Fetch keepalive：卸载场景的可靠传输

概述`fetch(url, { keepalive: true })` 允许在页面卸载后继续发送请求，适用于统计上报与短数据写入。请求体大小存在约 64KiB 限制，应避免长传输。用法/示例addEventListener('visibilitychange', () => { if (document.visibilityState === 'hidden') { fetch('/rum', { method: 'POST', body: JSON.stringify(metrics), keepalive: true, headers: { 'Content-Type': 'application/json' } }) } }) 工程建议控制请求体大小并压缩；失败时在下次会话补偿上报。对关键数据优先使用前台发送或 Background Sync；校验服务端的幂等与去重。与 `navigator.sendBeacon` 比较并按场景选型；`keepalive` 支持更多方法与头部。参考与验证MDN：`fetch()` keepalive — https://developer.mozilla.org/docs/Web/API/fetch#keepaliveweb.dev：Backgrounding — https://web.dev/articles/page-lifecycle-api