instant.page：提高網站頁面載入速度

<font><i>/*! instant.page v1.0.0 - (C) 2019 Alexandre Dieulot - https://instant.page/license */</i></font><font>

let urlToPreload
let mouseoverTimer
let lastTouchTimestamp

<b>const</b> prefetcher = document.createElement('link')
<b>const</b> isSupported = prefetcher.relList && prefetcher.relList.supports && prefetcher.relList.supports('prefetch')
<b>const</b> allowQueryString = 'instantAllowQueryString' in document.body.dataset

<b>if</b> (isSupported) {
prefetcher.rel = 'prefetch'
document.head.appendChild(prefetcher)

<b>const</b> eventListenersOptions = {
capture: <b>true</b>,
passive: <b>true</b>,
}
document.addEventListener('touchstart', touchstartListener, eventListenersOptions)
document.addEventListener('mouseover', mouseoverListener, eventListenersOptions)
}

function touchstartListener(event) {
</font><font><i>/* Chrome on Android calls mouseover before touchcancel so `lastTouchTimestamp`
* must be assigned on touchstart to be measured on mouseover. */</i></font><font>
lastTouchTimestamp = performance.now()

<b>const</b> linkElement = event.target.closest('a')

<b>if</b> (!linkElement) {
<b>return</b>
}

<b>if</b> (!isPreloadable(linkElement)) {
<b>return</b>
}

linkElement.addEventListener('touchcancel', touchendAndTouchcancelListener, {passive: <b>true</b>})
linkElement.addEventListener('touchend', touchendAndTouchcancelListener, {passive: <b>true</b>})

urlToPreload = linkElement.href
preload(linkElement.href)
}

function touchendAndTouchcancelListener() {
urlToPreload = undefined
stopPreloading()
}

function mouseoverListener(event) {
<b>if</b> (performance.now() - lastTouchTimestamp < 1100) {
<b>return</b>
}

<b>const</b> linkElement = event.target.closest('a')

<b>if</b> (!linkElement) {
<b>return</b>
}

<b>if</b> (!isPreloadable(linkElement)) {
<b>return</b>
}

linkElement.addEventListener('mouseout', mouseoutListener, {passive: <b>true</b>})

urlToPreload = linkElement.href

mouseoverTimer = setTimeout(() => {
preload(linkElement.href)
mouseoverTimer = undefined
}, 65)
}

function mouseoutListener(event) {
<b>if</b> (event.relatedTarget && event.target.closest('a') == event.relatedTarget.closest('a')) {
<b>return</b>
}

<b>if</b> (mouseoverTimer) {
clearTimeout(mouseoverTimer)
mouseoverTimer = undefined
}
<b>else</b> {
urlToPreload = undefined
stopPreloading()
}
}

function isPreloadable(linkElement) {
<b>if</b> (urlToPreload == linkElement.href) {
<b>return</b>
}

<b>const</b> urlObject = <b>new</b> URL(linkElement.href)

<b>if</b> (urlObject.origin != location.origin) {
<b>return</b>
}

<b>if</b> (!allowQueryString && urlObject.search) {
<b>return</b>
}

<b>if</b> (urlObject.pathname + urlObject.search == location.pathname + location.search && urlObject.hash) {
<b>return</b>
}

<b>if</b> ('noInstant' in linkElement.dataset) {
<b>return</b>
}

<b>return</b> <b>true</b>
}

function preload(url) {
prefetcher.href = url
}

function stopPreloading() {
</font><font><i>/* The spec says an empty string should abort the prefetching
* but Firefox 64 interprets it as a relative URL to prefetch. */</i></font><font>
prefetcher.removeAttribute('href')
}
</font>