/**
 * @license
 * Copyright Google LLC All Rights Reserved.
 *
 * Use of this source code is governed by an MIT-style license that can be
 * found in the LICENSE file at https://angular.io/license
 */

/**
 * Suppress closure compiler errors about unknown 'global' variable
 * @fileoverview
 * @suppress {undefinedVars}
 */

/**
 * Zone is a mechanism for intercepting and keeping track of asynchronous work.
 *
 * A Zone is a global object which is configured with rules about how to intercept and keep track
 * of the asynchronous callbacks. Zone has these responsibilities:
 *
 * 1. Intercept asynchronous task scheduling
 * 2. Wrap callbacks for error-handling and zone tracking across async operations.
 * 3. Provide a way to attach data to zones
 * 4. Provide a context specific last frame error handling
 * 5. (Intercept blocking methods)
 *
 * A zone by itself does not do anything, instead it relies on some other code to route existing
 * platform API through it. (The zone library ships with code which monkey patches all of the
 * browsers's asynchronous API and redirects them through the zone for interception.)
 *
 * In its simplest form a zone allows one to intercept the scheduling and calling of asynchronous
 * operations, and execute additional code before as well as after the asynchronous task. The rules
 * of interception are configured using [ZoneConfig]. There can be many different zone instances in
 * a system, but only one zone is active at any given time which can be retrieved using
 * [Zone#current].
 *
 *
 *
 * ## Callback Wrapping
 *
 * An important aspect of the zones is that they should persist across asynchronous operations. To
 * achieve this, when a future work is scheduled through async API, it is necessary to capture, and
 * subsequently restore the current zone. For example if a code is running in zone `b` and it
 * invokes `setTimeout` to scheduleTask work later, the `setTimeout` method needs to 1) capture the
 * current zone and 2) wrap the `wrapCallback` in code which will restore the current zone `b` once
 * the wrapCallback executes. In this way the rules which govern the current code are preserved in
 * all future asynchronous tasks. There could be a different zone `c` which has different rules and
 * is associated with different asynchronous tasks. As these tasks are processed, each asynchronous
 * wrapCallback correctly restores the correct zone, as well as preserves the zone for future
 * asynchronous callbacks.
 *
 * Example: Suppose a browser page consist of application code as well as third-party
 * advertisement code. (These two code bases are independent, developed by different mutually
 * unaware developers.) The application code may be interested in doing global error handling and
 * so it configures the `app` zone to send all of the errors to the server for analysis, and then
 * executes the application in the `app` zone. The advertising code is interested in the same
 * error processing but it needs to send the errors to a different third-party. So it creates the
 * `ads` zone with a different error handler. Now both advertising as well as application code
 * create many asynchronous operations, but the [Zone] will ensure that all of the asynchronous
 * operations created from the application code will execute in `app` zone with its error
 * handler and all of the advertisement code will execute in the `ads` zone with its error handler.
 * This will not only work for the async operations created directly, but also for all subsequent
 * asynchronous operations.
 *
 * If you think of chain of asynchronous operations as a thread of execution (bit of a stretch)
 * then [Zone#current] will act as a thread local variable.
 *
 *
 *
 * ## Asynchronous operation scheduling
 *
 * In addition to wrapping the callbacks to restore the zone, all operations which cause a
 * scheduling of work for later are routed through the current zone which is allowed to intercept
 * them by adding work before or after the wrapCallback as well as using different means of
 * achieving the request. (Useful for unit testing, or tracking of requests). In some instances
 * such as `setTimeout` the wrapping of the wrapCallback and scheduling is done in the same
 * wrapCallback, but there are other examples such as `Promises` where the `then` wrapCallback is
 * wrapped, but the execution of `then` is triggered by `Promise` scheduling `resolve` work.
 *
 * Fundamentally there are three kinds of tasks which can be scheduled:
 *
 * 1. [MicroTask] used for doing work right after the current task. This is non-cancelable which is
 *    guaranteed to run exactly once and immediately.
 * 2. [MacroTask] used for doing work later. Such as `setTimeout`. This is typically cancelable
 *    which is guaranteed to execute at least once after some well understood delay.
 * 3. [EventTask] used for listening on some future event. This may execute zero or more times, with
 *    an unknown delay.
 *
 * Each asynchronous API is modeled and routed through one of these APIs.
 *
 *
 * ### [MicroTask]
 *
 * [MicroTask]s represent work which will be done in current VM turn as soon as possible, before VM
 * yielding.
 *
 *
 * ### [MacroTask]
 *
 * [MacroTask]s represent work which will be done after some delay. (Sometimes the delay is
 * approximate such as on next available animation frame). Typically these methods include:
 * `setTimeout`, `setImmediate`, `setInterval`, `requestAnimationFrame`, and all browser specific
 * variants.
 *
 *
 * ### [EventTask]
 *
 * [EventTask]s represent a request to create a listener on an event. Unlike the other task
 * events they may never be executed, but typically execute more than once. There is no queue of
 * events, rather their callbacks are unpredictable both in order and time.
 *
 *
 * ## Global Error Handling
 *
 *
 * ## Composability
 *
 * Zones can be composed together through [Zone.fork()]. A child zone may create its own set of
 * rules. A child zone is expected to either:
 *
 * 1. Delegate the interception to a parent zone, and optionally add before and after wrapCallback
 *    hooks.
 * 2. Process the request itself without delegation.
 *
 * Composability allows zones to keep their concerns clean. For example a top most zone may choose
 * to handle error handling, while child zones may choose to do user action tracking.
 *
 *
 * ## Root Zone
 *
 * At the start the browser will run in a special root zone, which is configured to behave exactly
 * like the platform, making any existing code which is not zone-aware behave as expected. All
 * zones are children of the root zone.
 *
 */
interface Zone {
  /**
   *
   * @returns {Zone} The parent Zone.
   */
  parent: Zone|null;
  /**
   * @returns {string} The Zone name (useful for debugging)
   */
  name: string;

  /**
   * Returns a value associated with the `key`.
   *
   * If the current zone does not have a key, the request is delegated to the parent zone. Use
   * [ZoneSpec.properties] to configure the set of properties associated with the current zone.
   *
   * @param key The key to retrieve.
   * @returns {any} The value for the key, or `undefined` if not found.
   */
  get(key: string): any;

  /**
   * Returns a Zone which defines a `key`.
   *
   * Recursively search the parent Zone until a Zone which has a property `key` is found.
   *
   * @param key The key to use for identification of the returned zone.
   * @returns {Zone} The Zone which defines the `key`, `null` if not found.
   */
  getZoneWith(key: string): Zone|null;

  /**
   * Used to create a child zone.
   *
   * @param zoneSpec A set of rules which the child zone should follow.
   * @returns {Zone} A new child zone.
   */
  fork(zoneSpec: ZoneSpec): Zone;

