This means you could have a purely isolated front end running at, and an isolated back end running at xyz.com. And this problem has a number of use cases.įirst, you could have a decoupled client-server architecture for your web application. So now the problem is that your applications need resources from providers of different origins. So if your web app is running at, and you try to request a resource located at xyz.com, the browser (where your is running and making requests out to xyz.com) will kick in its same-origin policy. The domain name and port number together represent the part of the URL that's visible to you or other users. The scheme represents the protocol of the URL-for instance if it's HTTP or HTTPS. Origin typically constitutes a combination of the domain name, port number, and scheme. This policy states that if your application that's running on a browser requests a resource from a server that has a different origin than your application, the browser will block that request by default. This is why browsers by default impose the same-origin policy on all client applications. Their existence has shaped the way web applications work, especially the security aspect of it. Web browsers have been around for decades. That's where some complications arise because there are a lot of things that front-end developers don't have a hundred percent control over. But the client side of a web application always runs on a browser. This allows the front end and back end of an application to communicate and share resources with one another. But all in all, it sends back a response to the client. Now, this action could be anything, from getting data from the database to writing into the database to updating an entry in the database-literally anything. The server receives this request, understands it, and performs an action accordingly. So the client, or the front end of an application, makes an HTTP request to the server. Modern web applications typically follow the client-server architecture. Resource Sharing Between Client and Server But before we talk about these two things, let's understand the purpose of CORS and why it exists in the first place. When we answer those questions, it becomes really easy to figure out why CORS errors occur and how to fix them. So there are two things we need to focus on to get the hang of CORS: First, who sets this rule? And second, how? It's a rule that determines if a browser is allowed to retrieve some data from a server. CORS stands for cross-origin resource sharing. Let's first understand what CORS actually means. Then I'll walk you through how to enable CORS in your Node.js and TypeScript application. In this post, I'll help you understand what CORS is and why it occurs. Even worse, developers spend hours fixing it, only to realize they've been thinking in the wrong direction. A lot of developers struggle to understand what it means. Or perhaps you've had the front-end folks of your team complain about it a million times!ĬORS is often trickier than it appears. If you've built robust Node.js APIs with TypeScript before, I'm sure you've run into the infamous CORS error at least once.
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