jesperorb/cors.md

## cors.md

      
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              cors.md
            
          
    Handle CORS Client-side


Cross-origin resource sharing (CORS) is a mechanism that allows restricted resources (e.g. fonts) on a web page to be requested from another domain outside the domain from which the first resource was served. This is set on the server-side and there is nothing you can do from the client-side to change that setting, that is up to the server/API. There are some ways to get around it tho.

Sources : MDN - HTTP Access Control | Wiki - CORS
CORS is set server-side by supplying each request with additional headers which allow requests to be requested outside of the own domain, for example to your localhost. This is primarily set by the header:
Access-Control-Allow-Origin
The header specifies which origins (domains/servers) the information can be accessed from. To enable CORS you usually set it to allow access from all origins with a wildcard (*):
Access-Control-Allow-Origin: *
or you can tell the server to serve content to specific domains, every other domain will be blocked from showing the content in a browser:
Access-Control-Allow-Origin: https://developer.mozilla.org
Bypass client-side for production

Proxy

WARNING: Great services, but you are dependent on these services to work, if they break or go down, so does your app
You can use a service that proxies your request and automatically enable CORS for your:

https://cors-anywhere.herokuapp.com/
More Proxies here

Then you have to call your API by prepending one of these URLs to your request, for example:

https://cors-anywhere.herokuapp.com/http://yourapi.com

JSONP

WARNING: This isn't allowed on every API and may break when calling certain APIS
You can bypass CORS in production using JSONP which stands for JSON with Padding and is kinda also a 'hack'. But it is a widely used hack which many APIs support. You are not sending a pure JSON-request but you are wrapping your data in a function that gets evaluated. JSONP explained in the link below:
JSONP explained in layman terms @ Stack Overflow
jQuery $.ajax() JSONP

The simplest way to handle JSON is through the $.ajax()-function in jQuery as it handles the real dirty parts automatically:
$.ajax({
  method: 'GET',
  url: 'http://localhost:3000',
  dataType: 'jsonp', //change the datatype to 'jsonp' works in most cases
  success: (res) => {
   console.log(res);
  }
})
jQuery: Working with JSONP
fetch-jsonp library

There is no native implementation of JSONP in either XMLHttpRequest or fetch. If you want to use fetch you can use this 1kb library which handle JSONP with fetch:
fetch-jsonp @ GitHub

Link the code in your index.html:

<script src="https://cdnjs.cloudflare.com/ajax/libs/fetch-jsonp/1.0.6/fetch-jsonp.min.js"></script>

And use like fetch but with the function fetchJsonp:

fetchJsonp('http://localhost:3000')
  .then(res => res.json())
  .then(json => console.log(json));
Vanilla JavaScript jsonp

It can be done in regular JavaScript as well of course:

Simple JSONP in vanilla JS

Bypass during development 🔨

WARNING: All users who uses your site must use this hack so this is only intended for bypassing temporarily and for testing during development. You can't assume it will work for your end users. This is just a client-side quick fix and it doesn't change the way the server handles the request. This will make your site work on your computer and every other browser that also has this extension installed.
If you have Chrome you can use a extensions which 'hacks' the response/request. Be sure to disable it when not testing as it can break other sites, GitHub have been known to have problems with this extension if you have it enabled when browsing or using GitHub.
Allow-Control-Allow-Origin: * @ Chrome Web Store