instantpage.js

/*! instant.page v2.0.0 - (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')
const isDataSaverEnabled = navigator.connection && navigator.connection.saveData
const allowQueryString = 'instantAllowQueryString' in document.body.dataset
const allowExternalLinks = 'instantAllowExternalLinks' in document.body.dataset
const useWhitelist = 'instantWhitelist' in document.body.dataset

let delayOnHover = 65
let useMousedown = false
let useMousedownOnly = false
if ('instantIntensity' in document.body.dataset) {
  if (document.body.dataset.instantIntensity.substr(0, 'mousedown'.length) == 'mousedown') {
    useMousedown = true
    if (document.body.dataset.instantIntensity == 'mousedown-only') {
      useMousedownOnly = true
    }
  }
  else {
    const milliseconds = parseInt(document.body.dataset.instantIntensity)
    if (!isNaN(milliseconds)) {
      delayOnHover = milliseconds
    }
  }
}

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

  const eventListenersOptions = {
    capture: true,
    passive: true,
  }

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

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

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

  urlToPreload = linkElement.href

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

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

  if (!isPreloadable(linkElement)) {
    return
  }

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

  urlToPreload = linkElement.href

  preload(linkElement.href)
}

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

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

  urlToPreload = undefined

  stopPreloading()
}

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

  if (urlToPreload == 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) {
  prefetcher.href = url
}

function stopPreloading() {
  prefetcher.removeAttribute('href')
}