some progress

node_modules/mongoose/lib/cursor/AggregationCursor.js

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+/*!
+ * Module dependencies.
+ */
+
+'use strict';
+
+const MongooseError = require('../error/mongooseError');
+const Readable = require('stream').Readable;
+const eachAsync = require('../helpers/cursor/eachAsync');
+const immediate = require('../helpers/immediate');
+const util = require('util');
+
+/**
+ * An AggregationCursor is a concurrency primitive for processing aggregation
+ * results one document at a time. It is analogous to QueryCursor.
+ *
+ * An AggregationCursor fulfills the Node.js streams3 API,
+ * in addition to several other mechanisms for loading documents from MongoDB
+ * one at a time.
+ *
+ * Creating an AggregationCursor executes the model's pre aggregate hooks,
+ * but **not** the model's post aggregate hooks.
+ *
+ * Unless you're an advanced user, do **not** instantiate this class directly.
+ * Use [`Aggregate#cursor()`](/docs/api/aggregate.html#aggregate_Aggregate-cursor) instead.
+ *
+ * @param {Aggregate} agg
+ * @inherits Readable https://nodejs.org/api/stream.html#class-streamreadable
+ * @event `cursor`: Emitted when the cursor is created
+ * @event `error`: Emitted when an error occurred
+ * @event `data`: Emitted when the stream is flowing and the next doc is ready
+ * @event `end`: Emitted when the stream is exhausted
+ * @api public
+ */
+
+function AggregationCursor(agg) {
+  // set autoDestroy=true because on node 12 it's by default false
+  // gh-10902 need autoDestroy to destroy correctly and emit 'close' event
+  Readable.call(this, { autoDestroy: true, objectMode: true });
+
+  this.cursor = null;
+  this.agg = agg;
+  this._transforms = [];
+  const model = agg._model;
+  delete agg.options.cursor.useMongooseAggCursor;
+  this._mongooseOptions = {};
+
+  _init(model, this, agg);
+}
+
+util.inherits(AggregationCursor, Readable);
+
+/*!
+ * ignore
+ */
+
+function _init(model, c, agg) {
+  if (!model.collection.buffer) {
+    model.hooks.execPre('aggregate', agg, function() {
+      c.cursor = model.collection.aggregate(agg._pipeline, agg.options || {});
+      c.emit('cursor', c.cursor);
+    });
+  } else {
+    model.collection.emitter.once('queue', function() {
+      model.hooks.execPre('aggregate', agg, function() {
+        c.cursor = model.collection.aggregate(agg._pipeline, agg.options || {});
+        c.emit('cursor', c.cursor);
+      });
+    });
+  }
+}
+
+/**
+ * Necessary to satisfy the Readable API
+ * @method _read
+ * @memberOf AggregationCursor
+ * @instance
+ * @api private
+ */
+
+AggregationCursor.prototype._read = function() {
+  const _this = this;
+  _next(this, function(error, doc) {
+    if (error) {
+      return _this.emit('error', error);
+    }
+    if (!doc) {
+      _this.push(null);
+      _this.cursor.close(function(error) {
+        if (error) {
+          return _this.emit('error', error);
+        }
+      });
+      return;
+    }
+    _this.push(doc);
+  });
+};
+
+if (Symbol.asyncIterator != null) {
+  const msg = 'Mongoose does not support using async iterators with an ' +
+    'existing aggregation cursor. See https://bit.ly/mongoose-async-iterate-aggregation';
+
+  AggregationCursor.prototype[Symbol.asyncIterator] = function() {
+    throw new MongooseError(msg);
+  };
+}
+
+/**
+ * Registers a transform function which subsequently maps documents retrieved
+ * via the streams interface or `.next()`
+ *
+ * #### Example:
+ *
+ *     // Map documents returned by `data` events
+ *     Thing.
+ *       find({ name: /^hello/ }).
+ *       cursor().
+ *       map(function (doc) {
+ *        doc.foo = "bar";
+ *        return doc;
+ *       })
+ *       on('data', function(doc) { console.log(doc.foo); });
+ *
+ *     // Or map documents returned by `.next()`
+ *     const cursor = Thing.find({ name: /^hello/ }).
