cors

What is cors?

The cors npm package is a middleware for enabling Cross-Origin Resource Sharing (CORS) in web applications using Node.js and Express. It allows you to specify which domains can access resources on your server, which HTTP methods are allowed, and other security-related configurations to prevent or allow cross-site requests.

What are cors's main functionalities?

Basic Usage

This code sample demonstrates the basic usage of the cors package where it enables CORS for all routes and origins by default.

const express = require('express');
const cors = require('cors');

const app = express();

app.use(cors());

app.get('/data', (req, res) => {
  res.json({ message: 'This route is CORS-enabled for all origins!' });
});

app.listen(3000, () => {
  console.log('Server running on port 3000');
});

Configuring CORS for Specific Origins

This code sample shows how to configure CORS to allow requests from a specific origin, in this case, 'http://example.com'.

const express = require('express');
const cors = require('cors');

const app = express();

const corsOptions = {
  origin: 'http://example.com',
  optionsSuccessStatus: 200
};

app.use(cors(corsOptions));

app.get('/data', (req, res) => {
  res.json({ message: 'This route is CORS-enabled for http://example.com!' });
});

app.listen(3000, () => {
  console.log('Server running on port 3000');
});

Enabling CORS for Dynamic Origins

This code sample illustrates how to enable CORS based on a dynamic list of origins. The origins are checked against a whitelist, and CORS is configured accordingly.

const express = require('express');
const cors = require('cors');

const app = express();

const whitelist = ['http://example1.com', 'http://example2.com'];
const corsOptionsDelegate = function (req, callback) {
  let corsOptions;
  if (whitelist.indexOf(req.header('Origin')) !== -1) {
    corsOptions = { origin: true };
  } else {
    corsOptions = { origin: false };
  }
  callback(null, corsOptions);
};

app.use(cors(corsOptionsDelegate));

app.get('/data', (req, res) => {
  res.json({ message: 'This route has CORS enabled for specific origins!' });
});

app.listen(3000, () => {
  console.log('Server running on port 3000');
});

Other packages similar to cors

helmet

Helmet is a middleware for Express applications that sets various HTTP headers to help protect your app from some well-known web vulnerabilities. While it does not provide the same CORS functionality out of the box, it contributes to the overall security of the application, similar to how CORS restricts resource sharing for security purposes.

http-proxy-middleware

This package provides a way to create a proxy middleware for Express applications. It can be used to proxy requests to other servers, effectively bypassing CORS restrictions by making it seem like all requests originate from the same domain. It serves a different purpose but can be used to solve similar cross-origin issues in development environments.

README

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 Asynchronously
  • Enabling CORS Pre-Flight
• 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

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

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

var whitelist = ['http://example1.com', 'http://example2.com']
var corsOptions = {
  origin: function (origin, callback) {
    if (whitelist.indexOf(origin) !== -1) {
      callback(null, true)
    } else {
      callback(new Error('Not allowed by CORS'))
    }
  }
}

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')
})

If you do not want to block REST tools or server-to-server requests, add a !origin check in the origin function like so:

var corsOptions = {
  origin: function (origin, callback) {
    if (whitelist.indexOf(origin) !== -1 || !origin) {
      callback(null, true)
    } else {
      callback(new Error('Not allowed by CORS'))
    }
  }
}

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

Configuring CORS Asynchronously

var express = require('express')
var cors = require('cors')
var 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. 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.
• 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 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:

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

License

MIT License

Author

Troy Goode ([e-mail])