![]() So blocking performed by the browser after reading response headers. No 'Access-Control-Allow-Origin' header is present.: if there are no headers at all: XMLHttpRequest cannot load. Only then browser searches for ACAO a header in the response and if there is no header with current origin it blocks content usage in HTML or JS code and throws an error in the developer's console, e.g.a.com, or wildcard * that will allow all origins. The base header is Access-Control-Allow-Origin ( ACAO). Web-server should always answer with content but can add some extra headers, or may not.The browser asks the web server for resources regardless of the same or different origins are used.It all works in a CONFUSING way: when HTML or JavaScript asks for resource: To allow CORS, web-server, in responses to simple requests should add special HTTP response header that describes what set of origins which are permitted to get this resource. controlCORS#Preflighted_requestsĪll requests that are not simple are non-simple□ CORS on simple requests However, If you are paranoid, and worry about extra cases refer to browser documentation, e.g. So if you write a simple blog and don't see an explanation, just carefully check the rules above. 99% of cases are covered with the rules above. I would say it should never happen to you. NOTE: This is a base rule, but also there might be some rare extra situations when requests are non-simple. You can find their list and allowed values on fetch spec: If you need to set a header by yourself still, and still wish to keep the request simple you are allowed to white-listed request headers and their values, they called CORS-safelisted. All request makes do the same and have very same and simple interfaces. It will save your life from a lot of issues. Never use superagent/axios (or even worsen, ancient and heavy ajax) unless you know why you need it. By the way, the request maker can set it without your agreement, so better start with pure browser-native XHR of fetch API, unless you know why you need more complex requesters. Default headers sent by the browser are OK, we are talking only about headers set by you from your request maker (for example one of XHR/fetch/axios/superagent/jQuery Ajax etc). ☝Another tricky important condition - to be simple requests must have no manually set headers. So multipart/form-data POST is simple, but application/json POST is not simple! Also application/xml POST is not simple! You could check content type in Chrome DevTools -> Network tab, click on request (Make sure no filters are Applied and All badge selected), look at Request headers It is very important to know that CORS works differently on two kinds of requests: simple, and non-simple.Īnd only that of these which have one of the next values in Content-Type request header: ![]() Leter I will show how to implement it, but first, we need to consider more important things. To remove the SOP restriction developers use a special header-based mechanism called Cross-Origin Resource Sharing ( CORS).ĬORS should be implemented on the side of the webserver that serves resources and only there! In our case it is b.com's webserver. Origins are different so the browser would normally drop an exception in console (F12 in Chrome): has been blocked by cors policy. In the examples, a.com is an origin of the page which does request and b.com is an origin of the requested resource. Normally the browser will block the request according to the same-origin policy ( SOP). The page has some JavaScript file that inside of it's code does a REST API call using fetch/XHR/jQuery ajax.The page has a font loaded from CSS src: url("b.com/font.ttf").Imagine font or REST API is located on a domain b.com. Imagine a browser requests a font or calls some REST API by using JavaScript from a page served on a.com. Here I will explain why it happens and how it protects a user. Have you ever seen an error in a browser console: Access to fetch at '' from origin '' has been blocked by CORS You can help by donating to Ukrainian's army. We are uniting against Putin’s invasion and violence, in support of the people in Ukraine. Russians ruthlessly kill all civilians in Ukraine including childs and destroy their cities. Today, 14th May 2023, Ukraine is still bravely fighting for democratic values, human rights and peace in whole world.
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