+ *       cursor().
+ *       map(function (doc) {
+ *         doc.foo = "bar";
+ *         return doc;
+ *       });
+ *     cursor.next(function(error, doc) {
+ *       console.log(doc.foo);
+ *     });
+ *
+ * @param {Function} fn
+ * @return {AggregationCursor}
+ * @memberOf AggregationCursor
+ * @api public
+ * @method map
+ */
+
+Object.defineProperty(AggregationCursor.prototype, 'map', {
+  value: function(fn) {
+    this._transforms.push(fn);
+    return this;
+  },
+  enumerable: true,
+  configurable: true,
+  writable: true
+});
+
+/**
+ * Marks this cursor as errored
+ * @method _markError
+ * @instance
+ * @memberOf AggregationCursor
+ * @api private
+ */
+
+AggregationCursor.prototype._markError = function(error) {
+  this._error = error;
+  return this;
+};
+
+/**
+ * Marks this cursor as closed. Will stop streaming and subsequent calls to
+ * `next()` will error.
+ *
+ * @param {Function} callback
+ * @return {Promise}
+ * @api public
+ * @method close
+ * @emits close
+ * @see AggregationCursor.close https://mongodb.github.io/node-mongodb-native/4.9/classes/AggregationCursor.html#close
+ */
+
+AggregationCursor.prototype.close = async function close() {
+  if (typeof arguments[0] === 'function') {
+    throw new MongooseError('AggregationCursor.prototype.close() no longer accepts a callback');
+  }
+  try {
+    await this.cursor.close();
+  } catch (error) {
+    this.listeners('error').length > 0 && this.emit('error', error);
+    throw error;
+  }
+  this.emit('close');
+};
+
+/**
+ * Get the next document from this cursor. Will return `null` when there are
+ * no documents left.
+ *
+ * @return {Promise}
+ * @api public
+ * @method next
+ */
+
+AggregationCursor.prototype.next = async function next() {
+  if (typeof arguments[0] === 'function') {
+    throw new MongooseError('AggregationCursor.prototype.next() no longer accepts a callback');
+  }
+  return new Promise((resolve, reject) => {
+    _next(this, (err, res) => {
+      if (err != null) {
+        return reject(err);
+      }
+      resolve(res);
+    });
+  });
+};
+
+/**
+ * Execute `fn` for every document in the cursor. If `fn` returns a promise,
+ * will wait for the promise to resolve before iterating on to the next one.
+ * Returns a promise that resolves when done.
+ *
+ * @param {Function} fn
+ * @param {Object} [options]
+ * @param {Number} [options.parallel] the number of promises to execute in parallel. Defaults to 1.
+ * @param {Function} [callback] executed when all docs have been processed
+ * @return {Promise}
+ * @api public
+ * @method eachAsync
+ */
+
+AggregationCursor.prototype.eachAsync = function(fn, opts, callback) {
+  const _this = this;
+  if (typeof opts === 'function') {
+    callback = opts;
+    opts = {};
+  }
+  opts = opts || {};
+
+  return eachAsync(function(cb) { return _next(_this, cb); }, fn, opts, callback);
+};
+
+/**
+ * Returns an asyncIterator for use with [`for/await/of` loops](https://thecodebarbarian.com/getting-started-with-async-iterators-in-node-js)
+ * You do not need to call this function explicitly, the JavaScript runtime
+ * will call it for you.
+ *
+ * #### Example:
+ *
+ *     // Async iterator without explicitly calling `cursor()`. Mongoose still
+ *     // creates an AggregationCursor instance internally.
+ *     const agg = Model.aggregate([{ $match: { age: { $gte: 25 } } }]);
+ *     for await (const doc of agg) {
+ *       console.log(doc.name);
+ *     }
+ *
+ *     // You can also use an AggregationCursor instance for async iteration
+ *     const cursor = Model.aggregate([{ $match: { age: { $gte: 25 } } }]).cursor();
+ *     for await (const doc of cursor) {
+ *       console.log(doc.name);
+ *     }
+ *
+ * Node.js 10.x supports async iterators natively without any flags. You can
+ * enable async iterators in Node.js 8.x using the [`--harmony_async_iteration` flag](https://github.com/tc39/proposal-async-iteration/issues/117#issuecomment-346695187).
