What setCallback is used for in Laravel json response

Question

I am going through a Laravel tutorial on how to build a simple API. But I dont understand what the setCallback() is used for and how?

Eg:

public function index()
{
    $data = Input::get('data');

    If ( $data == ''){
        // query DB get all results
    }else{
        // query DB and get specific result
    }
    return Response::json(array(
            'error' => false,
            'stores' => $data->toArray()),
        200
    )->setCallback(Input::get('callback'));
}

The ->setCallback(Input::get('callback')); part

Answer 1

setCallback is not necessary. It's an optional feature for supporting JSONP requests in an API intended to be used by cross-domain JavaScript.

Answer 2

It's usually used for Jsonp support. For example with request parameter

callback=jsonp_e3bb6459d76a30

->setCallback(Input::get('callback')); -> ->setCallback(jsonp_e3bb6459d76a30);

Then you response

{"result":"","message":"","code":0} will be converted to

/**/jsonp_e3bb6459d76a30({"result":"","message":"","code":0});

It will also change the response Content-type header to text/javascript; charset=UTF-8

---

Further more, you can check the source code for what it really do in the background.

// ...\vendor\symfony\http-foundation\JsonResponse.php

    public function setCallback($callback = null)
    {
        if (null !== $callback) {
            // partially taken from https://geekality.net/2011/08/03/valid-javascript-identifier/
            // partially taken from https://github.com/willdurand/JsonpCallbackValidator
            //      JsonpCallbackValidator is released under the MIT License. See https://github.com/willdurand/JsonpCallbackValidator/blob/v1.1.0/LICENSE for details.
            //      (c) William Durand <[email protected]>
            $pattern = '/^[$_\p{L}][$_\p{L}\p{Mn}\p{Mc}\p{Nd}\p{Pc}\x{200C}\x{200D}]*(?:\[(?:"(?:\\\.|[^"\\\])*"|\'(?:\\\.|[^\'\\\])*\'|\d+)\])*?$/u';
            $reserved = [
                'break', 'do', 'instanceof', 'typeof', 'case', 'else', 'new', 'var', 'catch', 'finally', 'return', 'void', 'continue', 'for', 'switch', 'while',
                'debugger', 'function', 'this', 'with', 'default', 'if', 'throw', 'delete', 'in', 'try', 'class', 'enum', 'extends', 'super',  'const', 'export',
                'import', 'implements', 'let', 'private', 'public', 'yield', 'interface', 'package', 'protected', 'static', 'null', 'true', 'false',
            ];
            $parts = explode('.', $callback);
            foreach ($parts as $part) {
                if (!preg_match($pattern, $part) || \in_array($part, $reserved, true)) {
                    throw new \InvalidArgumentException('The callback name is not valid.');
                }
            }
        }

        $this->callback = $callback;

        return $this->update();
    }


    protected function update()
    {
        if (null !== $this->callback) {
            // Not using application/javascript for compatibility reasons with older browsers.
            $this->headers->set('Content-Type', 'text/javascript');

            return $this->setContent(sprintf('/**/%s(%s);', $this->callback, $this->data));
        }

        // Only set the header when there is none or when it equals 'text/javascript' (from a previous update with callback)
        // in order to not overwrite a custom definition.
        if (!$this->headers->has('Content-Type') || 'text/javascript' === $this->headers->get('Content-Type')) {
            $this->headers->set('Content-Type', 'application/json');
        }

        return $this->setContent($this->data);
    }