What Causes the \'Origin is Not Allowed by Access-Control-Allow-Origin\' Error?

Origins of the "origin is not allowed by Access-Control-Allow-Origin" Error

The error "Origin http://localhost:8080 is not allowed by Access-Control-Allow-Origin" arises due to the limitations of Javascript when making requests outside the domain hosting the script. Here are some specific scenarios where this error can occur:

• Requesting from example.com to test.com
• Requesting from example.com to inner.example.com
• Requesting from example.com:80 to example.com:81
• Requesting from example.com to example.com: accepting

Root Cause: Same Origin Policy

Javascript enforces the "same origin policy" to protect against malicious scripts that could transmit sensitive data to a remote server. This policy restricts cross-domain requests, ensuring that scripts can only access data from the same origin as the website they are hosted on.

Solution: JSONP or Server-Side Proxy

One workaround is JSONP, which allows you to load scripts from different domains. However, this requires the server to be configured for JSONP. If not, a server-side proxy (e.g., PHP, ASP) can be used to handle cross-domain requests.

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