+ *
+ * **Note:** This function is not set if `Symbol.asyncIterator` is undefined. If
+ * `Symbol.asyncIterator` is undefined, that means your Node.js version does not
+ * support async iterators.
+ *
+ * @method [Symbol.asyncIterator]
+ * @memberOf AggregationCursor
+ * @instance
+ * @api public
+ */
+
+if (Symbol.asyncIterator != null) {
+  AggregationCursor.prototype[Symbol.asyncIterator] = function() {
+    return this.transformNull()._transformForAsyncIterator();
+  };
+}
+
+/*!
+ * ignore
+ */
+
+AggregationCursor.prototype._transformForAsyncIterator = function() {
+  if (this._transforms.indexOf(_transformForAsyncIterator) === -1) {
+    this.map(_transformForAsyncIterator);
+  }
+  return this;
+};
+
+/*!
+ * ignore
+ */
+
+AggregationCursor.prototype.transformNull = function(val) {
+  if (arguments.length === 0) {
+    val = true;
+  }
+  this._mongooseOptions.transformNull = val;
+  return this;
+};
+
+/*!
+ * ignore
+ */
+
+function _transformForAsyncIterator(doc) {
+  return doc == null ? { done: true } : { value: doc, done: false };
+}
+
+/**
+ * Adds a [cursor flag](https://mongodb.github.io/node-mongodb-native/4.9/classes/AggregationCursor.html#addCursorFlag).
+ * Useful for setting the `noCursorTimeout` and `tailable` flags.
+ *
+ * @param {String} flag
+ * @param {Boolean} value
+ * @return {AggregationCursor} this
+ * @api public
+ * @method addCursorFlag
+ */
+
+AggregationCursor.prototype.addCursorFlag = function(flag, value) {
+  const _this = this;
+  _waitForCursor(this, function() {
+    _this.cursor.addCursorFlag(flag, value);
+  });
+  return this;
+};
+
+/*!
+ * ignore
+ */
+
+function _waitForCursor(ctx, cb) {
+  if (ctx.cursor) {
+    return cb();
+  }
+  ctx.once('cursor', function() {
+    cb();
+  });
+}
+
+/**
+ * Get the next doc from the underlying cursor and mongooseify it
+ * (populate, etc.)
+ * @param {Any} ctx
+ * @param {Function} cb
+ * @api private
+ */
+
+function _next(ctx, cb) {
+  let callback = cb;
+  if (ctx._transforms.length) {
+    callback = function(err, doc) {
+      if (err || (doc === null && !ctx._mongooseOptions.transformNull)) {
+        return cb(err, doc);
+      }
+      cb(err, ctx._transforms.reduce(function(doc, fn) {
+        return fn(doc);
+      }, doc));
+    };
+  }
+
+  if (ctx._error) {
+    return immediate(function() {
+      callback(ctx._error);
+    });
+  }
+
+  if (ctx.cursor) {
+    return ctx.cursor.next().then(
+      doc => {
+        if (!doc) {
+          return callback(null, null);
+        }
+
+        callback(null, doc);
+      },
+      err => callback(err)
+    );
+  } else {
+    ctx.once('cursor', function() {
+      _next(ctx, cb);
+    });
+  }
+}
+
+module.exports = AggregationCursor;

node_modules/mongoose/lib/cursor/ChangeStream.js

@@ -0,0 +1,151 @@
+'use strict';
+
+/*!
+ * Module dependencies.
+ */
+
+const EventEmitter = require('events').EventEmitter;
+
+/*!
+ * ignore
+ */
+
+class ChangeStream extends EventEmitter {
+  constructor(changeStreamThunk, pipeline, options) {
+    super();
+
+    this.driverChangeStream = null;
+    this.closed = false;
+    this.bindedEvents = false;
+    this.pipeline = pipeline;
+    this.options = options;
+
+    if (options && options.hydrate && !options.model) {
+      throw new Error(
+        'Cannot create change stream with `hydrate: true` ' +
+        'unless calling `Model.watch()`'
+      );
+    }
+
+    // This wrapper is necessary because of buffering.
