提高转化率的预请求库 instant.page

instant.page 是一个较新的功能库，该库支持现代浏览器，小而美。并且无侵入式。 只要在项目的 之前加入以下代码，便会得到收益。

<script
  src="//instant.page/5.1.0"
  type="module"
  integrity="sha384-by67kQnR+pyfy8yWP4kPO12fHKRLHZPfEsiSXR8u2IKcTdxD805MGUXBzVPnkLHw"
></script>

instant.page 认为当用户的鼠标停留某个标签 65 毫秒的时候，有两分之一的几率他们会点击那个链接，所以 instant.page 在这个时候开始预加载。

该方案不适合单页面应用，但是该库很棒的运用了 prefetch，是在你悬停于链接超过 65ms 时候，把已经放入的 head 最后的 link 改为悬停链接的 href。

不得不说，该库利用了很多浏览器新的的机制。包括使用 type=module 来拒绝旧的浏览器执行，利用 dataset 读取 instantIntensity 来控制延迟时间。

/*! instant.page v5.1.0 - (C) 2019-2020 Alexandre Dieulot - https://instant.page/license */

let mouseoverTimer
let lastTouchTimestamp
const prefetches = new Set()
const prefetchElement = document.createElement('link')
const isSupported = prefetchElement.relList && prefetchElement.relList.supports && prefetchElement.relList.supports('prefetch')
                    && window.IntersectionObserver && 'isIntersecting' in IntersectionObserverEntry.prototype
const allowQueryString = 'instantAllowQueryString' in document.body.dataset
const allowExternalLinks = 'instantAllowExternalLinks' in document.body.dataset
const useWhitelist = 'instantWhitelist' in document.body.dataset
const mousedownShortcut = 'instantMousedownShortcut' in document.body.dataset
const DELAY_TO_NOT_BE_CONSIDERED_A_TOUCH_INITIATED_ACTION = 1111

let delayOnHover = 65
let useMousedown = false
let useMousedownOnly = false
let useViewport = false

if ('instantIntensity' in document.body.dataset) {
  const intensity = document.body.dataset.instantIntensity

  if (intensity.substr(0, 'mousedown'.length) == 'mousedown') {
    useMousedown = true
    if (intensity == 'mousedown-only') {
      useMousedownOnly = true
    }
  }
  else if (intensity.substr(0, 'viewport'.length) == 'viewport') {
    if (!(navigator.connection && (navigator.connection.saveData || (navigator.connection.effectiveType && navigator.connection.effectiveType.includes('2g'))))) {
      if (intensity == "viewport") {
        /* Biggest iPhone resolution (which we want): 414 × 896 = 370944
         * Small 7" tablet resolution (which we don’t want): 600 × 1024 = 614400
         * Note that the viewport (which we check here) is smaller than the resolution due to the UI’s chrome */
        if (document.documentElement.clientWidth * document.documentElement.clientHeight < 450000) {
          useViewport = true
        }
      }
      else if (intensity == "viewport-all") {
        useViewport = true
      }
    }
  }
  else {
    const milliseconds = parseInt(intensity)
    if (!isNaN(milliseconds)) {
      delayOnHover = milliseconds
    }
  }
}

if (isSupported) {
  const eventListenersOptions = {
    capture: true,
    passive: true,
  }

  if (!useMousedownOnly) {
    document.addEventListener('touchstart', touchstartListener, eventListenersOptions)
  }

  if (!useMousedown) {
    document.addEventListener('mouseover', mouseoverListener, eventListenersOptions)
  }
  else if (!mousedownShortcut) {
      document.addEventListener('mousedown', mousedownListener, eventListenersOptions)
  }

  if (mousedownShortcut) {
    document.addEventListener('mousedown', mousedownShortcutListener, eventListenersOptions)
  }

  if (useViewport) {
    let triggeringFunction
    if (window.requestIdleCallback) {
      triggeringFunction = (callback) => {
        requestIdleCallback(callback, {
          timeout: 1500,
        })
      }
    }
    else {
      triggeringFunction = (callback) => {
        callback()
      }
    }

    triggeringFunction(() => {
      const intersectionObserver = new IntersectionObserver((entries) => {
        entries.forEach((entry) => {
          if (entry.isIntersecting) {
            const linkElement = entry.target
            intersectionObserver.unobserve(linkElement)
            preload(linkElement.href)
          }
        })
      })

      document.querySelectorAll('a').forEach((linkElement) => {
        if (isPreloadable(linkElement)) {
          intersectionObserver.observe(linkElement)
        }
      })
    })
  }
}

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

  const linkElement = event.target.closest('a')

  if (!isPreloadable(linkElement)) {
    return
  }

  preload(linkElement.href)
}

function mouseoverListener(event) {
  if (performance.now() - lastTouchTimestamp < DELAY_TO_NOT_BE_CONSIDERED_A_TOUCH_INITIATED_ACTION) {
    return
  }

  const linkElement = event.target.closest('a')

  if (!isPreloadable(linkElement)) {
    return
  }

  linkElement.addEventListener('mouseout', mouseoutListener, {passive: true})

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

function mousedownListener(event) {
  const linkElement = event.target.closest('a')

  if (!isPreloadable(linkElement)) {
    return
  }

  preload(linkElement.href)
}

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

  if (mouseoverTimer) {
    clearTimeout(mouseoverTimer)
    mouseoverTimer = undefined
  }
}

function mousedownShortcutListener(event) {
  if (performance.now() - lastTouchTimestamp < DELAY_TO_NOT_BE_CONSIDERED_A_TOUCH_INITIATED_ACTION) {
    return
  }

  const linkElement = event.target.closest('a')

  if (event.which > 1 || event.metaKey || event.ctrlKey) {
    return
  }

  if (!linkElement) {
    return
  }

  linkElement.addEventListener('click', function (event) {
    if (event.detail == 1337) {
      return
    }

    event.preventDefault()
  }, {capture: true, passive: false, once: true})

  const customEvent = new MouseEvent('click', {view: window, bubbles: true, cancelable: false, detail: 1337})
  linkElement.dispatchEvent(customEvent)
}

function isPreloadable(linkElement) {
  if (!linkElement || !linkElement.href) {
    return
  }

  if (useWhitelist && !('instant' in linkElement.dataset)) {
    return
  }

  if (!allowExternalLinks && linkElement.origin != location.origin && !('instant' in linkElement.dataset)) {
    return
  }

  if (!['http:', 'https:'].includes(linkElement.protocol)) {
    return
  }

  if (linkElement.protocol == 'http:' && location.protocol == 'https:') {
    return
  }

  if (!allowQueryString && linkElement.search && !('instant' in linkElement.dataset)) {
    return
  }

  if (linkElement.hash && linkElement.pathname + linkElement.search == location.pathname + location.search) {
    return
  }

  if ('noInstant' in linkElement.dataset) {
    return
  }

  return true
}

function preload(url) {
  if (prefetches.has(url)) {
    return
  }

  const prefetcher = document.createElement('link')
  prefetcher.rel = 'prefetch'
  prefetcher.href = url
  document.head.appendChild(prefetcher)

  prefetches.add(url)
}