  /**
   * Wraps a callback function in a new function which will properly restore the current zone upon
   * invocation.
   *
   * The wrapped function will properly forward `this` as well as `arguments` to the `callback`.
   *
   * Before the function is wrapped the zone can intercept the `callback` by declaring
   * [ZoneSpec.onIntercept].
   *
   * @param callback the function which will be wrapped in the zone.
   * @param source A unique debug location of the API being wrapped.
   * @returns {function(): *} A function which will invoke the `callback` through [Zone.runGuarded].
   */
  wrap<F extends Function>(callback: F, source: string): F;

  /**
   * Invokes a function in a given zone.
   *
   * The invocation of `callback` can be intercepted by declaring [ZoneSpec.onInvoke].
   *
   * @param callback The function to invoke.
   * @param applyThis
   * @param applyArgs
   * @param source A unique debug location of the API being invoked.
   * @returns {any} Value from the `callback` function.
   */
  run<T>(callback: Function, applyThis?: any, applyArgs?: any[], source?: string): T;

  /**
   * Invokes a function in a given zone and catches any exceptions.
   *
   * Any exceptions thrown will be forwarded to [Zone.HandleError].
   *
   * The invocation of `callback` can be intercepted by declaring [ZoneSpec.onInvoke]. The
   * handling of exceptions can be intercepted by declaring [ZoneSpec.handleError].
   *
   * @param callback The function to invoke.
   * @param applyThis
   * @param applyArgs
   * @param source A unique debug location of the API being invoked.
   * @returns {any} Value from the `callback` function.
   */
  runGuarded<T>(callback: Function, applyThis?: any, applyArgs?: any[], source?: string): T;

  /**
   * Execute the Task by restoring the [Zone.currentTask] in the Task's zone.
   *
   * @param task to run
   * @param applyThis
   * @param applyArgs
   * @returns {any} Value from the `task.callback` function.
   */
  runTask<T>(task: Task, applyThis?: any, applyArgs?: any): T;

  /**
   * Schedule a MicroTask.
   *
   * @param source
   * @param callback
   * @param data
   * @param customSchedule
   */
  scheduleMicroTask(
      source: string, callback: Function, data?: TaskData,
      customSchedule?: (task: Task) => void): MicroTask;

  /**
   * Schedule a MacroTask.
   *
   * @param source
   * @param callback
   * @param data
   * @param customSchedule
   * @param customCancel
   */
  scheduleMacroTask(
      source: string, callback: Function, data?: TaskData, customSchedule?: (task: Task) => void,
      customCancel?: (task: Task) => void): MacroTask;

  /**
   * Schedule an EventTask.
   *
   * @param source
   * @param callback
   * @param data
   * @param customSchedule
   * @param customCancel
   */
  scheduleEventTask(
      source: string, callback: Function, data?: TaskData, customSchedule?: (task: Task) => void,
      customCancel?: (task: Task) => void): EventTask;

  /**
   * Schedule an existing Task.
   *
   * Useful for rescheduling a task which was already canceled.
   *
   * @param task
   */
  scheduleTask<T extends Task>(task: T): T;

  /**
   * Allows the zone to intercept canceling of scheduled Task.
   *
   * The interception is configured using [ZoneSpec.onCancelTask]. The default canceler invokes
   * the [Task.cancelFn].
   *
   * @param task
   * @returns {any}
   */
  cancelTask(task: Task): any;
}

interface ZoneType {
  /**
   * @returns {Zone} Returns the current [Zone]. The only way to change
   * the current zone is by invoking a run() method, which will update the current zone for the
   * duration of the run method callback.
   */
  current: Zone;

  /**
   * @returns {Task} The task associated with the current execution.
   */
  currentTask: Task|null;

  /**
   * Verify that Zone has been correctly patched. Specifically that Promise is zone aware.
   */
  assertZonePatched(): void;

  /**
   *  Return the root zone.
   */
  root: Zone;

  /**
   * load patch for specified native module, allow user to
   * define their own patch, user can use this API after loading zone.js
   */
  __load_patch(name: string, fn: _PatchFn, ignoreDuplicate?: boolean): void;

  /**
   * Zone symbol API to generate a string with __zone_symbol__ prefix
   */
  __symbol__(name: string): string;
}

/**
 * Patch Function to allow user define their own monkey patch module.
 */
type _PatchFn = (global: Window, Zone: ZoneType, api: _ZonePrivate) => void;

/**
 * _ZonePrivate interface to provide helper method to help user implement
 * their own monkey patch module.
 */
interface _ZonePrivate {
  currentZoneFrame: () => _ZoneFrame;
  symbol: (name: string) => string;
  scheduleMicroTask: (task?: MicroTask) => void;
  onUnhandledError: (error: Error) => void;
  microtaskDrainDone: () => void;
  showUncaughtError: () => boolean;
  patchEventTarget: (global: any, api: _ZonePrivate, apis: any[], options?: any) => boolean[];
  patchOnProperties: (obj: any, properties: string[]|null, prototype?: any) => void;
  patchThen: (ctro: Function) => void;
  patchMethod:
      (target: any, name: string,
       patchFn: (delegate: Function, delegateName: string, name: string) =>
           (self: any, args: any[]) => any) => Function | null;
  bindArguments: (args: any[], source: string) => any[];
  patchMacroTask:
      (obj: any, funcName: string, metaCreator: (self: any, args: any[]) => any) => void;
  patchEventPrototype: (_global: any, api: _ZonePrivate) => void;
  isIEOrEdge: () => boolean;
  ObjectDefineProperty:
      (o: any, p: PropertyKey, attributes: PropertyDescriptor&ThisType<any>) => any;
  ObjectGetOwnPropertyDescriptor: (o: any, p: PropertyKey) => PropertyDescriptor | undefined;
  ObjectCreate(o: object|null, properties?: PropertyDescriptorMap&ThisType<any>): any;
  ArraySlice(start?: number, end?: number): any[];
  patchClass: (className: string) => void;
  wrapWithCurrentZone: (callback: any, source: string) => any;
  filterProperties: (target: any, onProperties: string[], ignoreProperties: any[]) => string[];
  attachOriginToPatched: (target: any, origin: any) => void;
  _redefineProperty: (target: any, callback: string, desc: any) => void;
  nativeScheduleMicroTask: (func: Function) => void;
  patchCallbacks:
      (api: _ZonePrivate, target: any, targetName: string, method: string,
       callbacks: string[]) => void;
  getGlobalObjects: () => {
    globalSources: any, zoneSymbolEventNames: any, eventNames: string[], isBrowser: boolean,
        isMix: boolean, isNode: boolean, TRUE_STR: string, FALSE_STR: string,
        ZONE_SYMBOL_PREFIX: string, ADD_EVENT_LISTENER_STR: string,
        REMOVE_EVENT_LISTENER_STR: string
  } | undefined;
}