+    changeStreamThunk((err, driverChangeStream) => {
+      if (err != null) {
+        this.emit('error', err);
+        return;
+      }
+
+      this.driverChangeStream = driverChangeStream;
+      this.emit('ready');
+    });
+  }
+
+  _bindEvents() {
+    if (this.bindedEvents) {
+      return;
+    }
+
+    this.bindedEvents = true;
+
+    if (this.driverChangeStream == null) {
+      this.once('ready', () => {
+        this.driverChangeStream.on('close', () => {
+          this.closed = true;
+        });
+
+        ['close', 'change', 'end', 'error'].forEach(ev => {
+          this.driverChangeStream.on(ev, data => {
+            // Sometimes Node driver still polls after close, so
+            // avoid any uncaught exceptions due to closed change streams
+            // See tests for gh-7022
+            if (ev === 'error' && this.closed) {
+              return;
+            }
+            if (data != null && data.fullDocument != null && this.options && this.options.hydrate) {
+              data.fullDocument = this.options.model.hydrate(data.fullDocument);
+            }
+            this.emit(ev, data);
+          });
+        });
+      });
+
+      return;
+    }
+
+    this.driverChangeStream.on('close', () => {
+      this.closed = true;
+    });
+
+    ['close', 'change', 'end', 'error'].forEach(ev => {
+      this.driverChangeStream.on(ev, data => {
+        // Sometimes Node driver still polls after close, so
+        // avoid any uncaught exceptions due to closed change streams
+        // See tests for gh-7022
+        if (ev === 'error' && this.closed) {
+          return;
+        }
+        this.emit(ev, data);
+      });
+    });
+  }
+
+  hasNext(cb) {
+    return this.driverChangeStream.hasNext(cb);
+  }
+
+  next(cb) {
+    if (this.options && this.options.hydrate) {
+      if (cb != null) {
+        const originalCb = cb;
+        cb = (err, data) => {
+          if (err != null) {
+            return originalCb(err);
+          }
+          if (data.fullDocument != null) {
+            data.fullDocument = this.options.model.hydrate(data.fullDocument);
+          }
+          return originalCb(null, data);
+        };
+      }
+
+      let maybePromise = this.driverChangeStream.next(cb);
+      if (maybePromise && typeof maybePromise.then === 'function') {
+        maybePromise = maybePromise.then(data => {
+          if (data.fullDocument != null) {
+            data.fullDocument = this.options.model.hydrate(data.fullDocument);
+          }
+          return data;
+        });
+      }
+      return maybePromise;
+    }
+
+    return this.driverChangeStream.next(cb);
+  }
+
+  on(event, handler) {
+    this._bindEvents();
+    return super.on(event, handler);
+  }
+
+  once(event, handler) {
+    this._bindEvents();
+    return super.once(event, handler);
+  }
+
+  _queue(cb) {
+    this.once('ready', () => cb());
+  }
+
+  close() {
+    this.closed = true;
+    if (this.driverChangeStream) {
+      this.driverChangeStream.close();
+    }
+  }
+}
+
+/*!
+ * ignore
+ */
+
+module.exports = ChangeStream;

node_modules/mongoose/lib/cursor/QueryCursor.js

@@ -0,0 +1,554 @@
+/*!
+ * Module dependencies.
+ */
+
+'use strict';
+
+const MongooseError = require('../error/mongooseError');
+const Readable = require('stream').Readable;
+const eachAsync = require('../helpers/cursor/eachAsync');
+const helpers = require('../queryhelpers');
+const immediate = require('../helpers/immediate');
+const util = require('util');
+
+/**
+ * A QueryCursor is a concurrency primitive for processing query results
+ * one document at a time. A QueryCursor fulfills the Node.js streams3 API,
+ * in addition to several other mechanisms for loading documents from MongoDB
+ * one at a time.
