Error "SyntaxError: "[object Object]" is not valid JSON"

Question

I have a problem with the following code. The console.log output is:

My requested URL via a JavaScript Ajax request is the "login.php":

 <?php include('init.php');
    use Login\LoginService;

    #include(__DIR__.'/Login/LoginService.php');

    global $pdo;
    session_start();

    $username = $_POST['username'];
    $pass = $_POST['password'];
    if (!empty($username)) {
        $test = new LoginService();
        $user = $test->getUsersLogin($username);
        if (!empty($user) && $user[0]['login'] == $username) {
            $json = json_encode(array("success" => 1));
            echo $json;
        } else {
            $json = json_encode(array("success" => 0));
            echo $json;
        }
    }
    ?>

My Ajax request via JavaScript:

$(() => {
    $('.login-form').on('submit', function (e) {
        e.preventDefault();

        $.ajax({
            type: "POST",
            dataType: "json",
            timeout: 500,
            url: '/src/login.php',
            data: $(this).serialize(),

            success: (data) => {
                try {
                    var jso = JSON.parse(data);
                    console.log(jso);
                } catch (e) {
                    console.log(e);
                    return false;
                }
            },
            error: (data) => {
                console.log(JSON.parse(data));
            }
        });
    });
});

Why is the response from the PHP {"success":1} not right? What is the problem?

SyntaxError: "[object Object]" is not valid JSON

You're not sending the form data => !empty($username) fails -=> PHP sends back an empty response => Unexpected end of JSON input — user5734311
– user5734311, Commented Sep 1, 2022 at 9:49
Whe I add the dataType: 'json' to my javascript. I become the following error: "[object Object]" is not valid JSON — Jonathan Fuchs
– Jonathan Fuchs, Commented Sep 1, 2022 at 9:51
@JonathanFuchs Because then data has already been parsed as JSON, so parsing an already parsed object will fail. — Ivar
– Ivar, Commented Sep 1, 2022 at 9:53
I add the data tag and the response is following: {"success":1} But the error: SyntaxError: "[object Object]" is not valid JSON — Jonathan Fuchs
– Jonathan Fuchs, Commented Sep 1, 2022 at 9:54
Useful tip: know the difference between JSON (a string) and a Javascript Object - specifically how they appear in the browser console when debugging. Then you can console.log(data) and you'll know immediately that it's already an object. — fdomn-m
– fdomn-m, Commented Sep 1, 2022 at 10:04

ADyson · Accepted Answer · 2022-09-01 10:01:17Z

39

If you write dataType: "json" then jQuery will automatically parse your response as JSON before it comes to the "success" function. This is described in the jQuery $.ajax documentation.

Therefore, data is already an object. You cannot pass an object into JSON.parse() - it requires a string.

Instead of

var jso = JSON.parse(data); console.log(jso);

you can just write

console.log(data);

answered Sep 1, 2022 at 10:01

ADyson

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2 Comments

user1579234 Dec 15, 2024 at 20:09

This worked for me. Content-Type of my Post request is: application/json console.log(JSON.parse(JSON.stringify(req.body)));

ADyson Dec 15, 2024 at 20:30

@user1579234 if you're going to write console.log(JSON.parse(JSON.stringify(req.body)));, then you could have written console.log(req.body); and got the same result. If you parse a JSON string, and then immediately turn it back into a string again, as your example does, then you're simply ending up back exactly where you started from. So in your case, req.body must be a JSON string when it arrives in your code. This answer is about what happens when jQuery automatically parses a JSON string into an object before it's passed to your code.

João Marcos Santos Teixeira · Accepted Answer · 2023-07-05 12:12:23Z

0

Try do it to avoid this error:

myFunction(data: string) {
  try {
    JSON.parse(data); 
    console.log(data);
  }
   catch (e) {
   console.log(e); 
  }
}

answered Jul 5, 2023 at 12:12

João Marcos Santos Teixeira

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