Developing software in the Real World

Handling JSON data errors in Slim 3

When you send JSON data into a Slim Framework application with a content-type of application/json, then Slim will decode it for you if you use getParsedBody():

Using curl to test:

If there’s an error however, you get this:

If you care about this, you can use json_last_error_msg() and return an error:

(note – in real code, you should check that the Accept header was a JSON one…)

Don’t forget the JSON_PRETTY_PRINT as a human is going to be reading this error, so make it easier for them.

Use jsonlint for more information

If you really want to provide great diagnostics, then use jsonlint:

Update your handler like this:

(lint() will return NULL or a ParsingException, so we don’t need to test for anything else.)

The result looks like this:

This is much more informative!

One thought on “Handling JSON data errors in Slim 3

  1. I think this should be something like this:

    class JsonLintMiddleware {

    public function __invoke(Request $request, Response $response, $next) {
    $input = $request->getBody();
    $parser = new JsonParser();
    $result = $parser->lint($input);
    if ($result instanceof ParsingException) {
    return $response->withJson(
    ['message' => $result->getMessage()],
    $response = $next($request, $response);
    return $response;

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