+ *
+ * QueryCursors execute the model's pre `find` hooks before loading any documents
+ * from MongoDB, and the model's post `find` hooks after loading each document.
+ *
+ * Unless you're an advanced user, do **not** instantiate this class directly.
+ * Use [`Query#cursor()`](/docs/api/query.html#query_Query-cursor) instead.
+ *
+ * @param {Query} query
+ * @param {Object} options query options passed to `.find()`
+ * @inherits Readable https://nodejs.org/api/stream.html#class-streamreadable
+ * @event `cursor`: Emitted when the cursor is created
+ * @event `error`: Emitted when an error occurred
+ * @event `data`: Emitted when the stream is flowing and the next doc is ready
+ * @event `end`: Emitted when the stream is exhausted
+ * @api public
+ */
+
+function QueryCursor(query, options) {
+  // set autoDestroy=true because on node 12 it's by default false
+  // gh-10902 need autoDestroy to destroy correctly and emit 'close' event
+  Readable.call(this, { autoDestroy: true, objectMode: true });
+
+  this.cursor = null;
+  this.query = query;
+  const _this = this;
+  const model = query.model;
+  this._mongooseOptions = {};
+  this._transforms = [];
+  this.model = model;
+  this.options = options || {};
+
+  model.hooks.execPre('find', query, (err) => {
+    if (err != null) {
+      _this._markError(err);
+      _this.listeners('error').length > 0 && _this.emit('error', err);
+      return;
+    }
+    this._transforms = this._transforms.concat(query._transforms.slice());
+    if (this.options.transform) {
+      this._transforms.push(options.transform);
+    }
+    // Re: gh-8039, you need to set the `cursor.batchSize` option, top-level
+    // `batchSize` option doesn't work.
+    if (this.options.batchSize) {
+      this.options.cursor = options.cursor || {};
+      this.options.cursor.batchSize = options.batchSize;
+
+      // Max out the number of documents we'll populate in parallel at 5000.
+      this.options._populateBatchSize = Math.min(this.options.batchSize, 5000);
+    }
+    model.collection.find(query._conditions, this.options, (err, cursor) => {
+      if (err != null) {
+        _this._markError(err);
+        _this.listeners('error').length > 0 && _this.emit('error', _this._error);
+        return;
+      }
+
+      if (_this._error) {
+        cursor.close(function() {});
+        _this.listeners('error').length > 0 && _this.emit('error', _this._error);
+      }
+      _this.cursor = cursor;
+      _this.emit('cursor', cursor);
+    });
+  });
+}
+
+util.inherits(QueryCursor, Readable);
+
+/**
+ * Necessary to satisfy the Readable API
+ * @method _read
+ * @memberOf QueryCursor
+ * @instance
+ * @api private
+ */
+
+QueryCursor.prototype._read = function() {
+  const _this = this;
+  _next(this, function(error, doc) {
+    if (error) {
+      return _this.emit('error', error);
+    }
+    if (!doc) {
+      _this.push(null);
+      _this.cursor.close(function(error) {
+        if (error) {
+          return _this.emit('error', error);
+        }
+      });
+      return;
+    }
+    _this.push(doc);
+  });
+};
+
+/**
+ * Registers a transform function which subsequently maps documents retrieved
+ * via the streams interface or `.next()`
+ *
+ * #### Example:
+ *
+ *     // Map documents returned by `data` events
+ *     Thing.
+ *       find({ name: /^hello/ }).
+ *       cursor().
+ *       map(function (doc) {
+ *        doc.foo = "bar";
+ *        return doc;
+ *       })
+ *       on('data', function(doc) { console.log(doc.foo); });
+ *
+ *     // Or map documents returned by `.next()`
+ *     const cursor = Thing.find({ name: /^hello/ }).
+ *       cursor().