/**
 * _ZoneFrame represents zone stack frame information
 */
interface _ZoneFrame {
  parent: _ZoneFrame|null;
  zone: Zone;
}

interface UncaughtPromiseError extends Error {
  zone: Zone;
  task: Task;
  promise: Promise<any>;
  rejection: any;
  throwOriginal?: boolean;
}

/**
 * Provides a way to configure the interception of zone events.
 *
 * Only the `name` property is required (all other are optional).
 */
interface ZoneSpec {
  /**
   * The name of the zone. Useful when debugging Zones.
   */
  name: string;

  /**
   * A set of properties to be associated with Zone. Use [Zone.get] to retrieve them.
   */
  properties?: {[key: string]: any};

  /**
   * Allows the interception of zone forking.
   *
   * When the zone is being forked, the request is forwarded to this method for interception.
   *
   * @param parentZoneDelegate Delegate which performs the parent [ZoneSpec] operation.
   * @param currentZone The current [Zone] where the current interceptor has been declared.
   * @param targetZone The [Zone] which originally received the request.
   * @param zoneSpec The argument passed into the `fork` method.
   */
  onFork?:
      (parentZoneDelegate: ZoneDelegate, currentZone: Zone, targetZone: Zone,
       zoneSpec: ZoneSpec) => Zone;

  /**
   * Allows interception of the wrapping of the callback.
   *
   * @param parentZoneDelegate Delegate which performs the parent [ZoneSpec] operation.
   * @param currentZone The current [Zone] where the current interceptor has been declared.
   * @param targetZone The [Zone] which originally received the request.
   * @param delegate The argument passed into the `wrap` method.
   * @param source The argument passed into the `wrap` method.
   */
  onIntercept?:
      (parentZoneDelegate: ZoneDelegate, currentZone: Zone, targetZone: Zone, delegate: Function,
       source: string) => Function;

  /**
   * Allows interception of the callback invocation.
   *
   * @param parentZoneDelegate Delegate which performs the parent [ZoneSpec] operation.
   * @param currentZone The current [Zone] where the current interceptor has been declared.
   * @param targetZone The [Zone] which originally received the request.
   * @param delegate The argument passed into the `run` method.
   * @param applyThis The argument passed into the `run` method.
   * @param applyArgs The argument passed into the `run` method.
   * @param source The argument passed into the `run` method.
   */
  onInvoke?:
      (parentZoneDelegate: ZoneDelegate, currentZone: Zone, targetZone: Zone, delegate: Function,
       applyThis: any, applyArgs?: any[], source?: string) => any;

  /**
   * Allows interception of the error handling.
   *
   * @param parentZoneDelegate Delegate which performs the parent [ZoneSpec] operation.
   * @param currentZone The current [Zone] where the current interceptor has been declared.
   * @param targetZone The [Zone] which originally received the request.
   * @param error The argument passed into the `handleError` method.
   */
  onHandleError?:
      (parentZoneDelegate: ZoneDelegate, currentZone: Zone, targetZone: Zone,
       error: any) => boolean;

  /**
   * Allows interception of task scheduling.
   *
   * @param parentZoneDelegate Delegate which performs the parent [ZoneSpec] operation.
   * @param currentZone The current [Zone] where the current interceptor has been declared.
   * @param targetZone The [Zone] which originally received the request.
   * @param task The argument passed into the `scheduleTask` method.
   */
  onScheduleTask?:
      (parentZoneDelegate: ZoneDelegate, currentZone: Zone, targetZone: Zone, task: Task) => Task;

  onInvokeTask?:
      (parentZoneDelegate: ZoneDelegate, currentZone: Zone, targetZone: Zone, task: Task,
       applyThis: any, applyArgs?: any[]) => any;

  /**
   * Allows interception of task cancellation.
   *
   * @param parentZoneDelegate Delegate which performs the parent [ZoneSpec] operation.
   * @param currentZone The current [Zone] where the current interceptor has been declared.
   * @param targetZone The [Zone] which originally received the request.
   * @param task The argument passed into the `cancelTask` method.
   */
  onCancelTask?:
      (parentZoneDelegate: ZoneDelegate, currentZone: Zone, targetZone: Zone, task: Task) => any;

  /**
   * Notifies of changes to the task queue empty status.
   *
   * @param parentZoneDelegate Delegate which performs the parent [ZoneSpec] operation.
   * @param currentZone The current [Zone] where the current interceptor has been declared.
   * @param targetZone The [Zone] which originally received the request.
   * @param hasTaskState
   */
  onHasTask?:
      (parentZoneDelegate: ZoneDelegate, currentZone: Zone, targetZone: Zone,
       hasTaskState: HasTaskState) => void;
}


/**
 *  A delegate when intercepting zone operations.
 *
 *  A ZoneDelegate is needed because a child zone can't simply invoke a method on a parent zone. For
 *  example a child zone wrap can't just call parent zone wrap. Doing so would create a callback
 *  which is bound to the parent zone. What we are interested in is intercepting the callback before
 *  it is bound to any zone. Furthermore, we also need to pass the targetZone (zone which received
 *  the original request) to the delegate.
 *
 *  The ZoneDelegate methods mirror those of Zone with an addition of extra targetZone argument in
 *  the method signature. (The original Zone which received the request.) Some methods are renamed
 *  to prevent confusion, because they have slightly different semantics and arguments.
 *
 *  - `wrap` => `intercept`: The `wrap` method delegates to `intercept`. The `wrap` method returns
 *     a callback which will run in a given zone, where as intercept allows wrapping the callback
 *     so that additional code can be run before and after, but does not associate the callback
 *     with the zone.
 *  - `run` => `invoke`: The `run` method delegates to `invoke` to perform the actual execution of
 *     the callback. The `run` method switches to new zone; saves and restores the `Zone.current`;
 *     and optionally performs error handling. The invoke is not responsible for error handling,
 *     or zone management.
 *
 *  Not every method is usually overwritten in the child zone, for this reason the ZoneDelegate
 *  stores the closest zone which overwrites this behavior along with the closest ZoneSpec.
 *
 *  NOTE: We have tried to make this API analogous to Event bubbling with target and current
 *  properties.
 *
 *  Note: The ZoneDelegate treats ZoneSpec as class. This allows the ZoneSpec to use its `this` to
 *  store internal state.
 */
interface ZoneDelegate {
  zone: Zone;
  fork(targetZone: Zone, zoneSpec: ZoneSpec): Zone;
  intercept(targetZone: Zone, callback: Function, source: string): Function;
  invoke(targetZone: Zone, callback: Function, applyThis?: any, applyArgs?: any[], source?: string):
      any;
  handleError(targetZone: Zone, error: any): boolean;
  scheduleTask(targetZone: Zone, task: Task): Task;
  invokeTask(targetZone: Zone, task: Task, applyThis?: any, applyArgs?: any[]): any;
  cancelTask(targetZone: Zone, task: Task): any;
  hasTask(targetZone: Zone, isEmpty: HasTaskState): void;
}

type HasTaskState = {
  microTask: boolean; macroTask: boolean; eventTask: boolean; change: TaskType;
};

