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
• Usage
  • Simple Usage
  • Enable CORS for a Single Route
  • Configuring CORS
  • Configuring CORS w/ Dynamic Origin
  • Enabling CORS Pre-Flight
  • Customizing CORS Settings Dynamically per Request
• Configuration Options
• Demo
• License
• Author

Installation

This is a Node.js module available through the npm registry. Installation is done using the npm install command:

$ npm install cors

Usage

Simple Usage (Enable All CORS Requests)

var express = require('express')
var cors = require('cors')
var 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')
})

Enable CORS for a Single Route

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

app.get('/products/:id', cors(), function (req, res, next) {
  res.json({msg: 'This is CORS-enabled for a Single Route'})
})

app.listen(80, function () {
  console.log('CORS-enabled web server listening on port 80')
})

Configuring CORS

See the configuration options for details.

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

var corsOptions = {
  origin: 'http://example.com',
  optionsSuccessStatus: 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')
})

Configuring CORS w/ Dynamic Origin

This module supports validating the origin dynamically using a function provided to the origin option. This function will be passed a string that is the origin (or undefined if the request has no origin), and a callback with the signature callback(error, origin).

The origin argument to the callback can be any value allowed for the origin option of the middleware, except a function. See the configuration options section for more information on all the possible value types.

This function is designed to allow the dynamic loading of allowed origin(s) from a backing datasource, like a database.

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

var corsOptions = {
  origin: function (origin, callback) {
    // db.loadOrigins is an example call to load
    // a list of origins from a backing database
    db.loadOrigins(function (error, origins) {
      callback(error, origins)
    })
  }
}

app.get('/products/:id', cors(corsOptions), function (req, res, next) {
  res.json({msg: 'This is CORS-enabled for an allowed 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')
var cors = require('cors')
var 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

NOTE: When using this middleware as an application level middleware (for example, app.use(cors())), pre-flight requests are already handled for all routes.

Customizing CORS Settings Dynamically per Request

For APIs that require different CORS configurations for specific routes or requests, you can dynamically generate CORS options based on the incoming request. The cors middleware allows you to achieve this by passing a function instead of static options. This function is called for each incoming request and must use the callback pattern to return the appropriate CORS options.

The function accepts:

1. req:

   • The incoming request object.

2. callback(error, corsOptions):

   • A function used to return the computed CORS options.
   • Arguments:
     • error: Pass null if there’s no error, or an error object to indicate a failure.
     • corsOptions: An object specifying the CORS policy for the current request.

Here’s an example that handles both public routes and restricted, credential-sensitive routes:

var dynamicCorsOptions = function(req, callback) {
  var corsOptions;
  if (req.path.startsWith('/auth/connect/')) {
    corsOptions = {
      origin: 'http://mydomain.com', // Allow only a specific origin
      credentials: true,            // Enable cookies and credentials
    };
  } else {
    corsOptions = { origin: '*' };   // Allow all origins for other routes
  }
  callback(null, corsOptions);
};

app.use(cors(dynamicCorsOptions));

app.get('/auth/connect/twitter', function (req, res) {
  res.send('CORS dynamically applied for Twitter authentication.');
});

app.get('/public', function (req, res) {
  res.send('Public data with open CORS.');
});

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. 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 (called as callback(err, origin), where origin is a non-function value of the origin option) 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.
• optionsSuccessStatus: Provides a status code to use for successful OPTIONS requests, since some legacy browsers (IE11, various SmartTVs) choke on 204.

The default configuration is the equivalent of:

{
  "origin": "*",
  "methods": "GET,HEAD,PUT,PATCH,POST,DELETE",
  "preflightContinue": false,
  "optionsSuccessStatus": 204
}

For details on the effect of each CORS header, read this article on web.dev.

Demo

A demo that illustrates CORS working (and not working) using React is available here: https://node-cors-client.netlify.com

Code for that demo can be found here:

• Client: https://github.com/troygoode/node-cors-client
• Server: https://github.com/troygoode/node-cors-server

License

MIT License

Author

Troy Goode (troygoode@gmail.com)