导航正时API是什么？

导航定时API通过提供精确的页面加载时间数据来帮助开发者测量网页性能。它利用JavaScript中的performance对象，记录从用户发起导航到页面完全加载各关键阶段的时间戳，如DNS查找、服务器响应和页面交互时间。借助这些时间戳差值，可计算出首字节时间、DOM加载时间和完整页面加载时间等指标。其优势在于跨浏览器标准化、无需额外库且能捕捉完整加载流程。但需注意旧版浏览器支持有限、跨域限制及某些环境下数据可能缺失的问题。

The Navigation Timing API is a browser tool that helps developers measure how fast a webpage loads. Instead of guessing or relying on rough estimates, this API gives precise timing data for different parts of the page load process — like when the DNS lookup starts, when the server responds, or when the page becomes interactive. It’s built into modern browsers and doesn’t require any libraries, making it a reliable way to track performance.

How Does the Navigation Timing API Work?

This API collects detailed timing information through the performance object in JavaScript. You can access it using performance.timing, which returns a bunch of timestamps marking key moments during navigation and loading.

For example:

• navigationStart: When the user initiates a navigation (like typing a URL or clicking a link).
• responseEnd: When the last byte of the response is received.
• domContentLoadedEventEnd: When the initial HTML document has been completely loaded and parsed.

By subtracting these timestamps from each other, you can calculate things like:

• Time to First Byte (TTFB): responseStart - domainLookupStart
• Page Load Time: loadEventEnd - navigationStart

These numbers are super useful if you're trying to understand or improve your site's performance.

What Can You Measure With It?

You can get a lot of insights from the Navigation Timing API. Here are some common metrics people track:

• DNS Lookup Time: How long it takes to resolve the domain name.
• TCP Connection Time: The time taken to establish a connection with the server.
• Time to First Byte (TTFB): How long before the browser receives the first byte of the page.
• DOM Load Time: From when the document starts loading until it finishes parsing.
• Full Page Load Time: Until everything (like images and scripts) is fully loaded.

Each of these gives you a piece of the performance puzzle. For instance, a slow DNS lookup might point to issues with your DNS provider, while a high TTFB could mean your server needs optimization.

Why Use Navigation Timing Instead of Other Methods?

There are other ways to measure page speed — like wrapping code in Date.now() calls or using third-party tools — but those usually don’t give you the full picture. Navigation Timing is better because:

• It's standardized across browsers.
• It captures every step from navigation to full load.
• It’s passive — no need to instrument every request manually.
• It works even if JavaScript runs late or fails.

Also, since it's part of the Performance API, it plays well with newer tools like PerformanceObserver, allowing you to monitor performance changes over time more accurately.

A Few Things to Keep in Mind

While the Navigation Timing API is powerful, there are a few gotchas:

• Some older browsers may not support all fields.
• If you're measuring from a service worker or iframe, the data might be limited.
• Privacy features in browsers (like cross-origin restrictions) can affect what data is available.

One easy mistake is assuming all values will always be there. So when you’re reading from performance.timing, check that the values make sense — sometimes they can be zero or missing, especially on redirects or secure connections.

基本上就这些。

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