/**
 * Task type: `microTask`, `macroTask`, `eventTask`.
 */
type TaskType = 'microTask'|'macroTask'|'eventTask';

/**
 * Task type: `notScheduled`, `scheduling`, `scheduled`, `running`, `canceling`, 'unknown'.
 */
type TaskState = 'notScheduled'|'scheduling'|'scheduled'|'running'|'canceling'|'unknown';


/**
 */
interface TaskData {
  /**
   * A periodic [MacroTask] is such which get automatically rescheduled after it is executed.
   */
  isPeriodic?: boolean;

  /**
   * Delay in milliseconds when the Task will run.
   */
  delay?: number;

  /**
   * identifier returned by the native setTimeout.
   */
  handleId?: number;
}

/**
 * Represents work which is executed with a clean stack.
 *
 * Tasks are used in Zones to mark work which is performed on clean stack frame. There are three
 * kinds of task. [MicroTask], [MacroTask], and [EventTask].
 *
 * A JS VM can be modeled as a [MicroTask] queue, [MacroTask] queue, and [EventTask] set.
 *
 * - [MicroTask] queue represents a set of tasks which are executing right after the current stack
 *   frame becomes clean and before a VM yield. All [MicroTask]s execute in order of insertion
 *   before VM yield and the next [MacroTask] is executed.
 * - [MacroTask] queue represents a set of tasks which are executed one at a time after each VM
 *   yield. The queue is ordered by time, and insertions can happen in any location.
 * - [EventTask] is a set of tasks which can at any time be inserted to the end of the [MacroTask]
 *   queue. This happens when the event fires.
 *
 */
interface Task {
  /**
   * Task type: `microTask`, `macroTask`, `eventTask`.
   */
  type: TaskType;

  /**
   * Task state: `notScheduled`, `scheduling`, `scheduled`, `running`, `canceling`, `unknown`.
   */
  state: TaskState;

  /**
   * Debug string representing the API which requested the scheduling of the task.
   */
  source: string;

  /**
   * The Function to be used by the VM upon entering the [Task]. This function will delegate to
   * [Zone.runTask] and delegate to `callback`.
   */
  invoke: Function;

  /**
   * Function which needs to be executed by the Task after the [Zone.currentTask] has been set to
   * the current task.
   */
  callback: Function;

  /**
   * Task specific options associated with the current task. This is passed to the `scheduleFn`.
   */
  data?: TaskData;

  /**
   * Represents the default work which needs to be done to schedule the Task by the VM.
   *
   * A zone may choose to intercept this function and perform its own scheduling.
   */
  scheduleFn?: (task: Task) => void;

  /**
   * Represents the default work which needs to be done to un-schedule the Task from the VM. Not all
   * Tasks are cancelable, and therefore this method is optional.
   *
   * A zone may chose to intercept this function and perform its own un-scheduling.
   */
  cancelFn?: (task: Task) => void;

  /**
   * @type {Zone} The zone which will be used to invoke the `callback`. The Zone is captured
   * at the time of Task creation.
   */
  readonly zone: Zone;

  /**
   * Number of times the task has been executed, or -1 if canceled.
   */
  runCount: number;

  /**
   * Cancel the scheduling request. This method can be called from `ZoneSpec.onScheduleTask` to
   * cancel the current scheduling interception. Once canceled the task can be discarded or
   * rescheduled using `Zone.scheduleTask` on a different zone.
   */
  cancelScheduleRequest(): void;
}

interface MicroTask extends Task {
  type: 'microTask';
}

interface MacroTask extends Task {
  type: 'macroTask';
}

interface EventTask extends Task {
  type: 'eventTask';
}

/** @internal */
type AmbientZone = Zone;

const Zone: ZoneType = (function(global: any) {
  const performance: {mark(name: string): void; measure(name: string, label: string): void;} =
      global['performance'];
  function mark(name: string) {
    performance && performance['mark'] && performance['mark'](name);
  }
  function performanceMeasure(name: string, label: string) {
    performance && performance['measure'] && performance['measure'](name, label);
  }
  mark('Zone');

  // Initialize before it's accessed below.
  // __Zone_symbol_prefix global can be used to override the default zone
  // symbol prefix with a custom one if needed.
  const symbolPrefix = global['__Zone_symbol_prefix'] || '__zone_symbol__';

  function __symbol__(name: string) {
    return symbolPrefix + name;
  }

  const checkDuplicate = global[__symbol__('forceDuplicateZoneCheck')] === true;
  if (global['Zone']) {
    // if global['Zone'] already exists (maybe zone.js was already loaded or
    // some other lib also registered a global object named Zone), we may need
    // to throw an error, but sometimes user may not want this error.
    // For example,
    // we have two web pages, page1 includes zone.js, page2 doesn't.
    // and the 1st time user load page1 and page2, everything work fine,
    // but when user load page2 again, error occurs because global['Zone'] already exists.
    // so we add a flag to let user choose whether to throw this error or not.
    // By default, if existing Zone is from zone.js, we will not throw the error.
    if (checkDuplicate || typeof global['Zone'].__symbol__ !== 'function') {
      throw new Error('Zone already loaded.');
    } else {
      return global['Zone'];
    }
  }

  class Zone implements AmbientZone {
    // tslint:disable-next-line:require-internal-with-underscore
    static __symbol__: (name: string) => string = __symbol__;

    static assertZonePatched() {
      if (global['Promise'] !== patches['ZoneAwarePromise']) {
        throw new Error(
            'Zone.js has detected that ZoneAwarePromise `(window|global).Promise` ' +
            'has been overwritten.\n' +
            'Most likely cause is that a Promise polyfill has been loaded ' +
            'after Zone.js (Polyfilling Promise api is not necessary when zone.js is loaded. ' +
            'If you must load one, do so before loading zone.js.)');
      }
    }

    static get root(): AmbientZone {
      let zone = Zone.current;
      while (zone.parent) {
        zone = zone.parent;
      }
      return zone;
    }

    static get current(): AmbientZone {
      return _currentZoneFrame.zone;
    }

    static get currentTask(): Task|null {
      return _currentTask;
    }

    // tslint:disable-next-line:require-internal-with-underscore
    static __load_patch(name: string, fn: _PatchFn, ignoreDuplicate = false): void {
      if (patches.hasOwnProperty(name)) {
        // `checkDuplicate` option is defined from global variable
        // so it works for all modules.
        // `ignoreDuplicate` can work for the specified module
        if (!ignoreDuplicate && checkDuplicate) {
          throw Error('Already loaded patch: ' + name);
        }
      } else if (!global['__Zone_disable_' + name]) {
        const perfName = 'Zone:' + name;
        mark(perfName);
        patches[name] = fn(global, Zone, _api);
        performanceMeasure(perfName, perfName);
      }
    }

