CORS for Beginners: Why the Browser Sometimes Blocks Your Request If you are learning frontend development, APIs, or JavaScript, there is a decent chance you have seen an error that looks something like this: "Blocked by CORS policy" "No 'Access-Control-Allow-Origin' header" "Cross-origin request blocked" And yes, the first time you see it, it feels like the browser woke up angry. The good news is that CORS is not random . It is a browser security rule with a specific purpose. In this post, we will break down what CORS is, why it exists, and how to think about it without drowning in jargon. Start with the short version CORS is a browser rule that controls when a webpage is allowed to make requests to a different origin. That raises another beginner question immediately: What is an origin ?…