Errors / JavaScript errors

Uncaught (in promise) SyntaxError: Unexpected end of JSON input

The page expected data from the server and got an empty answer.

Whatever the data was for does not appear; often a form looks sent and nothing happens.

Why it happens

• response.json() on a response with no body: a 204, a failed request, or an error the server answered with nothing.
• JSON.parse("") on a value read from storage or a data attribute that is empty.
• A response cut short by a timeout or a proxy.

How to fix it

1. Check response.ok and response.status before calling .json(); a 204 has no body by definition.
2. Guard empty strings before JSON.parse, and catch the error where the data is optional.
3. Look at the request in the network tab: its status and size say which case it is.

Whose problem it is

Either the site's own code or a third party's, depending on where it comes from.

Often seen with

• Unexpected token '<'The browser asked for a script and got a web page instead, usually an error page or the home page.
• Uncaught (in promise)Something the page asked for in the background failed, and no code was there to handle it.