    public get parent(): AmbientZone|null {
      return this._parent;
    }

    public get name(): string {
      return this._name;
    }


    private _parent: Zone|null;
    private _name: string;
    private _properties: {[key: string]: any};
    private _zoneDelegate: _ZoneDelegate;

    constructor(parent: Zone|null, zoneSpec: ZoneSpec|null) {
      this._parent = parent;
      this._name = zoneSpec ? zoneSpec.name || 'unnamed' : '<root>';
      this._properties = zoneSpec && zoneSpec.properties || {};
      this._zoneDelegate =
          new _ZoneDelegate(this, this._parent && this._parent._zoneDelegate, zoneSpec);
    }

    public get(key: string): any {
      const zone: Zone = this.getZoneWith(key) as Zone;
      if (zone) return zone._properties[key];
    }

    public getZoneWith(key: string): AmbientZone|null {
      let current: Zone|null = this;
      while (current) {
        if (current._properties.hasOwnProperty(key)) {
          return current;
        }
        current = current._parent;
      }
      return null;
    }

    public fork(zoneSpec: ZoneSpec): AmbientZone {
      if (!zoneSpec) throw new Error('ZoneSpec required!');
      return this._zoneDelegate.fork(this, zoneSpec);
    }

    public wrap<T extends Function>(callback: T, source: string): T {
      if (typeof callback !== 'function') {
        throw new Error('Expecting function got: ' + callback);
      }
      const _callback = this._zoneDelegate.intercept(this, callback, source);
      const zone: Zone = this;
      return function(this: unknown) {
        return zone.runGuarded(_callback, this, <any>arguments, source);
      } as any as T;
    }

    public run(callback: Function, applyThis?: any, applyArgs?: any[], source?: string): any;
    public run<T>(
        callback: (...args: any[]) => T, applyThis?: any, applyArgs?: any[], source?: string): T {
      _currentZoneFrame = {parent: _currentZoneFrame, zone: this};
      try {
        return this._zoneDelegate.invoke(this, callback, applyThis, applyArgs, source);
      } finally {
        _currentZoneFrame = _currentZoneFrame.parent!;
      }
    }

    public runGuarded(callback: Function, applyThis?: any, applyArgs?: any[], source?: string): any;
    public runGuarded<T>(
        callback: (...args: any[]) => T, applyThis: any = null, applyArgs?: any[],
        source?: string) {
      _currentZoneFrame = {parent: _currentZoneFrame, zone: this};
      try {
        try {
          return this._zoneDelegate.invoke(this, callback, applyThis, applyArgs, source);
        } catch (error) {
          if (this._zoneDelegate.handleError(this, error)) {
            throw error;
          }
        }
      } finally {
        _currentZoneFrame = _currentZoneFrame.parent!;
      }
    }


    runTask(task: Task, applyThis?: any, applyArgs?: any): any {
      if (task.zone != this) {
        throw new Error(
            'A task can only be run in the zone of creation! (Creation: ' +
            (task.zone || NO_ZONE).name + '; Execution: ' + this.name + ')');
      }
      // https://github.com/angular/zone.js/issues/778, sometimes eventTask
      // will run in notScheduled(canceled) state, we should not try to
      // run such kind of task but just return

      if (task.state === notScheduled && (task.type === eventTask || task.type === macroTask)) {
        return;
      }

      const reEntryGuard = task.state != running;
      reEntryGuard && (task as ZoneTask<any>)._transitionTo(running, scheduled);
      task.runCount++;
      const previousTask = _currentTask;
      _currentTask = task;
      _currentZoneFrame = {parent: _currentZoneFrame, zone: this};
      try {
        if (task.type == macroTask && task.data && !task.data.isPeriodic) {
          task.cancelFn = undefined;
        }
        try {
          return this._zoneDelegate.invokeTask(this, task, applyThis, applyArgs);
        } catch (error) {
          if (this._zoneDelegate.handleError(this, error)) {
            throw error;
          }
        }
      } finally {
        // if the task's state is notScheduled or unknown, then it has already been cancelled
        // we should not reset the state to scheduled
        if (task.state !== notScheduled && task.state !== unknown) {
          if (task.type == eventTask || (task.data && task.data.isPeriodic)) {
            reEntryGuard && (task as ZoneTask<any>)._transitionTo(scheduled, running);
          } else {
            task.runCount = 0;
            this._updateTaskCount(task as ZoneTask<any>, -1);
            reEntryGuard &&
                (task as ZoneTask<any>)._transitionTo(notScheduled, running, notScheduled);
          }
        }
        _currentZoneFrame = _currentZoneFrame.parent!;
        _currentTask = previousTask;
      }
    }

    scheduleTask<T extends Task>(task: T): T {
      if (task.zone && task.zone !== this) {
        // check if the task was rescheduled, the newZone
        // should not be the children of the original zone
        let newZone: any = this;
        while (newZone) {
          if (newZone === task.zone) {
            throw Error(`can not reschedule task to ${
                this.name} which is descendants of the original zone ${task.zone.name}`);
          }
          newZone = newZone.parent;
        }
      }
      (task as any as ZoneTask<any>)._transitionTo(scheduling, notScheduled);
      const zoneDelegates: _ZoneDelegate[] = [];
      (task as any as ZoneTask<any>)._zoneDelegates = zoneDelegates;
      (task as any as ZoneTask<any>)._zone = this;
      try {
        task = this._zoneDelegate.scheduleTask(this, task) as T;
      } catch (err) {
        // should set task's state to unknown when scheduleTask throw error
        // because the err may from reschedule, so the fromState maybe notScheduled
        (task as any as ZoneTask<any>)._transitionTo(unknown, scheduling, notScheduled);
        // TODO: @JiaLiPassion, should we check the result from handleError?
        this._zoneDelegate.handleError(this, err);
        throw err;
      }
      if ((task as any as ZoneTask<any>)._zoneDelegates === zoneDelegates) {
        // we have to check because internally the delegate can reschedule the task.
        this._updateTaskCount(task as any as ZoneTask<any>, 1);
      }
      if ((task as any as ZoneTask<any>).state == scheduling) {
        (task as any as ZoneTask<any>)._transitionTo(scheduled, scheduling);
      }
      return task;
    }

    scheduleMicroTask(
        source: string, callback: Function, data?: TaskData,
        customSchedule?: (task: Task) => void): MicroTask {
      return this.scheduleTask(
          new ZoneTask(microTask, source, callback, data, customSchedule, undefined));
    }

    scheduleMacroTask(
        source: string, callback: Function, data?: TaskData, customSchedule?: (task: Task) => void,
        customCancel?: (task: Task) => void): MacroTask {
      return this.scheduleTask(
          new ZoneTask(macroTask, source, callback, data, customSchedule, customCancel));
    }

    scheduleEventTask(
        source: string, callback: Function, data?: TaskData, customSchedule?: (task: Task) => void,
        customCancel?: (task: Task) => void): EventTask {
      return this.scheduleTask(
          new ZoneTask(eventTask, source, callback, data, customSchedule, customCancel));
    }

    cancelTask(task: Task): any {
      if (task.zone != this)
        throw new Error(
            'A task can only be cancelled in the zone of creation! (Creation: ' +
            (task.zone || NO_ZONE).name + '; Execution: ' + this.name + ')');

      if (task.state !== scheduled && task.state !== running) {
        return;
      }