+ *       map(function (doc) {
+ *         doc.foo = "bar";
+ *         return doc;
+ *       });
+ *     cursor.next(function(error, doc) {
+ *       console.log(doc.foo);
+ *     });
+ *
+ * @param {Function} fn
+ * @return {QueryCursor}
+ * @memberOf QueryCursor
+ * @api public
+ * @method map
+ */
+
+Object.defineProperty(QueryCursor.prototype, 'map', {
+  value: function(fn) {
+    this._transforms.push(fn);
+    return this;
+  },
+  enumerable: true,
+  configurable: true,
+  writable: true
+});
+
+/**
+ * Marks this cursor as errored
+ * @method _markError
+ * @memberOf QueryCursor
+ * @instance
+ * @api private
+ */
+
+QueryCursor.prototype._markError = function(error) {
+  this._error = error;
+  return this;
+};
+
+/**
+ * Marks this cursor as closed. Will stop streaming and subsequent calls to
+ * `next()` will error.
+ *
+ * @return {Promise}
+ * @api public
+ * @method close
+ * @emits close
+ * @see AggregationCursor.close https://mongodb.github.io/node-mongodb-native/4.9/classes/AggregationCursor.html#close
+ */
+
+QueryCursor.prototype.close = async function close() {
+  if (typeof arguments[0] === 'function') {
+    throw new MongooseError('QueryCursor.prototype.close() no longer accepts a callback');
+  }
+  try {
+    await this.cursor.close();
+    this.emit('close');
+  } catch (error) {
+    this.listeners('error').length > 0 && this.emit('error', error);
+    throw error;
+  }
+};
+
+/**
+ * Rewind this cursor to its uninitialized state. Any options that are present on the cursor will
+ * remain in effect. Iterating this cursor will cause new queries to be sent to the server, even
+ * if the resultant data has already been retrieved by this cursor.
+ *
+ * @return {AggregationCursor} this
+ * @api public
+ * @method rewind
+ */
+
+QueryCursor.prototype.rewind = function() {
+  const _this = this;
+  _waitForCursor(this, function() {
+    _this.cursor.rewind();
+  });
+  return this;
+};
+
+/**
+ * Get the next document from this cursor. Will return `null` when there are
+ * no documents left.
+ *
+ * @return {Promise}
+ * @api public
+ * @method next
+ */
+
+QueryCursor.prototype.next = async function next() {
+  if (arguments[0] === 'function') {
+    throw new MongooseError('QueryCursor.prototype.next() no longer accepts a callback');
+  }
+  return new Promise((resolve, reject) => {
+    _next(this, function(error, doc) {
+      if (error) {
+        return reject(error);
+      }
+      resolve(doc);
+    });
+  });
+};
+
+/**
+ * Execute `fn` for every document in the cursor. If `fn` returns a promise,
+ * will wait for the promise to resolve before iterating on to the next one.
+ * Returns a promise that resolves when done.
+ *
+ * #### Example:
+ *
+ *     // Iterate over documents asynchronously
+ *     Thing.
+ *       find({ name: /^hello/ }).
+ *       cursor().
+ *       eachAsync(async function (doc, i) {
+ *         doc.foo = doc.bar + i;
+ *         await doc.save();
+ *       })
+ *
+ * @param {Function} fn
+ * @param {Object} [options]
+ * @param {Number} [options.parallel] the number of promises to execute in parallel. Defaults to 1.
+ * @param {Number} [options.batchSize] if set, will call `fn()` with arrays of documents with length at most `batchSize`
+ * @param {Boolean} [options.continueOnError=false] if true, `eachAsync()` iterates through all docs even if `fn` throws an error. If false, `eachAsync()` throws an error immediately if the given function `fn()` throws an error.
+ * @param {Function} [callback] executed when all docs have been processed
+ * @return {Promise}
+ * @api public
+ * @method eachAsync
+ */
+
+QueryCursor.prototype.eachAsync = function(fn, opts, callback) {
+  const _this = this;
+  if (typeof opts === 'function') {
+    callback = opts;
+    opts = {};
+  }
+  opts = opts || {};
+
+  return eachAsync(function(cb) { return _next(_this, cb); }, fn, opts, callback);
+};
+
+/**
+ * The `options` passed in to the `QueryCursor` constructor.
