Troy Goode

npm install cors

09 May 2014

CORS is a popular node.js package I've written that provides an Express middleware you can use to enable CORS with various options. The code is open source (MIT License) and available on GitHub.

Installation (via npm)

$ npm install cors

Usage

Simple Usage (Enable All CORS Requests)

var express = require('express')
  , cors = require('cors')
  , app = express();

app.use(cors());
app.use(app.router);

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');
});

Enable CORS for a Single Route

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');
});

Configuring CORS

var express = require('express')
  , cors = require('cors')
  , app = express();

var corsOptions = {
  origin: 'http://example.com'
};

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');
});

Configuring CORS w/ Dynamic Origin

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(null, 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');
});

Enabling CORS Pre-Flight

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

Configuring CORS Asynchronously

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');
});

Configuration Options

  • origin: Configures the Access-Control-Allow-Origin CORS header. Expects a string (ex: "http://example.com"). Set to true to reflect the request origin, as defined by req.header('Origin'). Set to false to disable CORS. Can also be set to a function, which 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-Allow-Max-Age CORS header. Set to an integer to pass the header, otherwise it is omitted.

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

Demo

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: