instant.page/instantpage.js

/*! instant.page v0.0.0 - (C) 2019 Alexandre Dieulot - https://instant.page/license */

let urlToPreload
let mouseoverTimer
let lastTouchTimestamp

const prefetcher = document.createElement('link')
const isSupported = prefetcher.relList && prefetcher.relList.supports && prefetcher.relList.supports('prefetch')

if (isSupported) {
  prefetcher.rel = 'prefetch'
  document.head.appendChild(prefetcher)

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

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 (!linkElement) {
    return
  }

  if (!isPreloadable(linkElement)) {
    return
  }

  linkElement.addEventListener('touchcancel', touchendAndTouchcancelListener, {passive: true})
  linkElement.addEventListener('touchend', touchendAndTouchcancelListener, {passive: true})

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

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

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

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

  if (!linkElement) {
    return
  }

  if (!isPreloadable(linkElement)) {
    return
  }

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

  urlToPreload = linkElement.href

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

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

  if (mouseoverTimer) {
    clearTimeout(mouseoverTimer)
    mouseoverTimer = undefined
  }
  else {
    urlToPreload = undefined
    stopPreloading()
  }
}

function isPreloadable(linkElement) {
  if (urlToPreload == linkElement.href) {
    return false
  }

  const urlObject = new URL(linkElement.href)

  if (urlObject.origin != location.origin) {
    return false
  }

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

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

  return true
}

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

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