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Class AggregationCursor<TSchema>

The AggregationCursor class is an internal class that embodies an aggregation cursor on MongoDB allowing for iteration over the results returned from the underlying query. It supports one by one document iteration, conversion to an array or can be iterated as a Node 4.X or higher stream

Type parameters

  • TSchema = any

Hierarchy

Index

Events

CLOSE: "close" = ...

Properties

captureRejectionSymbol: typeof captureRejectionSymbol
captureRejections: boolean

Sets or gets the default captureRejection value for all emitters.

defaultMaxListeners: number
errorMonitor: typeof errorMonitor

This symbol shall be used to install a listener for only monitoring 'error' events. Listeners installed using this symbol are called before the regular 'error' listeners are called.

Installing a listener using this symbol does not change the behavior once an 'error' event is emitted, therefore the process will still crash if no regular 'error' listener is installed.

Accessors

  • get closed(): boolean
  • get id(): undefined | Long
  • get killed(): boolean
  • get loadBalanced(): boolean

Methods

  • [asyncIterator](): AsyncGenerator<TSchema, void, void>
  • addCursorFlag(flag: "tailable" | "oplogReplay" | "noCursorTimeout" | "awaitData" | "exhaust" | "partial", value: boolean): AggregationCursor<TSchema>
  • Add a cursor flag to the cursor

    Parameters

    • flag: "tailable" | "oplogReplay" | "noCursorTimeout" | "awaitData" | "exhaust" | "partial"

      The flag to set, must be one of following ['tailable', 'oplogReplay', 'noCursorTimeout', 'awaitData', 'partial' -.

    • value: boolean

      The flag boolean value.

    Returns AggregationCursor<TSchema>

  • bufferedCount(): number
  • close(): Promise<void>
  • emit<EventKey>(event: symbol | EventKey, ...args: Parameters<AbstractCursorEvents[EventKey]>): boolean
  • eventNames(): string[]
  • forEach(iterator: (doc: TSchema) => boolean | void): Promise<void>
  • Iterates over all the documents for this cursor using the iterator, callback pattern.

    If the iterator returns false, iteration will stop.

    deprecated
    • Will be removed in a future release. Use for await...of instead.

    Parameters

    • iterator: (doc: TSchema) => boolean | void

      The iteration callback.

        • (doc: TSchema): boolean | void
        • Parameters

          • doc: TSchema

          Returns boolean | void

    Returns Promise<void>

  • getMaxListeners(): number
  • hasNext(): Promise<boolean>
  • listenerCount<EventKey>(type: string | symbol | EventKey): number
  • Map all documents using the provided function If there is a transform set on the cursor, that will be called first and the result passed to this function's transform.

    remarks

    Note Cursors use null internally to indicate that there are no more documents in the cursor. Providing a mapping function that maps values to null will result in the cursor closing itself before it has finished iterating all documents. This will not result in a memory leak, just surprising behavior. For example:

    const cursor = collection.find({});
    cursor.map(() => null);

    const documents = await cursor.toArray();
    // documents is always [], regardless of how many documents are in the collection.

    Other falsey values are allowed:

    const cursor = collection.find({});
    cursor.map(() => '');

    const documents = await cursor.toArray();
    // documents is now an array of empty strings

    Note for Typescript Users: adding a transform changes the return type of the iteration of this cursor, it does not return a new instance of a cursor. This means when calling map, you should always assign the result to a new variable in order to get a correctly typed cursor variable. Take note of the following example:

    Type parameters

    • T

    Parameters

    • transform: (doc: TSchema) => T
        • (doc: TSchema): T
        • Parameters

          • doc: TSchema

          Returns T

    Returns AggregationCursor<T>

  • next(): Promise<null | TSchema>
  • Add a project stage to the aggregation pipeline

    remarks

    In order to strictly type this function you must provide an interface that represents the effect of your projection on the result documents.

