Time to first byte with javascript?

Question

Is there any modern browser that via javascript exposes time to first byte (TTFB) and/or time to last byte (TTLB) on a http request without resorting to any plugin?

What I would like is a javascript snippet that can access these values and post them back the the server for performance monitoring purposes.

Clarification: I am not looking for any js timers or developer tools. What I wonder and hoping is if there are any browsers that measures load times and exposes those value via javascript.

Answer 1

What you want is the W3C's PerformanceTiming interface. Browser support is good (see this survey from Sep 2011). Like you speculated in response to Shadow Wizard's answer, these times are captured by the browser and exposed to javascript in the window object. You can find them in window.performance.timing. The endpoint of your TTFB interval will be window.performance.timing.responseStart (defined as "the time immediately after the user agent receives the first byte of the response from the server, or from relevant application caches or from local resources"). There are some options for the starting point, depending on whether you're interested in time to unload the previous document, or time to resolve DNS, so you probably want to read the documentation and decide which one is right for your application.