JSON error lookup

Getting the text your code parsed

Many JSON errors are easy to understand once you see the text that was actually parsed, because it often isn’t what you expected, such as an HTML error page or an empty body. Log the start of it right before the parse call, then paste that here along with the error.

In JavaScript with fetch:

const res = await fetch(url);
const text = await res.text();
console.log(res.status, text.slice(0, 500));
const data = JSON.parse(text);

In Python with requests:

r = requests.get(url)
print(r.status_code, r.text[:500])
data = r.json()

In Go:

body, err := io.ReadAll(resp.Body)
if err != nil {
    return err
}
log.Printf("%d %.500s", resp.StatusCode, body)
err = json.Unmarshal(body, &data)

In PHP:

var_dump(substr($json, 0, 500));
$data = json_decode($json, true, 512, JSON_THROW_ON_ERROR);

The JavaScript, Python and Go versions print the status code too, because a 404, 401 or 500 often explains the error on its own.

Reading the position in the message

Most messages say where the parser stopped, but they don’t all count the same way. Take this JSON, where the comma after 7 is missing:

{"id": 7 "name": "Maya"}

The ones that give a position all point at the " before name, which is where the parser noticed something was wrong. The actual mistake is usually just before that spot. If your editor shows column numbers, the line and column will match it, but Chrome’s “position 9” is the tenth character, since it starts at 0.

The numbers can drift apart when the text has emoji or accented letters in it. Chrome counts each emoji as two characters and Rust counts bytes, so the same spot can get three different numbers. The lookup converts each of them for you.