getAllResponseHeaders

getAllResponseHeaders is a method of the XMLHttpRequest object in web development, primarily used to retrieve all response headers from an HTTP request as a single string. This method is useful for accessing metadata sent by the server in response to a client's request, which can include information such as content type, content length, caching policies, and custom headers.

When invoked, getAllResponseHeaders returns a string containing all the response headers, each separated by a carriage return and line feed (CRLF). The format typically adheres to "Header-Name: Header-Value" pairs, one per line. It is important to call this method only after the response has been received, usually once the readyState is 4 (DONE). Attempting to use getAllResponseHeaders() before the request is completed will result in an empty string.

This method is often used in scenarios where developers need to process or verify multiple response headers collectively, rather than handling individual headers using getResponseHeader(). It is supported across all modern browsers that implement XMLHttpRequest, making it a standard utility in AJAX-based interactions.

Security considerations include that getAllResponseHeaders can access only the headers accessible to the origin's CORS policy. If the server does not include proper Access-Control-Expose-Headers, certain headers may be hidden from scripts for security reasons.

In summary, getAllResponseHeaders is a browser-provided function that enables retrieval of all response headers from an XMLHttpRequest, facilitating comprehensive inspection of server response metadata in web applications.