+ *
+ * @api public
+ * @property options
+ */
+
+QueryCursor.prototype.options;
+
+/**
+ * Adds a [cursor flag](https://mongodb.github.io/node-mongodb-native/4.9/classes/FindCursor.html#addCursorFlag).
+ * Useful for setting the `noCursorTimeout` and `tailable` flags.
+ *
+ * @param {String} flag
+ * @param {Boolean} value
+ * @return {AggregationCursor} this
+ * @api public
+ * @method addCursorFlag
+ */
+
+QueryCursor.prototype.addCursorFlag = function(flag, value) {
+  const _this = this;
+  _waitForCursor(this, function() {
+    _this.cursor.addCursorFlag(flag, value);
+  });
+  return this;
+};
+
+/*!
+ * ignore
+ */
+
+QueryCursor.prototype.transformNull = function(val) {
+  if (arguments.length === 0) {
+    val = true;
+  }
+  this._mongooseOptions.transformNull = val;
+  return this;
+};
+
+/*!
+ * ignore
+ */
+
+QueryCursor.prototype._transformForAsyncIterator = function() {
+  if (this._transforms.indexOf(_transformForAsyncIterator) === -1) {
+    this.map(_transformForAsyncIterator);
+  }
+  return this;
+};
+
+/**
+ * Returns an asyncIterator for use with [`for/await/of` loops](https://thecodebarbarian.com/getting-started-with-async-iterators-in-node-js).
+ * You do not need to call this function explicitly, the JavaScript runtime
+ * will call it for you.
+ *
+ * #### Example:
+ *
+ *     // Works without using `cursor()`
+ *     for await (const doc of Model.find([{ $sort: { name: 1 } }])) {
+ *       console.log(doc.name);
+ *     }
+ *
+ *     // Can also use `cursor()`
+ *     for await (const doc of Model.find([{ $sort: { name: 1 } }]).cursor()) {
+ *       console.log(doc.name);
+ *     }
+ *
+ * Node.js 10.x supports async iterators natively without any flags. You can
+ * enable async iterators in Node.js 8.x using the [`--harmony_async_iteration` flag](https://github.com/tc39/proposal-async-iteration/issues/117#issuecomment-346695187).
+ *
+ * **Note:** This function is not if `Symbol.asyncIterator` is undefined. If
+ * `Symbol.asyncIterator` is undefined, that means your Node.js version does not
+ * support async iterators.
+ *
+ * @method [Symbol.asyncIterator]
+ * @memberOf QueryCursor
+ * @instance
+ * @api public
+ */
+
+if (Symbol.asyncIterator != null) {
+  QueryCursor.prototype[Symbol.asyncIterator] = function() {
+    return this.transformNull()._transformForAsyncIterator();
+  };
+}
+
+/*!
+ * ignore
+ */
+
+function _transformForAsyncIterator(doc) {
+  return doc == null ? { done: true } : { value: doc, done: false };
+}
+
+/**
+ * Get the next doc from the underlying cursor and mongooseify it
+ * (populate, etc.)
