How Can I Resolve Cross-Origin Request Errors When Fetching Data Using JavaScript?

Trying to Disable CORS with Fetch and Dealing with 'Access-Control-Allow-Origin'

You've encountered a common challenge when attempting cross-origin requests from your client JavaScript code using Fetch. The error message you received indicates that the endpoint you're trying to fetch from doesn't include the 'Access-Control-Allow-Origin' header, which is required for cross-origin access.

Problem: You're attempting to use the object { mode: 'no-cors' } in Fetch to disable CORS, but this approach is incorrect.

Why 'no-cors' Mode Doesn't Work:

Setting mode: 'no-cors' in Fetch doesn't disable CORS; instead, it tells browsers to prevent your frontend JavaScript from accessing the response body and headers. This setting is usually not desirable, as you typically want your code to be able to access the response.

Solution: CORS Proxy

The solution to this problem lies in using a CORS proxy. A CORS proxy acts as an intermediary between your frontend code and the remote endpoint, enabling cross-origin requests by modifying response headers.

By using a CORS proxy, you can forward your requests through the proxy, which will add the necessary Access-Control-Allow-Origin header to the response. This allows your frontend code to access the response body and headers as if it were coming from the same origin.

Deploying Your Own Proxy

An easy way to deploy your own CORS proxy is to follow these steps:

1. Clone the cors-anywhere GitHub repository (https://github.com/Rob--W/cors-anywhere).
2. Install dependencies using npm install.
3. Create a Heroku account and run heroku create.
4. Push your code to Heroku using git push heroku master.

Once deployed, you'll have a running CORS proxy at a URL like https://cryptic-headland-94862.herokuapp.com.

Prefixing Your Request URL with the Proxy URL

To use the proxy, simply prefix your request URL with the proxy URL. For example, if you want to fetch https://example.com, you would use the following URL:

https://cryptic-headland-94862.herokuapp.com/https://example.com

By prefixing the request URL with the proxy, you'll be able to make cross-origin requests successfully, thanks to the Access-Control-Allow-Origin header added by the proxy.

Additional Considerations:

• Postman can access the endpoint because it's a debugging tool that ignores same-origin restrictions.
• Using mode: 'no-cors' or mode: 'opaque' in Fetch is not the correct approach to disabling CORS.
• CORS is a security measure, and it's important not to bypass it casually. Always consider the security implications before disabling CORS restrictions.

The above is the detailed content of How Can I Resolve Cross-Origin Request Errors When Fetching Data Using JavaScript?. For more information, please follow other related articles on the PHP Chinese website!