Add What Counts As Cross-Origin With CORS? as an HTTP til

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_989 TILs and counting..._
+_990 TILs and counting..._
 
 ---
 
@@ -27,6 +27,7 @@ _989 TILs and counting..._
 * [Git](#git)
 * [Go](#go)
 * [HTML](#html)
+* [HTTP](#http)
 * [Internet](#internet)
 * [JavaScript](#javascript)
 * [jq](#jq)
@@ -300,6 +301,10 @@ _989 TILs and counting..._
 - [Render Text As Superscript](html/render-text-as-superscript.md)
 - [Submit A Form With A Button Outside The Form](html/submit-a-form-with-a-button-outside-the-form.md)
 
+### HTTP
+
+- [What Counts As Cross-Origin With CORS?](http/what-counts-as-cross-origin-with-cors.md)
+
 ### Internet
 
 - [Add Emoji To GitHub Repository Description](internet/add-emoji-to-github-repository-description.md)

http/what-counts-as-cross-origin-with-cors.md

@@ -0,0 +1,22 @@
+# What Counts As Cross-Origin With CORS?
+
+When it comes to HTTP, an
+[origin](https://developer.mozilla.org/en-US/docs/Glossary/origin) is defined
+by several different aspects of the URL. This is important for understanding
+what qualifies as _same_ and _cross_-origin when dealing with
+[CORS](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS) (Cross-Origin
+Resource Sharing).
+
+For something to be _same-origin_, it must have the same scheme (HTTP/HTTPS),
+the same host, and the same port. If any one of the scheme, host (including
+subdomains), or port is different, then it is not _same-origin_.
+
+Here are some examples of different origins:
+
+- `https://example.com` vs `http://example.com` (different scheme)
+- `https://example.com` vs `https://sub.example.com` (different host)
+- `https://example.com:3000` vs `https://example.com:5000` (different port)
+
+As long as the scheme, host, and port match, they are the same origin. The path
+(everything following the origin) doesn't factor into the question of same
+origin.