Preflight request
A preflight request is a browser-generated HTTP OPTIONS request that checks whether a subsequent cross-origin request is permitted.
Building Web APIs with ASP.NET Core
A preflight request is an HTTP OPTIONS request that a browser sends to determine whether a subsequent cross-origin request is permitted.
- How it is used
- Distinguishes the browser’s preliminary CORS check from a request that directly fetches a resource.
- How deep it goes
- Defines when a browser sends HTTP OPTIONS and why, without detailing the requirements for a simple request.
- Examples
- Shows ASP.NET Core CORS policy registration and middleware placement.
- What you can do after
- Operate/configure CORS in ASP.NET Core using a registered policy and middleware, while recognizing why a browser sends a preflight request.
Read how Building Web APIs with ASP.NET Core explains preflight request →
Spring Security in Action, Second Edition
A preflight request is a browser-generated HTTP request that tests whether a cross-origin request is permitted before the browser sends the original request.
- How it is used
- Explains why browsers make cross-origin OPTIONS requests and how those requests determine whether the original request proceeds.
- How deep it goes
- Describes browser responsibility, failure behavior, CORS headers, and MockMvc validation, without implementing browser-side preflight logic.
- Examples
- Uses a cross-origin POST example, browser console output, a request-flow figure, and a MockMvc OPTIONS test.
- What you can do after
- Diagnose/debug CORS behavior by inspecting OPTIONS requests and validating allowed-origin and allowed-method response headers.
Read how Spring Security in Action, Second Edition explains preflight request →
Build Financial Software with Generative AI (From Scratch)
A preflight request is a browser-generated cross-origin HTTP request that asks whether a subsequent request is permitted before sending it.
- How it is used
- Checks cross-origin API permission before the browser sends a request, helping diagnose and configure CORS failures.
- How deep it goes
- Explains the browser check and required response policy, then stops short of detailing the HTTP exchange itself.
- Examples
- Uses localhost dashboard-to-API calls, a missing Access-Control-Allow-Origin error, and FastAPI CORSMiddleware configuration.
- What you can do after
- Configure FastAPI CORSMiddleware with explicit origins, methods, headers, and credential behavior to resolve cross-origin API access.
Read how Build Financial Software with Generative AI (From Scratch) explains preflight request →
Related concepts
- HTTP methoduses · An HTTP method is a standardized operation token in an HTTP request that specifies the intended action on a resource.
- CORSuses · Cross-Origin Resource Sharing (CORS) is a web standard that uses HTTP headers to let servers specify which origins may access their resources through browsers.
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