      (task as ZoneTask<any>)._transitionTo(canceling, scheduled, running);
      try {
        this._zoneDelegate.cancelTask(this, task);
      } catch (err) {
        // if error occurs when cancelTask, transit the state to unknown
        (task as ZoneTask<any>)._transitionTo(unknown, canceling);
        this._zoneDelegate.handleError(this, err);
        throw err;
      }
      this._updateTaskCount(task as ZoneTask<any>, -1);
      (task as ZoneTask<any>)._transitionTo(notScheduled, canceling);
      task.runCount = 0;
      return task;
    }

    private _updateTaskCount(task: ZoneTask<any>, count: number) {
      const zoneDelegates = task._zoneDelegates!;
      if (count == -1) {
        task._zoneDelegates = null;
      }
      for (let i = 0; i < zoneDelegates.length; i++) {
        zoneDelegates[i]._updateTaskCount(task.type, count);
      }
    }
  }

  const DELEGATE_ZS: ZoneSpec = {
    name: '',
    onHasTask:
        (delegate: ZoneDelegate, _: AmbientZone, target: AmbientZone, hasTaskState: HasTaskState):
            void => delegate.hasTask(target, hasTaskState),
    onScheduleTask: (delegate: ZoneDelegate, _: AmbientZone, target: AmbientZone, task: Task):
        Task => delegate.scheduleTask(target, task),
    onInvokeTask:
        (delegate: ZoneDelegate, _: AmbientZone, target: AmbientZone, task: Task, applyThis: any,
         applyArgs: any): any => delegate.invokeTask(target, task, applyThis, applyArgs),
    onCancelTask: (delegate: ZoneDelegate, _: AmbientZone, target: AmbientZone, task: Task): any =>
        delegate.cancelTask(target, task)
  };

  class _ZoneDelegate implements ZoneDelegate {
    public zone: Zone;

    private _taskCounts:
        {microTask: number,
         macroTask: number,
         eventTask: number} = {'microTask': 0, 'macroTask': 0, 'eventTask': 0};

    private _parentDelegate: _ZoneDelegate|null;

    private _forkDlgt: _ZoneDelegate|null;
    private _forkZS: ZoneSpec|null;
    private _forkCurrZone: Zone|null;

    private _interceptDlgt: _ZoneDelegate|null;
    private _interceptZS: ZoneSpec|null;
    private _interceptCurrZone: Zone|null;

    private _invokeDlgt: _ZoneDelegate|null;
    private _invokeZS: ZoneSpec|null;
    private _invokeCurrZone: Zone|null;

    private _handleErrorDlgt: _ZoneDelegate|null;
    private _handleErrorZS: ZoneSpec|null;
    private _handleErrorCurrZone: Zone|null;

    private _scheduleTaskDlgt: _ZoneDelegate|null;
    private _scheduleTaskZS: ZoneSpec|null;
    private _scheduleTaskCurrZone: Zone|null;

    private _invokeTaskDlgt: _ZoneDelegate|null;
    private _invokeTaskZS: ZoneSpec|null;
    private _invokeTaskCurrZone: Zone|null;

    private _cancelTaskDlgt: _ZoneDelegate|null;
    private _cancelTaskZS: ZoneSpec|null;
    private _cancelTaskCurrZone: Zone|null;

    private _hasTaskDlgt: _ZoneDelegate|null;
    private _hasTaskDlgtOwner: _ZoneDelegate|null;
    private _hasTaskZS: ZoneSpec|null;
    private _hasTaskCurrZone: Zone|null;

    constructor(zone: Zone, parentDelegate: _ZoneDelegate|null, zoneSpec: ZoneSpec|null) {
      this.zone = zone;
      this._parentDelegate = parentDelegate;

      this._forkZS = zoneSpec && (zoneSpec && zoneSpec.onFork ? zoneSpec : parentDelegate!._forkZS);
      this._forkDlgt = zoneSpec && (zoneSpec.onFork ? parentDelegate : parentDelegate!._forkDlgt);
      this._forkCurrZone =
          zoneSpec && (zoneSpec.onFork ? this.zone : parentDelegate!._forkCurrZone);

      this._interceptZS =
          zoneSpec && (zoneSpec.onIntercept ? zoneSpec : parentDelegate!._interceptZS);
      this._interceptDlgt =
          zoneSpec && (zoneSpec.onIntercept ? parentDelegate : parentDelegate!._interceptDlgt);
      this._interceptCurrZone =
          zoneSpec && (zoneSpec.onIntercept ? this.zone : parentDelegate!._interceptCurrZone);

      this._invokeZS = zoneSpec && (zoneSpec.onInvoke ? zoneSpec : parentDelegate!._invokeZS);
      this._invokeDlgt =
          zoneSpec && (zoneSpec.onInvoke ? parentDelegate! : parentDelegate!._invokeDlgt);
      this._invokeCurrZone =
          zoneSpec && (zoneSpec.onInvoke ? this.zone : parentDelegate!._invokeCurrZone);

      this._handleErrorZS =
          zoneSpec && (zoneSpec.onHandleError ? zoneSpec : parentDelegate!._handleErrorZS);
      this._handleErrorDlgt =
          zoneSpec && (zoneSpec.onHandleError ? parentDelegate! : parentDelegate!._handleErrorDlgt);
      this._handleErrorCurrZone =
          zoneSpec && (zoneSpec.onHandleError ? this.zone : parentDelegate!._handleErrorCurrZone);

      this._scheduleTaskZS =
          zoneSpec && (zoneSpec.onScheduleTask ? zoneSpec : parentDelegate!._scheduleTaskZS);
      this._scheduleTaskDlgt = zoneSpec &&
          (zoneSpec.onScheduleTask ? parentDelegate! : parentDelegate!._scheduleTaskDlgt);
      this._scheduleTaskCurrZone =
          zoneSpec && (zoneSpec.onScheduleTask ? this.zone : parentDelegate!._scheduleTaskCurrZone);

      this._invokeTaskZS =
          zoneSpec && (zoneSpec.onInvokeTask ? zoneSpec : parentDelegate!._invokeTaskZS);
      this._invokeTaskDlgt =
          zoneSpec && (zoneSpec.onInvokeTask ? parentDelegate! : parentDelegate!._invokeTaskDlgt);
      this._invokeTaskCurrZone =
          zoneSpec && (zoneSpec.onInvokeTask ? this.zone : parentDelegate!._invokeTaskCurrZone);

