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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