According to David Flanagan's book "JavaScript: The Definitive Guide, 5th Edition", it is recommended to send request headers before making an AJAX request.
Is it necessary to do this for compatibility across different browsers?
request.setRequestHeader("User-Agent", "XMLHttpRequest");
request.setRequestHeader("Accept-Language", "en");
request.setRequestHeader("If-Modified-Since", lastRequestTime.toString());
When it comes to the last 2 headers, they are actually not essential for ensuring browser compatibility. These headers serve as indicators of preference (Accept-Language) and content optimization (If-Modified-Since).
request.setRequestHeader("Accept-Language", "en");
request.setRequestHeader("If-Modified-Since", lastRequestTime.toString());
The first header is used by servers to determine if a query originated from AJAX or regular navigation. Some older browsers may not automatically set this header, potentially causing issues with browser compatibility if your server relies on it being set. However, if your server does not depend on this header, then lack of it being set won't impact browser compatibility.
request.setRequestHeader("X-Requested-With", "XMLHttpRequest");
It is important to note that the first header should be X-Requested-With, not User-Agent.
Instead of reinventing the wheel, refer to jQuery for guidance:
Pay attention to "X-Requested-With" and consider setting it to "XMLHttpRequest". No need to worry about the "User-Agent".
Many individuals try to recreate jQuery without exploring its source code or that of other frameworks like DoJo or MooTools. Use the existing resources or utilize a framework to focus on creating a practical application rather than revisiting solved problems.
Read this series of blog posts on dailyjs where they develop a framework from the ground up:
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