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cors

middleware for dynamically or statically enabling CORS in express/connect applications

cors

CORS is a node.js package for providing a Connect/Express middleware that can be used to enable CORS with various options.

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Installation (via npm)

$ npm install cors
var express = require('express')
  , cors = require('cors')
  , app = express();
 
app.use(cors());
 
app.get('/products/:id', function(req, res, next){
  res.json({msg: 'This is CORS-enabled for all origins!'});
});
 
app.listen(80, function(){
  console.log('CORS-enabled web server listening on port 80');
});
var express = require('express')
  , cors = require('cors')
  , app = express();
 
app.get('/products/:id', cors(), function(req, res, next){
  res.json({msg: 'This is CORS-enabled for all origins!'});
});
 
app.listen(80, function(){
  console.log('CORS-enabled web server listening on port 80');
});
var express = require('express')
  , cors = require('cors')
  , app = express();
 
var corsOptions = {
  origin: 'http://example.com',
  optionsSucccessCode: 200 // some legacy browsers (IE11, various SmartTVs) choke on 204 
};
 
app.get('/products/:id', cors(corsOptions), function(req, res, next){
  res.json({msg: 'This is CORS-enabled for only example.com.'});
});
 
app.listen(80, function(){
  console.log('CORS-enabled web server listening on port 80');
});
var express = require('express')
  , cors = require('cors')
  , app = express();
 
var whitelist = ['http://example1.com', 'http://example2.com'];
var corsOptions = {
  origin: function(origin, callback){
    var originIsWhitelisted = whitelist.indexOf(origin) !== -1;
    callback(originIsWhitelisted ? null : 'Bad Request', originIsWhitelisted);
  }
};
 
app.get('/products/:id', cors(corsOptions), function(req, res, next){
  res.json({msg: 'This is CORS-enabled for a whitelisted domain.'});
});
 
app.listen(80, function(){
  console.log('CORS-enabled web server listening on port 80');
});

Certain CORS requests are considered 'complex' and require an initial OPTIONS request (called the "pre-flight request"). An example of a 'complex' CORS request is one that uses an HTTP verb other than GET/HEAD/POST (such as DELETE) or that uses custom headers. To enable pre-flighting, you must add a new OPTIONS handler for the route you want to support:

var express = require('express')
  , cors = require('cors')
  , app = express();
 
app.options('/products/:id', cors()); // enable pre-flight request for DELETE request 
app.del('/products/:id', cors(), function(req, res, next){
  res.json({msg: 'This is CORS-enabled for all origins!'});
});
 
app.listen(80, function(){
  console.log('CORS-enabled web server listening on port 80');
});

You can also enable pre-flight across-the-board like so:

app.options('*', cors()); // include before other routes
var express = require('express')
  , cors = require('cors')
  , app = express();
 
var whitelist = ['http://example1.com', 'http://example2.com'];
var corsOptionsDelegate = function(req, callback){
  var corsOptions;
  if(whitelist.indexOf(req.header('Origin')) !== -1){
    corsOptions = { origin: true }; // reflect (enable) the requested origin in the CORS response 
  }else{
    corsOptions = { origin: false }; // disable CORS for this request 
  }
  callback(null, corsOptions); // callback expects two parameters: error and options 
};
 
app.get('/products/:id', cors(corsOptionsDelegate), function(req, res, next){
  res.json({msg: 'This is CORS-enabled for a whitelisted domain.'});
});
 
app.listen(80, function(){
  console.log('CORS-enabled web server listening on port 80');
});
  • origin: Configures the Access-Control-Allow-Origin CORS header. Possible values:
  • Boolean - set origin to true to reflect the request origin, as defined by req.header('Origin'), or set it to false to disable CORS.
  • String - set origin to a specific origin. For example if you set it to "http://example.com" only requests from "http://example.com" will be allowed.
  • RegExp - set origin to a regular expression pattern which will be used to test the request origin. If it's a match, the request origin will be reflected. For example the pattern /example\.com$/ will reflect any request that is coming from an origin ending with "example.com".
  • Array - set origin to an array of valid origins. Each origin can be a String or a RegExp. For example ["http://example1.com", /\.example2\.com$/] will accept any request from "http://example1.com" or from a subdomain of "example2.com".
  • Function - set origin to a function implementing some custom logic. The function takes the request origin as the first parameter and a callback (which expects the signature err [object], allow [bool]) as the second.
  • methods: Configures the Access-Control-Allow-Methods CORS header. Expects a comma-delimited string (ex: 'GET,PUT,POST') or an array (ex: ['GET', 'PUT', 'POST']).
  • allowedHeaders: Configures the Access-Control-Allow-Headers CORS header. Expects a comma-delimited string (ex: 'Content-Type,Authorization') or an array (ex: ['Content-Type', 'Authorization']). If not specified, defaults to reflecting the headers specified in the request's Access-Control-Request-Headers header.
  • exposedHeaders: Configures the Access-Control-Expose-Headers CORS header. Expects a comma-delimited string (ex: 'Content-Range,X-Content-Range') or an array (ex: ['Content-Range', 'X-Content-Range']). If not specified, no custom headers are exposed.
  • credentials: Configures the Access-Control-Allow-Credentials CORS header. Set to true to pass the header, otherwise it is omitted.
  • maxAge: Configures the Access-Control-Max-Age CORS header. Set to an integer to pass the header, otherwise it is omitted.
  • preflightContinue: Pass the CORS preflight response to the next handler.

The default configuration is the equivalent of:

{
  "origin": "*",
  "methods": "GET,HEAD,PUT,PATCH,POST,DELETE",
  "preflightContinue": false
}

For details on the effect of each CORS header, read this article on HTML5 Rocks.

A demo that illustrates CORS working (and not working) using jQuery is available here: http://node-cors-client.herokuapp.com/

Code for that demo can be found here:

MIT License

Troy Goode (troygoode@gmail.com)