A preflight request is a type of HTTP request sent by a web browser to a server before making a main request. It’s used to determine if the server will accept the main request, and to gather information about the server’s capabilities and requirements.
Here’s a breakdown of how preflight requests work:
- Browser Determines Need for Preflight: The browser checks the main request’s HTTP method (e.g., PUT, DELETE, PATCH), headers, and other factors to determine if a preflight request is necessary.
- Browser Sends OPTIONS Request: If a preflight is needed, the browser sends an OPTIONS request to the server. This request includes the HTTP method, headers, and URL of the main request.
- Server Responds with Allowable Methods and Headers: The server responds to the OPTIONS request with a 200 OK status code and a list of HTTP methods and headers that it allows for the main request.
- Browser Makes Main Request: If the server’s response indicates that the main request is allowed, the browser sends it.
Here are some common scenarios where preflight requests are used:
- Cross-origin AJAX requests: When a web page makes an AJAX request to a server on a different domain, a preflight request is often sent to check if the server allows cross-origin requests.
- WebSockets: WebSockets are used for full-duplex communication between a web browser and a server. A preflight request is typically sent before establishing a WebSocket connection to ensure compatibility.
- Fetch API: The Fetch API is a modern API for making network requests in JavaScript. Preflight requests are automatically handled by the Fetch API.
In summary, preflight requests are a crucial part of the HTTP protocol, ensuring that web applications can communicate effectively across different domains and platforms.