+ * @param {Any} ctx
+ * @param {Function} cb
+ * @api private
+ */
+
+function _next(ctx, cb) {
+  let callback = cb;
+  if (ctx._transforms.length) {
+    callback = function(err, doc) {
+      if (err || (doc === null && !ctx._mongooseOptions.transformNull)) {
+        return cb(err, doc);
+      }
+      cb(err, ctx._transforms.reduce(function(doc, fn) {
+        return fn.call(ctx, doc);
+      }, doc));
+    };
+  }
+
+  if (ctx._error) {
+    return immediate(function() {
+      callback(ctx._error);
+    });
+  }
+
+  if (ctx.cursor) {
+    if (ctx.query._mongooseOptions.populate && !ctx._pop) {
+      ctx._pop = helpers.preparePopulationOptionsMQ(ctx.query,
+        ctx.query._mongooseOptions);
+      ctx._pop.__noPromise = true;
+    }
+    if (ctx.query._mongooseOptions.populate && ctx.options._populateBatchSize > 1) {
+      if (ctx._batchDocs && ctx._batchDocs.length) {
+        // Return a cached populated doc
+        return _nextDoc(ctx, ctx._batchDocs.shift(), ctx._pop, callback);
+      } else if (ctx._batchExhausted) {
+        // Internal cursor reported no more docs. Act the same here
+        return callback(null, null);
+      } else {
+        // Request as many docs as batchSize, to populate them also in batch
+        ctx._batchDocs = [];
+        ctx.cursor.next().then(
+          res => { _onNext.call({ ctx, callback }, null, res); },
+          err => { _onNext.call({ ctx, callback }, err); }
+        );
+        return;
+      }
+    } else {
+      return ctx.cursor.next().then(
+        doc => {
+          if (!doc) {
+            callback(null, null);
+            return;
+          }
+
+          if (!ctx.query._mongooseOptions.populate) {
+            return _nextDoc(ctx, doc, null, callback);
+          }
+
+          ctx.query.model.populate(doc, ctx._pop).then(
+            doc => {
+              _nextDoc(ctx, doc, ctx._pop, callback);
+            },
+            err => {
+              callback(err);
+            }
+          );
+        },
+        error => {
+          callback(error);
+        }
+      );
+    }
+  } else {
+    ctx.once('error', cb);
+
+    ctx.once('cursor', function(cursor) {
+      ctx.removeListener('error', cb);
+      if (cursor == null) {
+        return;
+      }
+      _next(ctx, cb);
+    });
+  }
+}
+
+/*!
+ * ignore
+ */
+
+function _onNext(error, doc) {
+  if (error) {
+    return this.callback(error);
+  }
+  if (!doc) {
+    this.ctx._batchExhausted = true;
+    return _populateBatch.call(this);
+  }
+
+  this.ctx._batchDocs.push(doc);
+
+  if (this.ctx._batchDocs.length < this.ctx.options._populateBatchSize) {
+    // If both `batchSize` and `_populateBatchSize` are huge, calling `next()` repeatedly may
+    // cause a stack overflow. So make sure we clear the stack regularly.
+    if (this.ctx._batchDocs.length > 0 && this.ctx._batchDocs.length % 1000 === 0) {
+      return immediate(() => this.ctx.cursor.next().then(
+        res => { _onNext.call(this, null, res); },
+        err => { _onNext.call(this, err); }
+      ));
+    }
+    this.ctx.cursor.next().then(
+      res => { _onNext.call(this, null, res); },
+      err => { _onNext.call(this, err); }
+    );
+  } else {
+    _populateBatch.call(this);
+  }
+}
+
+/*!
+ * ignore
+ */
+
+function _populateBatch() {
+  if (!this.ctx._batchDocs.length) {
+    return this.callback(null, null);
+  }
+  const _this = this;
+  this.ctx.query.model.populate(this.ctx._batchDocs, this.ctx._pop).then(
+    () => {
+      _nextDoc(_this.ctx, _this.ctx._batchDocs.shift(), _this.ctx._pop, _this.callback);
+    },
+    err => {
+      _this.callback(err);
+    }
+  );
+}
+
+/*!
+ * ignore
+ */
+
+function _nextDoc(ctx, doc, pop, callback) {
+  if (ctx.query._mongooseOptions.lean) {
+    return ctx.model.hooks.execPost('find', ctx.query, [[doc]], err => {
+      if (err != null) {
+        return callback(err);
+      }
+      callback(null, doc);
+    });
+  }
+
+  const { model, _fields, _userProvidedFields, options } = ctx.query;
+  helpers.createModelAndInit(model, doc, _fields, _userProvidedFields, options, pop, (err, doc) => {
+    if (err != null) {
+      return callback(err);
+    }
+    ctx.model.hooks.execPost('find', ctx.query, [[doc]], err => {
+      if (err != null) {
+        return callback(err);
+      }
+      callback(null, doc);
+    });
+  });
+}
+
+/*!
+ * ignore
+ */
+
+function _waitForCursor(ctx, cb) {
+  if (ctx.cursor) {
+    return cb();
+  }
+  ctx.once('cursor', function(cursor) {
+    if (cursor == null) {
+      return;
+    }
+    cb();
+  });
+}
+
+module.exports = QueryCursor;