CORS, Cross-origin resource sharing (CORS) is a mechanism that allows restricted resources(e.g. fonts, JavaScript, etc.) on a web page to be requested from another domain outside the domain from which the resource originated.

Sometimes in IONIC if we are sending an AJAX request to server we get an error like this

XMLHttpRequest cannot load http://api.ionic.com/endpoint.
No 'Access-Control-Allow-Origin' header is present on the requested resource.
Origin 'http://localhost:8100' is therefore not allowed access.

To handle such an error we have to enable CORS.

How to handle CORS in ionic

To enable CORS create another parameter inside ionic.project file as follow:

{
  "name": "ProjectName",
  "app_id": "",
  "proxies": [
    {
      "path": "/api",
      "proxyUrl": "http://SERVER-URL/api"
    }
  ]
}

Now where you want to send request on an url send it as

http://localhost:8100/api

It will proxy requests out to http://SERVER_URL/api on your behalf.

Source :- http://blog.ionic.io/handling-cors-issues-in-ionic/