      this._cancelTaskZS =
          zoneSpec && (zoneSpec.onCancelTask ? zoneSpec : parentDelegate!._cancelTaskZS);
      this._cancelTaskDlgt =
          zoneSpec && (zoneSpec.onCancelTask ? parentDelegate! : parentDelegate!._cancelTaskDlgt);
      this._cancelTaskCurrZone =
          zoneSpec && (zoneSpec.onCancelTask ? this.zone : parentDelegate!._cancelTaskCurrZone);

      this._hasTaskZS = null;
      this._hasTaskDlgt = null;
      this._hasTaskDlgtOwner = null;
      this._hasTaskCurrZone = null;

      const zoneSpecHasTask = zoneSpec && zoneSpec.onHasTask;
      const parentHasTask = parentDelegate && parentDelegate._hasTaskZS;
      if (zoneSpecHasTask || parentHasTask) {
        // If we need to report hasTask, than this ZS needs to do ref counting on tasks. In such
        // a case all task related interceptors must go through this ZD. We can't short circuit it.
        this._hasTaskZS = zoneSpecHasTask ? zoneSpec : DELEGATE_ZS;
        this._hasTaskDlgt = parentDelegate;
        this._hasTaskDlgtOwner = this;
        this._hasTaskCurrZone = zone;
        if (!zoneSpec!.onScheduleTask) {
          this._scheduleTaskZS = DELEGATE_ZS;
          this._scheduleTaskDlgt = parentDelegate!;
          this._scheduleTaskCurrZone = this.zone;
        }
        if (!zoneSpec!.onInvokeTask) {
          this._invokeTaskZS = DELEGATE_ZS;
          this._invokeTaskDlgt = parentDelegate!;
          this._invokeTaskCurrZone = this.zone;
        }
        if (!zoneSpec!.onCancelTask) {
          this._cancelTaskZS = DELEGATE_ZS;
          this._cancelTaskDlgt = parentDelegate!;
          this._cancelTaskCurrZone = this.zone;
        }
      }
    }

    fork(targetZone: Zone, zoneSpec: ZoneSpec): AmbientZone {
      return this._forkZS ? this._forkZS.onFork!(this._forkDlgt!, this.zone, targetZone, zoneSpec) :
                            new Zone(targetZone, zoneSpec);
    }

    intercept(targetZone: Zone, callback: Function, source: string): Function {
      return this._interceptZS ?
          this._interceptZS.onIntercept!
          (this._interceptDlgt!, this._interceptCurrZone!, targetZone, callback, source) :
          callback;
    }

    invoke(
        targetZone: Zone, callback: Function, applyThis: any, applyArgs?: any[],
        source?: string): any {
      return this._invokeZS ? this._invokeZS.onInvoke!
                              (this._invokeDlgt!, this._invokeCurrZone!, targetZone, callback,
                               applyThis, applyArgs, source) :
                              callback.apply(applyThis, applyArgs);
    }

    handleError(targetZone: Zone, error: any): boolean {
      return this._handleErrorZS ?
          this._handleErrorZS.onHandleError!
          (this._handleErrorDlgt!, this._handleErrorCurrZone!, targetZone, error) :
          true;
    }

    scheduleTask(targetZone: Zone, task: Task): Task {
      let returnTask: ZoneTask<any> = task as ZoneTask<any>;
      if (this._scheduleTaskZS) {
        if (this._hasTaskZS) {
          returnTask._zoneDelegates!.push(this._hasTaskDlgtOwner!);
        }
        // clang-format off
        returnTask = this._scheduleTaskZS.onScheduleTask !(
            this._scheduleTaskDlgt !, this._scheduleTaskCurrZone !, targetZone, task) as ZoneTask<any>;
        // clang-format on
        if (!returnTask) returnTask = task as ZoneTask<any>;
      } else {
        if (task.scheduleFn) {
          task.scheduleFn(task);
        } else if (task.type == microTask) {
          scheduleMicroTask(<MicroTask>task);
        } else {
          throw new Error('Task is missing scheduleFn.');
        }
      }
      return returnTask;
    }

    invokeTask(targetZone: Zone, task: Task, applyThis: any, applyArgs?: any[]): any {
      return this._invokeTaskZS ? this._invokeTaskZS.onInvokeTask!
                                  (this._invokeTaskDlgt!, this._invokeTaskCurrZone!, targetZone,
                                   task, applyThis, applyArgs) :
                                  task.callback.apply(applyThis, applyArgs);
    }

    cancelTask(targetZone: Zone, task: Task): any {
      let value: any;
      if (this._cancelTaskZS) {
        value = this._cancelTaskZS.onCancelTask!
                (this._cancelTaskDlgt!, this._cancelTaskCurrZone!, targetZone, task);
      } else {
        if (!task.cancelFn) {
          throw Error('Task is not cancelable');
        }
        value = task.cancelFn(task);
      }
      return value;
    }

    hasTask(targetZone: Zone, isEmpty: HasTaskState) {
      // hasTask should not throw error so other ZoneDelegate
      // can still trigger hasTask callback
      try {
        this._hasTaskZS &&
            this._hasTaskZS.onHasTask!
            (this._hasTaskDlgt!, this._hasTaskCurrZone!, targetZone, isEmpty);
      } catch (err) {
        this.handleError(targetZone, err);
      }
    }

    // tslint:disable-next-line:require-internal-with-underscore
    _updateTaskCount(type: TaskType, count: number) {
      const counts = this._taskCounts;
      const prev = counts[type];
      const next = counts[type] = prev + count;
      if (next < 0) {
        throw new Error('More tasks executed then were scheduled.');
      }
      if (prev == 0 || next == 0) {
        const isEmpty: HasTaskState = {
          microTask: counts['microTask'] > 0,
          macroTask: counts['macroTask'] > 0,
          eventTask: counts['eventTask'] > 0,
          change: type
        };
        this.hasTask(this.zone, isEmpty);
      }
    }
  }

  class ZoneTask<T extends TaskType> implements Task {
    public type: T;
    public source: string;
    public invoke: Function;
    public callback: Function;
    public data: TaskData|undefined;
    public scheduleFn: ((task: Task) => void)|undefined;
    public cancelFn: ((task: Task) => void)|undefined;
    // tslint:disable-next-line:require-internal-with-underscore
    _zone: Zone|null = null;
    public runCount: number = 0;
    // tslint:disable-next-line:require-internal-with-underscore
    _zoneDelegates: _ZoneDelegate[]|null = null;
    // tslint:disable-next-line:require-internal-with-underscore
    _state: TaskState = 'notScheduled';