    By default chaining a projection to your cursor changes the returned type to the generic Document type. You should specify a parameterized type to have assertions on your final results.

    example
    // Best way
    const docs: AggregationCursor<{ a: number }> = cursor.project<{ a: number }>({ _id: 0, a: true });
    // Flexible way
    const docs: AggregationCursor<Document> = cursor.project({ _id: 0, a: true });
    remarks

    In order to strictly type this function you must provide an interface that represents the effect of your projection on the result documents.

    Note for Typescript Users: adding a transform changes the return type of the iteration of this cursor, it does not return a new instance of a cursor. This means when calling project, you should always assign the result to a new variable in order to get a correctly typed cursor variable. Take note of the following example:

    example
    const cursor: AggregationCursor<{ a: number; b: string }> = coll.aggregate([]);
    const projectCursor = cursor.project<{ a: number }>({ _id: 0, a: true });
    const aPropOnlyArray: {a: number}[] = await projectCursor.toArray();

    // or always use chaining and save the final cursor

    const cursor = coll.aggregate().project<{ a: string }>({
    _id: 0,
    a: { $convert: { input: '$a', to: 'string' }
    }});

    Type parameters

    Parameters

    Returns AggregationCursor<T>

  • readBufferedDocuments(number?: number): TSchema[]
  • removeAllListeners<EventKey>(event?: string | symbol | EventKey): AggregationCursor<TSchema>
  • rewind(): void
  • 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.

    Returns void

  • toArray(): Promise<TSchema[]>
  • Returns an array of documents. The caller is responsible for making sure that there is enough memory to store the results. Note that the array only contains partial results when this cursor had been previously accessed. In that case, cursor.rewind() can be used to reset the cursor.

    Returns Promise<TSchema[]>

  • tryNext(): Promise<null | TSchema>
  • getEventListeners(emitter: EventEmitter | _DOMEventTarget, name: string | symbol): Function[]
  • Returns a copy of the array of listeners for the event named eventName.

    For EventEmitters this behaves exactly the same as calling .listeners on the emitter.

    For EventTargets this is the only way to get the event listeners for the event target. This is useful for debugging and diagnostic purposes.

    const { getEventListeners, EventEmitter } = require('events');

    {
    const ee = new EventEmitter();
    const listener = () => console.log('Events are fun');
    ee.on('foo', listener);
    getEventListeners(ee, 'foo'); // [listener]
    }
    {
    const et = new EventTarget();
    const listener = () => console.log('Events are fun');
    et.addEventListener('foo', listener);
    getEventListeners(et, 'foo'); // [listener]
    }
    since

    v15.2.0, v14.17.0

    Parameters

    • emitter: EventEmitter | _DOMEventTarget
    • name: string | symbol

    Returns Function[]

  • listenerCount(emitter: EventEmitter, eventName: string | symbol): number
  • A class method that returns the number of listeners for the given eventNameregistered on the given emitter.

    const { EventEmitter, listenerCount } = require('events');
    const myEmitter = new EventEmitter();
    myEmitter.on('event', () => {});
    myEmitter.on('event', () => {});
    console.log(listenerCount(myEmitter, 'event'));
    // Prints: 2
    since

    v0.9.12

    deprecated

    Since v3.2.0 - Use listenerCount instead.

    Parameters

    • emitter: EventEmitter

      The emitter to query

    • eventName: string | symbol

      The event name

    Returns number

  • on(emitter: EventEmitter, eventName: string, options?: StaticEventEmitterOptions): AsyncIterableIterator<any>
  • const { on, EventEmitter } = require('events');
    

    (async () => { const ee = new EventEmitter();

    // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); });

    for await (const event of on(ee, 'foo')) { // The execution of this inner block is synchronous and it // processes one event at a time (even with await). Do not use // if concurrent execution is required. console.log(event); // prints ['bar'] [42] } // Unreachable here })();


    Returns an `AsyncIterator` that iterates `eventName` events. It will throw
    if the `EventEmitter` emits `'error'`. It removes all listeners when
    exiting the loop. The `value` returned by each iteration is an array
    composed of the emitted event arguments.

