measure-before-unload.js

(function() {
  let userThinkTime = 0

  ;['alert', 'confirm', 'prompt'].forEach(function(methodName) {
    window[methodName] = (function(method) {
      return function () {
        userThinkTime -= performance.now()
        var returnValue = method.apply(window, arguments)
        userThinkTime += performance.now()
        return returnValue
      }
    })(window[methodName])
  })

  window.addEventListener = (function(addEventListener) {
    const methodName = 'onbeforeunload'
    const handlers = []
    addEventListener.call(window, methodName, function () {
      let duration = 0
      duration -= performance.now()
      for (var i = 0; i < handlers.length; i++)
        try {
          handlers[i]()
        } catch (e) {
          console.error(e)
        }
      duration += performance.now()
      console.info(methodName, duration, 'userThinkTime', userThinkTime)
    })

    return function () {
      if (arguments[0] === methodName) {
        handlers.push(arguments[1])
      } else {
        addEventListener.apply(window, arguments)
      }
    }
  })(window.addEventListener)
})()