    constructor(
        type: T, source: string, callback: Function, options: TaskData|undefined,
        scheduleFn: ((task: Task) => void)|undefined, cancelFn: ((task: Task) => void)|undefined) {
      this.type = type;
      this.source = source;
      this.data = options;
      this.scheduleFn = scheduleFn;
      this.cancelFn = cancelFn;
      if (!callback) {
        throw new Error('callback is not defined');
      }
      this.callback = callback;
      const self = this;
      // TODO: @JiaLiPassion options should have interface
      if (type === eventTask && options && (options as any).useG) {
        this.invoke = ZoneTask.invokeTask;
      } else {
        this.invoke = function() {
          return ZoneTask.invokeTask.call(global, self, this, <any>arguments);
        };
      }
    }

    static invokeTask(task: any, target: any, args: any): any {
      if (!task) {
        task = this;
      }
      _numberOfNestedTaskFrames++;
      try {
        task.runCount++;
        return task.zone.runTask(task, target, args);
      } finally {
        if (_numberOfNestedTaskFrames == 1) {
          drainMicroTaskQueue();
        }
        _numberOfNestedTaskFrames--;
      }
    }

    get zone(): Zone {
      return this._zone!;
    }

    get state(): TaskState {
      return this._state;
    }

    public cancelScheduleRequest() {
      this._transitionTo(notScheduled, scheduling);
    }

    // tslint:disable-next-line:require-internal-with-underscore
    _transitionTo(toState: TaskState, fromState1: TaskState, fromState2?: TaskState) {
      if (this._state === fromState1 || this._state === fromState2) {
        this._state = toState;
        if (toState == notScheduled) {
          this._zoneDelegates = null;
        }
      } else {
        throw new Error(`${this.type} '${this.source}': can not transition to '${
            toState}', expecting state '${fromState1}'${
            fromState2 ? ' or \'' + fromState2 + '\'' : ''}, was '${this._state}'.`);
      }
    }

    public toString() {
      if (this.data && typeof this.data.handleId !== 'undefined') {
        return this.data.handleId.toString();
      } else {
        return Object.prototype.toString.call(this);
      }
    }

    // add toJSON method to prevent cyclic error when
    // call JSON.stringify(zoneTask)
    public toJSON() {
      return {
        type: this.type,
        state: this.state,
        source: this.source,
        zone: this.zone.name,
        runCount: this.runCount
      };
    }
  }


  //////////////////////////////////////////////////////
  //////////////////////////////////////////////////////
  ///  MICROTASK QUEUE
  //////////////////////////////////////////////////////
  //////////////////////////////////////////////////////
  const symbolSetTimeout = __symbol__('setTimeout');
  const symbolPromise = __symbol__('Promise');
  const symbolThen = __symbol__('then');
  let _microTaskQueue: Task[] = [];
  let _isDrainingMicrotaskQueue: boolean = false;
  let nativeMicroTaskQueuePromise: any;

  function nativeScheduleMicroTask(func: Function) {
    if (!nativeMicroTaskQueuePromise) {
      if (global[symbolPromise]) {
        nativeMicroTaskQueuePromise = global[symbolPromise].resolve(0);
      }
    }
    if (nativeMicroTaskQueuePromise) {
      let nativeThen = nativeMicroTaskQueuePromise[symbolThen];
      if (!nativeThen) {
        // native Promise is not patchable, we need to use `then` directly
        // issue 1078
        nativeThen = nativeMicroTaskQueuePromise['then'];
      }
      nativeThen.call(nativeMicroTaskQueuePromise, func);
    } else {
      global[symbolSetTimeout](func, 0);
    }
  }

  function scheduleMicroTask(task?: MicroTask) {
    // if we are not running in any task, and there has not been anything scheduled
    // we must bootstrap the initial task creation by manually scheduling the drain
    if (_numberOfNestedTaskFrames === 0 && _microTaskQueue.length === 0) {
      // We are not running in Task, so we need to kickstart the microtask queue.
      nativeScheduleMicroTask(drainMicroTaskQueue);
    }
    task && _microTaskQueue.push(task);
  }

  function drainMicroTaskQueue() {
    if (!_isDrainingMicrotaskQueue) {
      _isDrainingMicrotaskQueue = true;
      while (_microTaskQueue.length) {
        const queue = _microTaskQueue;
        _microTaskQueue = [];
        for (let i = 0; i < queue.length; i++) {
          const task = queue[i];
          try {
            task.zone.runTask(task, null, null);
          } catch (error) {
            _api.onUnhandledError(error as Error);
          }
        }
      }
      _api.microtaskDrainDone();
      _isDrainingMicrotaskQueue = false;
    }
  }

  //////////////////////////////////////////////////////
  //////////////////////////////////////////////////////
  ///  BOOTSTRAP
  //////////////////////////////////////////////////////
  //////////////////////////////////////////////////////


  const NO_ZONE = {name: 'NO ZONE'};
  const notScheduled: 'notScheduled' = 'notScheduled', scheduling: 'scheduling' = 'scheduling',
                      scheduled: 'scheduled' = 'scheduled', running: 'running' = 'running',
                      canceling: 'canceling' = 'canceling', unknown: 'unknown' = 'unknown';
  const microTask: 'microTask' = 'microTask', macroTask: 'macroTask' = 'macroTask',
                   eventTask: 'eventTask' = 'eventTask';

  const patches: {[key: string]: any} = {};
  const _api: _ZonePrivate = {
    symbol: __symbol__,
    currentZoneFrame: () => _currentZoneFrame,
    onUnhandledError: noop,
    microtaskDrainDone: noop,
    scheduleMicroTask: scheduleMicroTask,
    showUncaughtError: () => !(Zone as any)[__symbol__('ignoreConsoleErrorUncaughtError')],
    patchEventTarget: () => [],
    patchOnProperties: noop,
    patchMethod: () => noop,
    bindArguments: () => [],
    patchThen: () => noop,
    patchMacroTask: () => noop,
    patchEventPrototype: () => noop,
    isIEOrEdge: () => false,
    getGlobalObjects: () => undefined,
    ObjectDefineProperty: () => noop,
    ObjectGetOwnPropertyDescriptor: () => undefined,
    ObjectCreate: () => undefined,
    ArraySlice: () => [],
    patchClass: () => noop,
    wrapWithCurrentZone: () => noop,
    filterProperties: () => [],
    attachOriginToPatched: () => noop,
    _redefineProperty: () => noop,
    patchCallbacks: () => noop,
    nativeScheduleMicroTask: nativeScheduleMicroTask
  };
  let _currentZoneFrame: _ZoneFrame = {parent: null, zone: new Zone(null, null)};
  let _currentTask: Task|null = null;
  let _numberOfNestedTaskFrames = 0;

  function noop() {}

  performanceMeasure('Zone', 'Zone');
  return global['Zone'] = Zone;
})(typeof window !== 'undefined' && window || typeof self !== 'undefined' && self || global);
