# 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.

_Sources_ : [MDN - HTTP Access Control](https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS) | [Wiki - CORS](https://en.wikipedia.org/wiki/Cross-origin_resource_sharing)

**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:
```http
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 (*)_:
```http
Access-Control-Allow-Origin: *
```

## Bypass during development :hammer:

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.

[Allow-Control-Allow-Origin: * @ Chrome Web Store](https://chrome.google.com/webstore/detail/allow-control-allow-origi/nlfbmbojpeacfghkpbjhddihlkkiljbi?hl=en)

**All 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 browser-side fix and it doesn't change the way the server handles the request**


## Bypass client-side for production

### Proxy

You can use a service that proxies your request and automatically enable `CORS` for your:
* https://crossorigin.me/ 
* https://cors-anywhere.herokuapp.com/

Then you have to call your API by prepending one of these URLs to your request, for example:
* https://crossorigin.me/http://yourapi.com
* https://cors-anywhere.herokuapp.com/http://yourapi.com
 

### JSONP

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](http://stackoverflow.com/questions/3839966/can-anyone-explain-what-jsonp-is-in-layman-terms)


#### jQuery $.ajax() JSONP

The simplest way to handle JSON is through the `$.ajax()`-function in _jQuery_ as it handles the real dirty parts automatically:

```js
$.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](https://learn.jquery.com/ajax/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](https://github.com/camsong/fetch-jsonp)

1. Link the code in your `index.html`:
```html
<script src="https://cdnjs.cloudflare.com/ajax/libs/fetch-jsonp/1.0.6/fetch-jsonp.min.js"></script>
```

2. And use like fetch but with the function `fetchJsonp`:
```js
fetchJsonp('http://localhost:3000')
  .then(res => res.json())
  .then(json => console.log(json));
```

## Handle CORS via a Node proxy server

We are entering backend territory, be aware. You can redirect the traffic from your choosen API through your own `node`-server and this will allow you to set the headers that control `CORS` yourself. You can set up a `node`-server that makes a request to to the API via the package `request` for you. This is basically what the links further up under [#proxy](Proxy) does.  

Instructions for setting this up yourself are in the repository in the link below.

* [__node-api-proxy__ by _jesperorb_](https://github.com/jesperorb/node-api-proxy)

This will allow you to make your calls to `http://localhost:8000` instead of the API-URL (or to whatever URL you decide to host it on).