app.options('*', cors()) code example

Example 1: access-control-allow-origin nodejs express

// Add headers
app.use(function (req, res, next) {

    // Website you wish to allow to connect
    res.setHeader('Access-Control-Allow-Origin', 'http://localhost:8888');

    // Request methods you wish to allow
    res.setHeader('Access-Control-Allow-Methods', 'GET, POST, OPTIONS, PUT, PATCH, DELETE');

    // Request headers you wish to allow
    res.setHeader('Access-Control-Allow-Headers', 'X-Requested-With,content-type');

    // Set to true if you need the website to include cookies in the requests sent
    // to the API (e.g. in case you use sessions)
    res.setHeader('Access-Control-Allow-Credentials', true);

    // Pass to next layer of middleware
    next();
});

Example 2: cors express

var allowedOrigins = ['http://localhost:3000',
                      'http://yourapp.com'];
app.use(cors({  
  origin: function(origin, callback){
    // allow requests with no origin     
    // (like mobile apps or curl requests)    
    if(!origin) 
      return callback(null, true);    
    if(allowedOrigins.indexOf(origin) === -1){
      var msg = 'The CORS policy for this site does not ' +                
          'allow access from the specified Origin.';      
      return callback(new Error(msg), false);    
    }    
    return callback(null, true);  
  }
}));