    An `AbortSignal` can be used to cancel waiting on events:

    ```js
    const { on, EventEmitter } = require('events');
    const ac = new AbortController();

    (async () => {
    const ee = new EventEmitter();

    // Emit later on
    process.nextTick(() => {
    ee.emit('foo', 'bar');
    ee.emit('foo', 42);
    });

    for await (const event of on(ee, 'foo', { signal: ac.signal })) {
    // The execution of this inner block is synchronous and it
    // processes one event at a time (even with await). Do not use
    // if concurrent execution is required.
    console.log(event); // prints ['bar'] [42]
    }
    // Unreachable here
    })();

    process.nextTick(() => ac.abort());
    since

    v13.6.0, v12.16.0

    Parameters

    • emitter: EventEmitter
    • eventName: string

      The name of the event being listened for

    • Optional options: StaticEventEmitterOptions

    Returns AsyncIterableIterator<any>

    that iterates eventName events emitted by the emitter

  • once(emitter: _NodeEventTarget, eventName: string | symbol, options?: StaticEventEmitterOptions): Promise<any[]>
  • once(emitter: _DOMEventTarget, eventName: string, options?: StaticEventEmitterOptions): Promise<any[]>
  • Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.

    This method is intentionally generic and works with the web platform EventTarget interface, which has no special'error' event semantics and does not listen to the 'error' event.

    const { once, EventEmitter } = require('events');

    async function run() {
    const ee = new EventEmitter();

    process.nextTick(() => {
    ee.emit('myevent', 42);
    });

    const [value] = await once(ee, 'myevent');
    console.log(value);

    const err = new Error('kaboom');
    process.nextTick(() => {
    ee.emit('error', err);
    });

    try {
    await once(ee, 'myevent');
    } catch (err) {
    console.log('error happened', err);
    }
    }

    run();

    The special handling of the 'error' event is only used when events.once()is used to wait for another event. If events.once() is used to wait for the 'error' event itself, then it is treated as any other kind of event without special handling:

    const { EventEmitter, once } = require('events');

    const ee = new EventEmitter();

    once(ee, 'error')
    .then(([err]) => console.log('ok', err.message))
    .catch((err) => console.log('error', err.message));

    ee.emit('error', new Error('boom'));

    // Prints: ok boom

    An AbortSignal can be used to cancel waiting for the event:

    const { EventEmitter, once } = require('events');

    const ee = new EventEmitter();
    const ac = new AbortController();

    async function foo(emitter, event, signal) {
    try {
    await once(emitter, event, { signal });
    console.log('event emitted!');
    } catch (error) {
    if (error.name === 'AbortError') {
    console.error('Waiting for the event was canceled!');
    } else {
    console.error('There was an error', error.message);
    }
    }
    }

    foo(ee, 'foo', ac.signal);
    ac.abort(); // Abort waiting for the event
    ee.emit('foo'); // Prints: Waiting for the event was canceled!
    since

    v11.13.0, v10.16.0

    Parameters

    • emitter: _NodeEventTarget
    • eventName: string | symbol
    • Optional options: StaticEventEmitterOptions

    Returns Promise<any[]>

  • Parameters

    • emitter: _DOMEventTarget
    • eventName: string
    • Optional options: StaticEventEmitterOptions

    Returns Promise<any[]>

  • setMaxListeners(n?: number, ...eventTargets: (EventEmitter | _DOMEventTarget)[]): void
  • const {
    setMaxListeners,
    EventEmitter
    } = require('events');

    const target = new EventTarget(); const emitter = new EventEmitter();

    setMaxListeners(5, target, emitter); ```

    since

    v15.4.0

    Parameters

    • Optional n: number

      A non-negative number. The maximum number of listeners per EventTarget event.

    • Rest ...eventTargets: (EventEmitter | _DOMEventTarget)[]

    Returns void

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