how to use cors code example

Example 1: 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);  
  }
}));

Example 2: what is cors

“CORS” stands for Cross-Origin Resource Sharing. 
It allows you to make requests from one website to another website 
in the browser, which is normally prohibited by another browser policy 
called the Same-Origin Policy (SOP).

Example 3: what is CORS

Use CORS to allow cross-origin access. 
CORS is a part of HTTP that lets servers specify any other hosts 
from which a browser should permit loading of content.

How to block cross-origin access
To prevent cross-origin writes, 
	check an unguessable token in the request — known as a Cross-Site Request Forgery (CSRF) token. 
    prevent cross-origin reads of pages that require this token.
To prevent cross-origin reads of a resource, 
	ensure that it is not embeddable.
    prevent embedding because embedding a resource always leaks some information about it.
To prevent cross-origin embeds, 
	ensure that your resource cannot be interpreted
    Browsers may not respect the Content-Type header. 
For example, if you point a <script> tag at an HTML document, the browser will try to parse the HTML as JavaScript. When your resource is not an entry point to your site, you can also use a CSRF token to prevent embedding.