The London Perl and Raku Workshop takes place on 26th Oct 2024. If your company depends on Perl, please consider sponsoring and/or attending.


Catalyst::View::JSON - JSON view for your data


  # lib/MyApp/View/
  package MyApp::View::JSON;
  use base qw( Catalyst::View::JSON );

  # configure in lib/
      'View::JSON' => {
          allow_callback  => 1,    # defaults to 0
          callback_param  => 'cb', # defaults to 'callback'
          expose_stash    => [ qw(foo bar) ], # defaults to everything

  sub hello : Local {
      my($self, $c) = @_;
      $c->stash->{message} = 'Hello World!';


Catalyst::View::JSON is a Catalyst View handler that returns stash data in JSON format.



Flag to allow callbacks by adding callback=function. Defaults to 0 (doesn't allow callbacks). See "CALLBACKS" for details.


Name of URI parameter to specify JSON callback function name. Defaults to callback. Only effective when allow_callback is turned on.


Scalar, List or regular expression object, to specify which stash keys are exposed as a JSON response. Defaults to everything. Examples configuration:

  # use 'json_data' value as a data to return
  expose_stash => 'json_data',

  # only exposes keys 'foo' and 'bar'
  expose_stash => [ qw( foo bar ) ],

  # only exposes keys that matches with /^json_/
  expose_stash => qr/^json_/,

Suppose you have data structure of the following.

  $c->stash->{foo} = [ 1, 2 ];
  $c->stash->{bar} = 2;

By default, this view will return:


When you set expose_stash => [ 'foo' ], it'll return


and in the case of expose_stash => 'foo', it'll just return


instead of the whole object (hashref in perl). This option will be useful when you share the method with different views (e.g. TT) and don't want to expose non-irrelevant stash variables as in JSON.

  no_x_json_header: 1

By default this plugin sets X-JSON header if the requested client is a Prototype.js with X-JSON support. By setting 1, you can opt-out this behavior so that you can do eval() by your own. Defaults to 0.


An optional hashref that supplies arguments to JSON::MaybeXS used when creating a new object.


If versions of this view older than 0.36, there was some code that added a UTF-8 BOM marker to the end of the JSON string when the user agent was Safari. After looking at a lot of existing code I don't think this is needed anymore so we removed it by default. However if this turns out to be a problem you can re enable it by setting this attribute to true. Possible a breaking change so we offer this workaround.

You may also override the method 'user_agent_bom_test' which received the current request user agent string to try and better determine if this is needed. Patches for this welcomed.



Standard target of $c->forward used to prepare a response


The methods accepts either of the following argument signatures in order to promote compatibility with the semi standard render method as define in numerous Catalyst views on CPAN:

    my $json_string = $c->view('JSON')->render($c, undef, $data);
    my $json_string = $c->view('JSON')->render($c, $data);

Given '$data' returns the JSON serialized version, or throws and error.


By default it uses JSON::MaybeXS::encode_json to serialize perl data structure into JSON data format. If you want to avoid this and encode with your own encoder (like passing different options to JSON::MaybeXS etc.), you can implement the encode_json method in your View class.

  package MyApp::View::JSON;
  use base qw( Catalyst::View::JSON );

  use JSON::MaybeXS ();

  sub encode_json {
      my($self, $c, $data) = @_;
      my $encoder = JSON::MaybeXS->new->(ascii => 1, pretty => 1, allow_nonref => 1);



NOTE Starting in release v5.90080 Catalyst encodes all text like body returns as UTF8. It however ignores content types like application/json and assumes that a correct JSON serializer is doing what it is supposed to do, which is encode UTF8 automatically. In general this is what this view does so you shoulding need to mess with the encoding flag here unless you have some odd case.

Also, the comment about regard 'browser gotcha's' was written a number of years ago and I can't say one way or another if those gotchas continue to be common in the wild.

NOTE Setting this configuation has no bearing on how the actual serialized string is encoded. This ONLY sets the content type header in your response. By default we set the 'utf8' flag on JSON::MaybeXS so that the string generated and set to your response body is proper UTF8 octets that can be transmitted over HTTP. If you are planning to do some alternative encoding you should turn off this default via the json_encoder_args:

      json_encoder_args => +{utf8=>0} );

NOTE In 2015 the use of UTF8 as encoding is widely standard so it is very likely you should need to do nothing to get the correct encoding. The following documentation will remain for historical value and backcompat needs.

Due to the browser gotchas like those of Safari and Opera, sometimes you have to specify a valid charset value in the response's Content-Type header, e.g. text/javascript; charset=utf-8.

