


How Can I Bypass 'No Access-Control-Allow-Origin' Errors When Using Fetch?
Working with CORS in Fetch Requests
While accessing cross-origin resources using fetch, it's common to encounter the "No access-control-allow-origin" error. This prevents client-side JavaScript from accessing responses due to cross-origin restrictions.
Passing { mode: 'no-cors' } to Fetch
Contrary to the expectation, { mode: 'no-cors' } won't alleviate the issue. Instead, it strictly blocks JavaScript access to response body and header contents.
The Solution: CORS Proxy
To overcome this, a CORS proxy can be employed. A proxy sits between the client and the target website. It takes the request, forwards it to the target site, and receives the response. Crucially, the proxy adds the 'Access-Control-Allow-Origin' response header, which allows the client code to access the response.
Why Postman Can Access the Endpoint
While Postman allows access to the endpoint without an 'Access-Control-Allow-Origin' header, web browsers impose cross-origin restrictions. This header is mandatory for client-side JavaScript to interact with the response.
Misconception about Disabling CORS
When aiming to "disable CORS," what is actually intended is disabling the same-origin policy. CORS, in fact, provides a way to loosen this policy by allowing certain cross-origin access.
When to Use { mode: 'no-cors' }
{ mode: 'no-cors' } should only be considered in specific scenarios:
- Embedding content into HTML elements using JavaScript, but not directly through their attributes.
- Caching a resource using Service Workers and the Cache Storage API.
However, even in these cases, there are limitations and important factors to consider.
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