catch(e)
{
//not a json
}
Fiddle here.

add this function to your code:
function isJSON(data) {
try {
JSON.parse(data);
return true;
} catch (ex) {
return false;
}
}
and simply to use it :
if(isJSON(data))
// it's json, do whatever you want
else
// it's not json, do whatever you want

pass a value to a variable in javascript where these values are responses in json

I want to pass a value to a variable in javascript. These values are responses in json.
$.getJSON('url.php?id=login&email='+document.getElementById("txtemail").value+'&password='+document.getElementById("txtpassword").value, function (data)
{
alert(data.status);
alert(data.msg);
alert(data.user_id);
alert(data.session_id);
});
I want to store session_id in a variable and then have to send it to a url. How can i do this?
Thanks for any help...

Do you mean something like this?
var ses_id = data.session_id;
jQuery.post("<url>?session_id="+ses_id);

have a look at http://api.jquery.com/jQuery.post/
you can use console.log to debug your javascript a little bit more efficiently. http://getfirebug.com/logging
compare this to the post of the previous poster:
$.getJSON('url.php', 'id=login&email='+document.getElementById("txtemail").value+'&password='+document.getElementById("txtpassword").value,
function (data) {
console.log(data);
var session_id = data.session_id;
$.post('url2.php', session_id, function() {
alert('success');
});
}
});