Catalyst::View::JSON comes with the configuration variable encoding which defaults to utf-8. You can change it via YourApp->config or even runtime, using component.


This assumes you set your stash data in raw euc-jp bytes, or Unicode flagged variable. In case of Unicode flagged variable, Catalyst::View::JSON automatically encodes the data into your encoding value (euc-jp in this case) before emitting the data to the browser.

Another option would be to use JavaScript-UCS as an encoding (and pass Unicode flagged string to the stash). That way all non-ASCII characters in the output JSON will be automatically encoded to JavaScript Unicode encoding like \uXXXX. You have to install Encode::JavaScript::UCS to use the encoding.


By default it returns raw JSON data so your JavaScript app can deal with using XMLHttpRequest calls. Adding callbacks (JSONP) to the API gives more flexibility to the end users of the API: overcome the cross-domain restrictions of XMLHttpRequest. It can be done by appending script node with dynamic DOM manipulation, and associate callback handler to the returned data.

For example, suppose you have the following code.

  sub end : Private {
      my($self, $c) = @_;
      if ($c->req->param('output') eq 'json') {
      } else {

/foo/bar?output=json will just return the data set in $c->stash as JSON format, like:

  { result: "foo", message: "Hello" }

but /foo/bar?output=json&callback=handle_result will give you:

  handle_result({ result: "foo", message: "Hello" });

and you can write a custom handle_result function to handle the returned data asynchronously.

The valid characters you can use in the callback function are


but you can customize the behaviour by overriding the validate_callback_param method in your View::JSON class.

See and for more about JSONP.

NOTE For another way to enable JSONP in your application take a look at Plack::Middleware::JSONP


JSON use is still developing and has not been standardized. This section provides some notes on various libraries.

Dojo Toolkit: Setting's mimetype to 'text/json' in the JavaScript request will instruct to expect JSON data in the response body and auto-eval it. Dojo ignores the server response Content-Type. This works transparently with Catalyst::View::JSON.

Prototype.js: prototype.js will auto-eval JSON data that is returned in the custom X-JSON header. The reason given for this is to allow a separate HTML fragment in the response body, however this of limited use because IE 6 has a max header length that will cause the JSON evaluation to silently fail when reached. The recommend approach is to use Catalyst::View::JSON which will JSON format all the response data and return it in the response body.

In at least prototype 1.5.0 rc0 and above, prototype.js will send the X-Prototype-Version header. If this is encountered, a JavaScript eval will be returned in the X-JSON response header to automatically eval the response body, unless you set no_x_json_header to 1. If your version of prototype does not send this header, you can manually eval the response body using the following JavaScript:

  evalJSON: function(request) {
    try {
      return eval('(' + request.responseText + ')');
    } catch (e) {}
  // elsewhere
  var json = this.evalJSON(request);

NOTE The above comments were written a number of years ago and I would take then with a grain of salt so to speak. For now I will leave them in place but not sure they are meaningful in 2015.


Catalyst::View::JSON makes the data available as a (sort of) JavaScript to the client, so you might want to be careful about the security of your data.

Use callbacks only for public data

When you enable callbacks (JSONP) by setting allow_callback, all your JSON data will be available cross-site. This means embedding private data of logged-in user to JSON is considered bad.

  # MyApp.yaml
    allow_callback: 1

  sub foo : Local {
      my($self, $c) = @_;
      $c->stash->{address} = $c->user->street_address; # BAD

If you want to enable callbacks in a controller (for public API) and disable in another, you need to create two different View classes, like MyApp::View::JSON and MyApp::View::JSONP, because allow_callback is a static configuration of the View::JSON class.

See for more.

Avoid valid cross-site JSON requests

Even if you disable the callbacks, the nature of JavaScript still has a possibility to access private JSON data cross-site, by overriding Array constructor [].

  # MyApp.yaml
    expose_stash: json

  sub foo : Local {
      my($self, $c) = @_;
      $c->stash->{json} = [ $c->user->street_address ]; # BAD

When you return logged-in user's private data to the response JSON, you might want to disable GET requests (because script tag invokes GET requests), or include a random digest string and validate it.

See for more.


Tatsuhiko Miyagawa <>


This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.


Following people has been contributing patches, bug reports and suggestions for the improvement of Catalyst::View::JSON.

  John Wang
  Daisuke Murase
  Jun Kuriyama
  Tomas Doran


Catalyst, JSON::MaybeXS, Encode::JavaScript::UCS