Understanding Browser Navigation Timing API: All Key Metrics Explained
This article explains the Navigation Timing API's full set of timestamps—from navigationStart to loadEventEnd—detailing each metric's meaning and when it records a value, helping developers diagnose page‑load performance and how they differ for same‑origin versus cross‑origin requests, redirects, and cached resources.
The Navigation Timing API records timestamps for each stage of a page load, from the moment the previous page is unloaded to when the target page finishes loading.
Key timing points include:
navigationStart : Time when the previous page unload completes (or fetchStart if no previous page).
unloadEventEnd : Time when the unload event finishes for same-origin pages; 0 otherwise.
redirectStart and redirectEnd : Times marking the start and end of HTTP redirects for same-origin redirects; 0 if not applicable.
fetchStart : Time before the browser checks the cache or initiates the request.
domainLookupStart and domainLookupEnd : Times when DNS lookup begins and ends; fallback to fetchStart for cached resources.
connectStart and connectEnd : Times when the connection to the server is initiated and completed; fallback to domainLookupEnd for cached resources.
requestStart : Time when the request is sent, regardless of source.
responseStart : Time when the first byte of the response is received.
responseEnd : Time when the last byte is received or the connection closes.
domLoading , domInteractive , domContentLoadedEventStart , domContentLoadedEventEnd , domComplete : Various DOM processing milestones.
loadEventStart and loadEventEnd : Times before and after the load event fires.
The accompanying diagram illustrates these points.
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