load javascript or css file

loadFile

  function loadFile (url, callback) {
    var head = document.getElementsByTagName("head")[0] || document.documentElement
    var baseElement = head.getElementsByTagName("base")[0]

    var IS_CSS_RE = /\.css(?:\?|$)/i
    var currentlyAddingScript
    var interactiveScript

    var isOldWebKit = +navigator.userAgent
        .replace(/.*AppleWebKit\/(\d+)\..*/, "$1") < 536


    function request(url, callback) {
      var isCSS = IS_CSS_RE.test(url)
      var node = document.createElement(isCSS ? "link" : "script")

      addOnload(node, callback, isCSS, url)

      if (isCSS) {
        node.rel = "stylesheet"
        node.href = url
      } else {
        node.async = true
        node.src = url
      }

      currentlyAddingScript = node

      baseElement ?
          head.insertBefore(node, baseElement) :
          head.appendChild(node)

      currentlyAddingScript = null
    }

    function addOnload(node, callback, isCSS, url) {
      var supportOnload = "onload" in node

      if (isCSS && (isOldWebKit || !supportOnload)) {
        setTimeout(function() {
          pollCss(node, callback)
        }, 1)
        return
      }

      if (supportOnload) {
        node.onload = onload
        node.onerror = function() {
          onload()
        }
      }
      else {
        node.onreadystatechange = function() {
          if (/loaded|complete/.test(node.readyState)) {
            onload()
          }
        }
      }

      function onload() {
        node.onload = node.onerror = node.onreadystatechange = null

        if (!isCSS) {
          head.removeChild(node)
        }

        node = null

        callback()
      }
    }

    function pollCss(node, callback) {
      var sheet = node.sheet
      var isLoaded

      if (isOldWebKit) {
        if (sheet) {
          isLoaded = true
        }
      } else if (sheet) {
        try {
          if (sheet.cssRules) {
            isLoaded = true
          }
        } catch (ex) {
          if (ex.name === "NS_ERROR_DOM_SECURITY_ERR") {
            isLoaded = true
          }
        }
      }

      setTimeout(function() {
        if (isLoaded) {
          callback()
        }
        else {
          pollCss(node, callback)
        }
      }, 20)
    }

    function getCurrentScript() {
      if (currentlyAddingScript) {
        return currentlyAddingScript
      }

      if (interactiveScript && interactiveScript.readyState === "interactive") {
        return interactiveScript
      }

      var scripts = head.getElementsByTagName("script")

      for (var i = scripts.length - 1; i >= 0; i--) {
        var script = scripts[i]
        if (script.readyState === "interactive") {
          interactiveScript = script
          return interactiveScript
        }
      }
    }

    request(url, callback)
  }